Selected properties and life cycle assessment of mortars incorporating municipal solid waste bottom ash and silica fume for sustainable construction

The use of municipal solid waste bottom ash (MSWBA) as a sustainable alternative construction material is an effective approach to reduce natural resource depletion and advance circular economy concepts. The combined effects of MSWBA as a partial replacement of natural fine aggregate and silica fume (SF) as a supplementary cementitious material on the engineering performance and environmental sustainability of mortar were investigated in this study. Mortar mixtures were prepared with different binder content (389.6–524.9 kg/m 3), water to binder (w/b) ratio (0.526–0.663), MSWBA replacement level (60%–90%) and silica fume content (2%–8%). Ten mortar mixtures were prepared, including one control and nine modified mixtures. Tests for workability, setting time, compressive strength, flexural strength, ultrasonic pulse velocity (UPV), unit weight and water absorption were carried out for fresh and hardened properties evaluation. Correlation analyses, contour analysis and life cycle assessment (LCA) were also performed. The results showed that optimizing the w/b ratio produced satisfactory workability for all the mixtures. Although the early-age mechanical performance was reduced with high MSWBA replacement, mixtures with higher binder contents and silica fume had higher strength, density and durability. Among the MSWBA-modified mixtures, T4, T7, and T8 exhibited the most favorable overall combination of mechanical strength, UPV, and water-absorption performance. These optimized mixtures achieved comparatively high compressive and flexural strengths and UPV while maintaining relatively low water absorption, suggesting that the incorporation of silica fume helped mitigate the adverse effects associated with the porous nature of MSWBA. Correlation and contour analyses indicated that lower w/b ratios, particularly approximately 0.53–0.55, were associated with more favorable compressive-strength performance within the investigated mixture range. Results of the environmental assessment showed that the replacement of natural sand by MSWBA reduced the cradle-to-gate embodied carbon by up to 21.0%. This suggests the potential of optimized MSWBA–SF mortars to produce sustainable and low-carbon cementitious materials without compromising engineering performance.

The construction industry is a significant contributor to global greenhouse gas emissions, energy consumption, and solid waste generation, as well as one of the largest consumers of natural resources (Althoey et al., 2023; Ciutac et al., 2026). The depletion of non-renewable resources and the intensification of environmental impacts associated with extraction, manufacturing, and transportation activities have been significantly accelerated by the demand for cement and natural aggregates, which has been significantly increased by rapid urbanization and infrastructure development (Shukla et al., 2025; Sikhakhane et al., 2024). Portland cement production alone is responsible for approximately 7%–8% of global anthropogenic carbon dioxide (CO 2) emissions due to the energy-intensive clinker production process and limestone calcination (Althoey et al., 2023; Tran et al., 2025). In parallel, the extensive exploitation of natural river sand and crushed aggregates has contributed to resource depletion, ecosystem disturbance, and increased transportation-related emissions. Consequently, the development of sustainable cementitious materials that can reduce reliance on virgin resources while maintaining engineering performance has emerged as a significant research priority within the context of low-carbon construction and circular economy (Nilimaa, 2023; Valente et al., 2022). Municipal solid waste (MSW) generation has increased considerably worldwide due to population growth, urbanization, and changing consumption patterns. Waste-to-energy incineration has emerged as an effective approach for reducing landfill volumes and recovering energy; however, it generates significant quantities of municipal solid waste bottom ash (MSWBA), representing approximately 20%–30% of the original waste mass after combustion (Sivayogaraj and Elavenil, 2025; Chen et al., 2023). Traditionally, MSWBA has been disposed of in landfills, resulting in environmental concerns related to land occupation, potential heavy metal leaching, groundwater contamination, and long-term waste management costs (Bandarra et al., 2024; Sepúlveda Olea et al., 2024).

Therefore, the valorization of MSWBA as a secondary construction material has gained increasing attention as a sustainable strategy for reducing landfill dependency and conserving natural aggregate resources (Chen et al., 2025; Cho et al., 2020). The incorporation of MSWBA into mortar and concrete as a partial replacement for natural fine aggregate has demonstrated significant potential for sustainable construction applications. Previous studies have shown that appropriately processed MSWBA can be successfully incorporated into cementitious materials while maintaining acceptable engineering properties and reducing the consumption of virgin aggregates (Machaka et al., 2019; Woo et al., 2021). Nevertheless, the practical implementation of MSWBA remains challenging due to its low specific gravity, high water absorption, irregular particle morphology, porous structure, and variable chemical composition (Bawab et al., 2021; Chen et al., 2025). These characteristics may increase water demand, reduce packing efficiency, increase permeability, and negatively affect mechanical performance, particularly at high replacement levels (Chen et al., 2023; Cho et al., 2020). Therefore, appropriate material treatment and mixture optimization strategies are essential to overcome these limitations and maximize the potential of MSWBA-based cementitious materials. The incorporation of supplementary cementitious materials (SCMs) represents an effective strategy for improving the performance of recycled aggregate cementitious systems. Among available SCMs, silica fume (SF) has received considerable attention due to its ultrafine particle size, high amorphous silica content, and strong pozzolanic activity (Hamada et al., 2023; Lou et al., 2023). Silica fume reacts with calcium hydroxide generated during cement hydration to produce additional calcium silicate hydrate (C–S–H) gel while simultaneously acting as a micro-filler that improves particle packing and refines the capillary pore structure (Khan et al., 2023; Wang et al., 2022).

Numerous studies have demonstrated that SF incorporation enhances compressive strength, flexural strength, durability, and resistance to water penetration, particularly in cementitious composites containing porous or recycled aggregates (Ali et al., 2022; Wang et al., 2024). Accordingly, the combined utilization of MSWBA and SF provides a promising approach for improving mechanical performance while maintaining the environmental benefits associated with waste valorization. Despite the increasing interest in MSWBA-based cementitious materials, important research gaps remain. Previous studies have mainly focused on individual engineering properties, limited replacement levels, or specific material modifications rather than comprehensive mixture optimization approaches (Huynh and Ngo, 2022; Lu et al., 2024). Recent reviews have highlighted persistent challenges related to mixture proportion optimization, durability enhancement, and comprehensive evaluation of MSWBA-containing cementitious materials (Chen et al., 2025).

In particular, limited attention has been given to the combined influence of binder content, water-to-binder (w/b) ratio, silica fume dosage, and high-volume MSWBA replacement within a unified experimental framework. Furthermore, integrated assessments combining fresh-state properties, mechanical performance, durability-related characteristics, non-destructive evaluation, statistical optimization, and environmental assessment remain limited. Ultrasonic Pulse Velocity (UPV) provides valuable information regarding internal homogeneity, pore structure development, and material integrity, complementing conventional destructive mechanical testing methods (Ezzedine El Dandachy et al., 2024; Hachem et al., 2023). Beyond engineering performance, the environmental assessment of MSWBA-based construction materials requires further consideration. Most existing life-cycle assessment (LCA) studies rely on generic international inventory databases that may not accurately represent regional production technologies, transportation distances, or material supply chains. Previous research has demonstrated that aggregate extraction and transportation can significantly influence the embodied carbon of cementitious materials, particularly when materials are transported over long distances (D’Orazio et al., 2023; de Bortoli, 2023). This issue is particularly relevant in Lebanon, where the mountainous geography and concentration of construction activities in coastal urban areas often require the transportation of aggregates from inland quarry sites. Therefore, localsized life-cycle inventories are necessary to provide a more realistic evaluation of the environmental benefits associated with replacing natural aggregates with secondary materials such as MSWBA. Sustainable mortar design requires balancing competing engineering and environmental objectives. Increasing MSWBA replacement reduces natural aggregate consumption and promotes circular resource utilization; however, excessive replacement may compromise mechanical performance due to the porous nature of the ash. Conversely, increasing cement content generally enhances strength but increases embodied carbon because Portland cement remains the dominant contributor to emissions in cement-based materials. Therefore, identifying favorable combinations of binder content, silica fume dosage, MSWBA replacement level, and w/b ratio is essential for developing sustainable mortars capable of achieving both engineering reliability and environmental benefits (Najamuddin et al., 2024). Statistical correlation analyses and optimization techniques provide effective approaches for understanding interactions among these parameters and identifying favorable mixture conditions.

Accordingly, the primary objective of this study is to systematically investigate the combined influence of high-volume MSWBA replacement and silica fume modification on the fresh, hardened, non-destructive, and environmental performance of sustainable mortar mixtures. The novelty of the present work lies not in the individual use of experimental testing, correlation analysis, contour analysis, or life-cycle assessment, but in their integration within a unified mixture-design and assessment framework for high-volume MSWBA–SF mortars. Specifically, the study investigates high MSWBA replacement levels (60%–90%) while simultaneously varying binder content, water-to-binder (w/b) ratio, and silica fume dosage (2%–8%). The mixtures are comprehensively evaluated through workability, setting time, compressive strength, flexural strength, ultrasonic pulse velocity (UPV), unit weight, and water absorption. Correlation analysis and contour-based optimization are subsequently employed to identify relationships among mixture parameters and performance indicators and to determine favorable mixture regions. Finally, a cradle-to-gate (A1–A3) life-cycle assessment based on localized Lebanese inventory and transportation data is conducted to evaluate the environmental implications of natural sand substitution by MSWBA. By integrating high-volume waste utilization, silica fume modification, multi-parameter mixture design, comprehensive performance characterization, statistical optimization, and regionalized LCA, this study provides a systematic basis for evaluating the engineering–environmental trade-offs of MSWBA-based sustainable mortars.

Materials

Ordinary Portland Cement (OPC) and densified silica fume (SF) were used as the binder system in this investigation. Silica fume was incorporated as a supplementary cementitious material owing to its high amorphous silica content and fine particle size, which can contribute to microstructural densification and pozzolanic activity within the mortar matrix. The specific gravity of SF was taken as 2.20 based on reported values in the literature. Natural river sand, with a specific gravity of 2.65, was used as the fine aggregate in the control mixture. Municipal solid waste bottom ash (MSWBA), collected from the SICOMO plant located in Bar-Eliace, Lebanon, was used as a partial replacement for natural sand.

The raw MSWBA was subjected to a controlled pretreatment procedure prior to its incorporation into the mortar mixtures. The material was initially screened through a 4.5 mm mesh to remove oversized particles and unwanted impurities. It was then washed using a water-to-solid mass ratio of 2:1 under continuous agitation for 4 days. Following washing, the treated MSWBA was oven-dried at 105 °C ± 5 °C until a constant mass was achieved. The specific gravity of the treated MSWBA was 2.00. Water washing was adopted as a pretreatment because previous studies have demonstrated its potential to reduce soluble constituents and modify the leaching behavior of potentially hazardous constituents in municipal solid waste incineration bottom ash. In particular, Sun and Yi (2020), investigated the evolution of washing-water pH and its influence on the removal of heavy metals from incineration bottom ash, including Pb, Zn, Ni, Cd, Cu, and Cr, and demonstrated that washing conditions and duration can influence metal removal. Therefore, controlled washing conditions were considered appropriate for the pretreatment of the MSWBA used in the present study.

In the present study, the effectiveness of the washing treatment with respect to soluble constituents was assessed through conductivity measurements of the wash water before and after the 4-day treatment. The reduction in conductivity indicated a decrease in soluble ionic constituents following washing. However, detailed concentrations of individual heavy metals and their leaching behavior were not determined before and after washing in the present investigation. Therefore, the effectiveness of the treatment with respect to the removal or immobilization of specific potentially hazardous elements cannot be quantitatively established and is acknowledged as a limitation of the study. This limitation is relevant because the chemical composition and contaminant content of municipal solid waste incineration residues can vary considerably, and their suitability for construction applications requires appropriate chemical and environmental characterization (Sivayogaraj and Elavenil, 2025; Czop et al., 2021).

The chemical composition of the MSWBA was determined using X-ray fluorescence (XRF) analysis at the Sibline Cement Factory laboratory, Lebanon. Prior to analysis, the MSWBA samples were dried at 105 °C until a constant mass was achieved. The loss on ignition (LOI) was determined by heating a representative portion of the dried sample at 950 °C until a constant mass was obtained. The XRF results presented in Table 1 show that the MSWBA was primarily composed of CaO (35.93%), SiO 2 (21.10%), Al 2 O 3 (13.08%), and Fe 2 O 3 (8.07%), indicating a predominantly calcium- and aluminosilicate-based composition. The measured SO 3 content was 0.28%, while the chloride (Cl) content was 1.70%. The MSWBA also exhibited a relatively high LOI of 15.55%, which may be associated with residual unburned matter and/or other thermally decomposable constituents. The presence of chloride, sulfate-related compounds, and residual combustible constituents was considered when interpreting the potential use of MSWBA as a fine-aggregate replacement. Previous investigations of municipal solid waste incineration residues have similarly emphasized the importance of chemical composition, soluble salts, and potentially hazardous constituents when evaluating their use in cementitious materials. Although the chemical characterization provided information on the major oxide composition, chloride, sulfate-related content, and LOI of the MSWBA, potentially hazardous elements such as Pb, Cd, Cr, Ni, Zn, Cu, and As were not individually quantified in the present study. Accordingly, the present assessment does not establish the complete environmental safety or leaching compliance of the MSWBA. This limitation is particularly relevant at the high replacement levels investigated in this study, and further investigation involving detailed elemental and leaching characterization is recommended to fully assess the environmental suitability of the material.

A polycarboxylate ether-based superplasticizer was used for all mortar mixtures. The superplasticizer dosage was fixed at 4% of the total binder mass to achieve the target water-to-binder ratio (w/b) and maintain consistent workability throughout the experimental program.

Mix design

A comprehensive experimental investigation was conducted to examine the effects of municipal solid waste bottom ash (MSWBA) and silica fume (SF) on the fresh, physical, mechanical, and durability-related properties of mortar. Four principal mixture variables were considered: binder content, water-to-binder ratio (w/b), MSWBA replacement level, and silica fume content. The ranges and combinations of these variables were selected based on findings reported in previous studies, preliminary material trials, and the specific objective of evaluating the feasibility and limitations of incorporating MSWBA at substantially higher replacement levels than those commonly investigated in the literature. The experimental matrix was therefore designed as a targeted comparative investigation rather than as a formal statistical design-of-experiments methodology.

The MSWBA replacement levels of 60%, 75%, and 90% were deliberately selected to investigate high-volume incorporation and were defined on a volumetric basis. The MSWBA replacement was defined as the volume fraction of the total fine-aggregate fraction occupied by MSWBA, with the remaining volume occupied by natural sand. Because the specific gravity of the treated MSWBA (2.00) was lower than that of natural river sand (2.65), the corresponding masses of MSWBA and natural sand were calculated from their respective specific gravities while maintaining the prescribed aggregate replacement volume. Specifically, the absolute-volume method was used to first determine the total fine-aggregate volume, after which 60%, 75%, or 90% of this volume was allocated to MSWBA and the remaining 40%, 25%, or 10%, respectively, was allocated to natural sand. Thus, the reported MSWBA replacement percentages represent volume fractions and not mass fractions. Recent literature has generally focused on considerably lower MSWIBA replacement levels. A systematic review and meta-analysis reported that MSWIBA replacement levels above approximately 20% can be associated with greater reductions in mechanical performance and increased material-related challenges (Sivayogaraj and Elavenil, 2025). In contrast, Nadaf et al. (2026) investigated MSWIBA replacement levels up to 75% and reported that increasing cement content could partially mitigate the reduction in mechanical performance associated with high ash incorporation. Based on these findings, the present study extended the replacement range to 60%–90% to investigate the performance, limitations, and potential applicability of high-volume MSWBA incorporation.

Silica fume contents of 2%, 4%, and 8% were selected to investigate the potential contribution of a highly reactive and fine pozzolanic material to mortars containing high MSWBA contents. The three SF levels were intended to provide low, intermediate, and relatively high additions and to assess whether increasing SF content could contribute to matrix densification and improved mechanical performance. Previous experimental research on cementitious mortars has investigated low silica-fume additions and reported improvements in mechanical and microstructural properties associated with silica-fume incorporation (Etli et al., 2024). In addition, recent research on fly-ash-based geopolymer mortar investigated a broader silica-fume range of 2%–10%, demonstrating the relevance of evaluating both low and relatively high SF contents (Ergeshov et al., 2026). Accordingly, the 2%–8% range adopted in the present study was selected to provide a practical range of SF incorporation while avoiding excessive replacement of the primary binder.

The binder content was varied from 389.6 to 524.9 kg/m 3 to examine whether increasing the amount of cementitious material could partially compensate for the effects of high MSWBA replacement. This approach was supported by previous research indicating that increasing cement content can help mitigate the reduction in mechanical performance associated with high levels of MSWIBA incorporation (Nadaf et al., 2026). The selected binder contents also allowed different combinations of MSWBA and SF to be investigated while maintaining practical mortar proportions and adequate workability.

The w/b ratio was varied from approximately 0.53–0.66 to investigate the influence of effective water availability on workability and the resulting physical and mechanical properties. Because MSWBA has a relatively high water absorption capacity, preliminary absorption trials were conducted to quantify the amount of water retained by the MSWBA. To ensure precise mixture proportioning, a clear distinction is established among three water components: absorbed water (W absorbed), defined as the water retained within the pores of the aggregate materials; effective water (W effective), defined as the free water available in the mortar matrix for hydration and workability; and total water (W total), representing the absolute sum of both components. Thus, the total water can be expressed as:

The effective water content for each mixture was determined from the selected effective w/b ratio and the total binder content (cement + SF).

The absorption capacities of natural sand and treated MSWBA were determined from preliminary absorption measurements conducted before mixture preparation. The measured/adopted absorption values were 3% for natural sand and 15% for treated MSWBA. These values were subsequently used to calculate the absorbed-water component for each mixture based on the corresponding aggregate.

Thus, the water retained by MSWBA was accounted for separately during mixture preparation and was not included in the reported effective w/b ratio. The selected w/b values therefore reflect the combined requirements of workability, MSWBA water absorption, and the investigation of the influence of effective water content on mortar performance. The adopted range is also comparable to w/b ranges investigated in previous mortar studies incorporating silica-based materials (Etli et al., 2024).

The nine MSWBA-modified mixtures (T1–T9) were consequently formulated as targeted combinations of binder content (w/b) ratio, MSWBA replacement level, and SF content. The mixtures were selected to span the predefined ranges and to represent different combinations of low, intermediate, and high MSWBA and SF contents together with practical binder and effective water levels. The combinations were deliberately varied to investigate the combined response of the mortar to these mixture variables rather than to isolate the effect of a single parameter. The experimental matrix was not intended to represent a full factorial or formal statistical design; rather, it was established to provide representative combinations across the selected variable ranges while maintaining practical mortar proportions. A control mixture containing 100% natural sand and no SF or MSWBA was prepared as a reference for evaluating the changes associated with MSWBA and SF incorporation.

All mixtures were proportioned using the absolute-volume method to ensure volumetric consistency among the ten mortar mixtures (Control and T1–T9). The mix proportions and corresponding fresh-state flow values are summarized in Table 2. A constant dosage of 4% of the binder mass of a polycarboxylate-ether-based superplasticizer was used for all mixtures to promote comparable workability and uniform dispersion of the cementitious materials. The mortar ingredients were blended in a high-shear planetary mixer. The dry materials were mixed for 2 min, followed by the gradual addition of water and superplasticizer and mixing for a further 3 min until a homogeneous mortar was obtained.

Experimental matrix and variable interaction. It should be noted that the four mixture variables were not varied independently in the present experimental matrix. In several mixtures, changes in binder content were accompanied by changes in the w/b ratio, MSWBA replacement level, and/or SF content. Consequently, the experimental design does not permit complete isolation of the individual contribution of each mixture variable. In particular, the relatively higher binder contents generally correspond to lower w/b ratios in some mixtures, resulting in a potential confounding effect between binder content and water-to-binder ratio. Therefore, the observed changes in fresh, physical, mechanical, and durability-related properties are interpreted as the combined response to the mixture proportions, rather than being attributed exclusively to a single parameter. Comparisons between mixtures are consequently made with consideration of the differences in their overall composition. The experimental results should therefore be interpreted as identifying performance trends and promising mixture combinations rather than establishing independent causal relationships for each individual variable. A statistically controlled experimental design in which the variables are independently varied would be required to quantify their individual and interaction effects.

Specimen preparation and curing

The fresh mortar was cast in cube molds of 50 × 50 × 50 mm for the compressive strength tests and prism molds of 40 × 40 × 160 mm for the flexural strength and ultrasonic pulse velocity (UPV) tests. Manual compaction was employed during casting to reduce entrapped air and to provide adequate consolidation. Plastic sheets were placed over all the specimens immediately after casting to prevent early loss of moisture. After 24 h, the specimens were demolded and cured by immersing in water for 7 days to promote early hydration. Then, they were stored under controlled laboratory conditions (20 °C ± 2 °C and 60% ± 5% relative humidity) for a further 21 days until testing at 28 days. The selected curing regime was intended to provide sufficient moisture availability during the early hydration period while subsequently allowing the specimens to develop under controlled laboratory conditions. This approach is relevant to MSWBA-containing cementitious materials because water-treated MSWBA can participate in microstructural development and influence strength development during curing, with measurable effects observed at 28 days (Chen et al., 2025). Previous investigations have also demonstrated that the curing method can significantly influence the strength development of MSWIBA-containing cementitious materials (Huang et al., 2018; Liu et al., 2022). Accordingly, the 7-day water-curing period followed by controlled laboratory storage was adopted to ensure adequate early hydration while providing consistent conditions for evaluating the 28-day properties of the mortars. The 28-day testing age was selected to assess the developed mechanical and physical properties following the initial hydration and post-curing period. Nevertheless, longer-term curing ages were not investigated in the present study; therefore, the results should not be interpreted as representing long-term performance beyond 28 days.

Experimental testing

The particle size distribution of the constituent materials was established through sieve analysis according to ASTM C136 (ASTM C136-06, 2006). The fineness modulus (FM) of each blend of aggregates was determined from the cumulative percentage retained to evaluate the impact of aggregate grading on particle packing. The fresh-state workability of the mortar mixtures was assessed by the flow table test in accordance with ASTM C1437 (ASTM C1437, 2020), in which the mortar was subjected to 25 standard drops and the spread diameter was recorded (Figure 1a). Initial and final setting times were evaluated using a Vicat apparatus according to ASTM C191 (ASTM C191-21, 2021) (Figure 1b). To ensure valid application to the mortar system and comparative consistency across all mixes, setting-time specimens were prepared by testing representative paste/matrix portions of each batch immediately after mixing. Because a uniform superplasticizer dosage of 4% by binder mass was maintained across all formulations, the relative influence of high-volume MSWBA replacement and silica fume incorporation on setting behavior was evaluated under identical binder-matrix testing conditions, enabling consistent comparison with the reference control mortar. Given that ASTM C191 is primarily intended for hydraulic cement pastes, the obtained setting times are interpreted as comparative measurements of setting behavior among the investigated mixtures rather than as direct equivalents to setting times of neat cement paste.

Mechanical performance of the hardened state was tested by compressive and flexural strength testing. Compressive strength was measured on 50 × 50 × 50 mm cube specimens in accordance with ASTM C109 (ASTM C109/C109M-21, 2021) at curing ages of 7 and 28 days (Figure 1c). Representative mortar cube specimens prepared for testing are shown in Figure 1d. Flexural strength was determined using 40 × 40 × 160 mm prism specimens under three-point bending. Prior to flexural testing, ultrasonic pulse velocity (UPV) measurements were performed on the prism specimens in accordance with ASTM C597 (ASTM International, 2016) to assess their internal integrity and homogeneity using a non-destructive testing approach.

The physical properties of the hardened mortar, including dry unit weight, saturated surface-dry (SSD) unit weight, water absorption, and permeable void content, were determined at 28 days. Cube specimens were oven-dried at 105 °C ± 5 °C until a constant mass was reached and subsequently immersed in water for 24 h to obtain the SSD condition. All reported results represent the average of three replicate specimens for each mixture.

Correlation analyses were performed to investigate the relationships among the measured physical, mechanical, and durability properties. Linear regression models were developed using the average values of each mixture, and the coefficient of determination (R 2) was used to quantify the strength of the observed relationships.

Environmental life cycle inventory

A cradle-to-gate constituent-material LCA was conducted to assess the environmental implications of replacing natural fine aggregate with municipal solid waste bottom ash (MSWBA) in mortar. The environmental assessment was conducted following the principles and methodological requirements of ISO 14040 (2006); ISO 14044 (2006). The system boundary was defined as cradle-to-gate for the quantified constituent-material inventory and covered the upstream production or acquisition of the constituent materials, the inventory processes represented by the adopted emission factors, and the transportation processes explicitly included in the life-cycle inventory up to the mortar production facility.

For natural fine aggregate, the inventory factor included both the material production contribution (A1) and the specified transportation contribution (A2). For MSWBA, the material was treated as a secondary waste-derived material under the cut-off allocation approach, with no upstream environmental burden assigned to the ash itself. The laboratory-scale MSWBA pretreatment operations used in this experimental study (washing, mechanical agitation, liquid–solid separation, and oven drying) were not separately quantified in the baseline LCA because corresponding energy, water, and process inventories were not included in the adopted LCI. Accordingly, these operations are outside the quantified baseline inventory, although their potential industrial-scale environmental implications are discussed as a limitation. The assessment therefore represents the cradle-to-gate impacts captured by the defined inventory rather than a complete industrial-scale model of MSWBA processing. The assessment excluded transportation of fresh mortar to the construction site, mortar placement and construction activities, the use and maintenance stage, demolition, and end-of-life treatment or disposal of the mortar.

The functional unit was defined as 1 m 3 of mortar, and all mixtures were compared on this equivalent functional basis. The control mortar served as the reference mixture and contained natural fine aggregate without MSWBA replacement. Its calculated embodied carbon was used as the baseline against which the environmental performance of the MSWBA-containing mixtures was compared. The reported percentage changes in embodied carbon therefore represent the difference between each MSWBA-containing mixture and the control mixture on the common functional basis of 1 m 3 of mortar. The environmental benefit associated with MSWBA replacement arises primarily from the reduced quantity of natural fine aggregate required and the allocation assumption that assigns no upstream environmental burden to MSWBA under the cut-off approach.

The localized life-cycle inventory for each constituent material is summarized in Table 3. To improve regional representativeness and reduce the uncertainties related to generic international datasets, key inventory parameters were adapted to the Lebanese conditions. The emission factor for Ordinary Portland Cement (CEM I) was considered as 0.83 kg CO2e kg -1, consistent with those reported for Lebanese cement production by the Ministry of Environment and the United Nations Development Programme (MoE/UNDP/GEF, 2015). For natural fine aggregates, a conservative emission factor of 0.0185 kg CO 2 e kg −1 was used. This value includes an A1 production-stage contribution of 0.0050 kg CO 2 e kg −1 from crushed sandstone aggregate datasets in the ecoinvent Version 3.12 life-cycle inventory database (ecoinvent Association, 2025), supported by published life-cycle assessments of manufactured sand production (Zhu X. et al., 2023), and an A2 transportation contribution of 0.0135 kg CO 2 e kg −1.

Transportation emissions were calculated using a representative one-way haul distance of 75 km from inland quarry locations to coastal mortar production facilities. The transportation emission factor was 0.18 kg CO 2 e tonne −1 km −1. This assumption is consistent with the mountainous terrain, transportation conditions, and aggregate supply characteristics of northern Lebanon, where construction materials are often transported from inland sources to urban coastal areas (Abi Farraj et al., 2020). Previous studies have shown that transportation can constitute a significant portion of aggregate-related embodied emissions, highlighting the importance of using region-specific transportation assumptions in LCA studies (Pacheco et al., 2022; Zhu Y. et al., 2023).

The emission factors used in the environmental inventory for the mortar constituents—OPC, silica fume, water, natural fine aggregate, superplasticizer, and MSWBA—are summarized in Table 3, together with their respective data sources, geographic scopes, and allocation assumptions. For MSWBA, the adopted cut-off allocation assigns no upstream environmental burden to the secondary material itself; therefore, an emission factor of 0.0000 kg CO 2 e kg -1 was applied in the baseline LCA.

The total embodied carbon footprint (M CO2, kg CO 2 e/m 3) for each mixture were calculated using the following equations:

The functional unit was defined as 1 m 3 of mortar. Accordingly (M i) represents the mass of constituent (i) required to produce 1 m 3 of mortar (kg/m 3), and (EF i,CO2) represents the corresponding emission factor (kg CO 2 e kg -1 of material). The total calculated embodied carbon is therefore expressed in kg CO 2 e m -3 of mortar.

Statistical and contour analysis

To further evaluate the combined influence of the water-to-binder ratio and MSWBA replacement level on mortar performance, contour surface analyses were developed using the average compressive strength values obtained for each mixture. The contour plots were generated by interpolating the experimental data using a two-dimensional surface fitting algorithm, allowing visualization of the interactive effects of the design variables and identification of optimal performance regions. In addition, linear regression analyses were performed to investigate the relationships among the measured physical, mechanical, and durability properties. The coefficient of determination (R 2) was used to quantify the strength of the observed correlations.

Particle size distribution and aggregate skeleton characteristics

Particle size distribution (PSD) of the natural sand, raw municipal solid waste bottom ash (MSWBA), and sand–MSWBA blends was determined by sieve analysis according to ASTM C136. The grading curves of the natural sand, raw MSWBA, and investigated sand–MSWBA blends are presented in Figures 2a–e. The natural sand exhibited a relatively fine particle-size distribution, with a fineness modulus (FM) of 1.82, whereas the raw MSWBA exhibited a considerably coarser particle-size distribution, with an FM of 3.29. The higher FM of MSWBA indicates a greater proportion of relatively coarse particles compared with the natural sand. This difference is consistent with the heterogeneous and relatively coarse particle characteristics commonly reported for MSWBA (Sivayogaraj and Elavenil, 2025; Loginova et al., 2019).

The incorporation of MSWBA consequently modified the overall grading characteristics of the aggregate blends, as illustrated in Figures 2c–e. For the experimentally characterized blends for which FM values were determined, the FM increased from 1.82 for the natural sand to approximately 2.30 and 2.31 for the 40:60 and 25:75 sand–MSWBA blends, respectively. This shift in FM indicates that incorporation of MSWBA changed the relative distribution of particle sizes within the aggregate skeleton. Such changes in particle-size distribution can influence particle packing, interparticle interaction, water demand, and the resulting mechanical performance of cementitious materials containing MSWBA (Cheng et al., 2024; Loginova et al., 2019).

For comparative evaluation of aggregate grading, the particle-size distributions presented in Figures 2a–e were compared with the fine-aggregate grading limits specified in ASTM C33/C33M. The reference grading envelope was defined using cumulative percentage-passing limits of 100% for the 9.5-mm (3/8-in.) sieve, 95%–100% for the No. 4 (4.75 mm) sieve, 80%–100% for the No. 8 (2.36 mm) sieve, 50%–85% for the No. 16 (1.18 mm) sieve, 25%–60% for the No. 30 (600 μm) sieve, 5%–30% for the No. 50 (300 μm) sieve, and 0%–10% for the No. 100 (150 μm) sieve. These limits are shown in Figures 2a–e as the ASTM Min and ASTM Max curves. Because MSWBA is a processed waste-derived aggregate rather than a conventional natural fine aggregate, the ASTM C33/C33M limits were used only as a comparative reference and were not interpreted as establishing formal ASTM C33/C33M compliance of the MSWBA.

The grading curve of the natural sand in Figure 2a shows that it falls within the ASTM C33/C33M reference limits at several sieve sizes, including 4.75, 2.36, and 0.15 mm. However, the cumulative percentage passing exceeded the ASTM upper limits at the 1.18-, 0.60-, and 0.30-mm sieves, with values of 87.65%, 75.65%, and 50.65%, respectively, compared with maximum limits of 85%, 60%, and 30%. This indicates that the natural sand contains a relatively high proportion of particles in the intermediate-to-fine size range compared with the ASTM reference envelope. The raw MSWBA exhibited a distinctly different grading profile, as shown in Figure 2b. The cumulative percentage passing was 99.1%, 89.9%, 54.9%, 24.9%, 1.7%, and 0.8% at the 4.75-, 2.36-, 1.18-, 0.60-, 0.30-, and 0.15-mm sieves, respectively. Compared with the natural sand, the MSWBA therefore contained a substantially lower proportion of fine particles and a greater proportion of coarser particles. Its grading was within the ASTM reference range at the 4.75-, 2.36-, 1.18-, and 0.15-mm sieves, while the percentages passing the 0.60- and 0.30-mm sieves were slightly and substantially below the corresponding ASTM minimum limits, respectively. These results further confirm the relatively coarse nature of the raw MSWBA.

The incorporation of MSWBA produced intermediate grading characteristics in the sand–MSWBA blends, as illustrated in Figures 2c–e. For the 40:60 sand–MSWBA blend shown in Figure 2c, the cumulative percentage passing was 99.95%, 91.80%, 79.20%, 64.00%, 31.40%, and 3.80% at the 4.75-, 2.36-, 1.18-, 0.60-, 0.30-, and 0.15-mm sieves, respectively. The corresponding values for the 25:75 blend shown in Figure 2d were 98.95%, 95.05%, 79.45%, 59.30%, 32.50%, and 3.35%. Both blends therefore exhibited grading profiles broadly comparable with the ASTM C33/C33M reference envelope over most of the investigated sieve range, although the percentage passing the 0.30-mm sieve was slightly above the ASTM upper limit of 30% for both blends. The 10:90 sand–MSWBA blend shown in Figure 2e exhibited a further shift toward the coarser grading characteristics associated with the higher MSWBA content. Accordingly, the investigated blends should be described as broadly comparable to, rather than fully compliant with, the ASTM C33/C33M grading envelope. Overall, the sieve-analysis results demonstrate that increasing the MSWBA proportion progressively changes the aggregate skeleton from the relatively fine grading of the natural sand toward the coarser grading characteristics of MSWBA. This trend is evident from the shift in the grading curves from Figures 2a–e and, for the blends for which FM was determined, from the increase in FM from 1.82 for natural sand to approximately 2.30–2.31. The resulting modification of particle-size distribution may affect packing efficiency, void structure, water demand, and particle interaction within the mortar matrix. Nevertheless, ASTM C33/C33M grading limits alone cannot establish optimum packing or overall suitability of MSWBA because waste-derived aggregates may differ substantially from natural sand in particle morphology, porosity, surface texture, and absorption characteristics. Accordingly, the grading results were used primarily as a comparative characterization of the aggregate blends rather than as a stand-alone criterion for mixture selection. The effects of the observed grading differences were therefore considered together with the subsequent fresh, physical, mechanical, and durability-related results when evaluating the performance of the MSWBA-containing mortars.

Workability

The flow diameter of all nine mixtures were consistent in the range of 16–17 cm indicating the robustness of the mix design strategy across different binder levels. The effect of chemical dispersion was removed by maintaining the dosage of the superplasticizer (polycarboxylate based) at a fixed value of 4% of the total binder mass, leaving the water-to-binder (w/b) ratio as the only rheological control lever. This experimental approach demonstrates that the increased water demand associated with the ultrafine, high-specific-surface-area silica fume (SF) and the porous, angular morphology of the municipal solid waste bottom ash (MSWBA) can be effectively counteracted through accurate w/b ratio calibration. It should be noticed that there is a clear correlation between the binder content and the w/b requirements: the mixtures with a higher binder content (i.e., T7, T8, and T9 between 504.8 and 524.9 kg/m 3) allow for lower w/b ratios (0.526–0.550) than the low-binder series (i.e., T1–T3 with 389.6–406.3 kg/m 3 and w/b ratios close to 0.66). This behavior indicates increased efficiency in packing of particles, in which the increased volume of fine cementitious material leads to a more lubricated paste matrix, which reduces the void space to be filled by free water. The observed fresh-state stability indicates that the adopted mixture-proportioning strategy provided comparable workability and adequate specimen consolidation across the investigated mixtures. Therefore, differences in the subsequent hardened-state properties are interpreted primarily in relation to the measured differences in mixture composition and material characteristics rather than to differences in fresh-state flow or casting consistency.

Setting time

The initial and final setting times of the mortar mixtures were measured using Vicat needle apparatus in accordance with ASTM C191. Measurement results are shown in Figure 3. The initial setting time was found to be in the range of 25–35 min, whereas the final setting time was found to be in the range of 174–230 min. This shows a relatively uniform setting behavior for all the mixtures even with significant variations in binder content, silica fume dosage and the MSWBA replacement level. The slight difference in setting time implied that the use of 60%–90% MSWBA as a replacement of fine aggregate did not result in substantial changes in the measured setting times of the cementitious matrix. The high pozzolanic reactivity and ultrafine particle size of silica fume can promote nucleation and accelerate the formation of calcium silicate hydrate (C–S–H), but the dosage of silica fume (2%–8%) was not sufficient to cause significant changes in the overall setting characteristics. Moreover, the observed influence on setting behavior was relatively minor since MSWBA was used as aggregate filler and not as cement replacement. Similar observations were made in previous research (Huynh and Ngo, 2022; Irshidat et al., 2021) where the use of properly processed MSWBA as a fine aggregate replacement had a minor effect on the setting properties of cementitious materials. The relatively uniform setting characteristics indicate that the investigated mixtures exhibited comparable setting behavior despite the variations in binder content, silica fume dosage, and MSWBA replacement level. T7 showed the highest initial (35 min) and final (230 min) setting times among the investigated mixtures (Figure 3). This behavior may be associated with the combined influence of its relatively high binder content (524.9 kg/m 3), w/b ratio (0.550), and overall mixture composition. However, the observed differences in setting time were relatively small across the experimental mixtures. Since the setting-time measurements were performed on representative paste/matrix portions using the Vicat procedure, the results should be interpreted primarily as comparative indicators of setting behavior under the adopted laboratory conditions rather than as direct measures of field setting performance. Overall, the results indicate that incorporating 60%–90% MSWBA as a fine-aggregate replacement, together with 2%–8% SF, did not produce large changes in the measured setting times under the conditions investigated. Nevertheless, additional testing under field-representative conditions would be required to establish setting behavior for specific construction applications.

Unit weight (dry and SSD at 28 days)

Figure 4 depicts the 28-day dry and saturated surface-dry (SSD) unit weights of the mortar mixtures. The control mix had the highest unit weight and in general, the incorporation of Municipal Solid Waste Bottom Ash (MSWBA) decreased the densities for both dry and SSD conditions. The reduction is mainly due to the lower specific gravity (2.00) and more porous structure of MSWBA compared to natural river sand (specific gravity = 2.65), which results in lighter mortar composites. Similar observations have been observed in Irshidat et al. (2021), Huynh and Ngo (2022); Chen et al. (2025). Among the modified mixtures, T7, T8 and T9 gave the highest unit weights close to the control mixture, although they had high levels of MSWBA replacement. The behavior of the samples can be explained by their relatively high binder contents (approx. 505–525 kg/m 3) and the presence of silica fume whose fine particle size may improve particle packing and contribute to matrix densification. Its established pozzolanic behavior may also contribute to matrix refinement, as reported in previous cementitious systems; however, this mechanism was not directly characterized in the present study. Therefore, the incorporation of supplementary cementitious materials may partially compensate for the lower density associated with MSWBA and may contribute to improved particle packing and matrix density. The unit-weight results correlate well with the respective water absorption results, where the denser mixtures showed lower absorption values, which is an indicator of a less permeable microstructure. The results suggest that the reduction in density due to the high volume inclusion of MSWBA with acceptable physical properties can be significantly reduced by optimizing the binder content and silica fume dosage.

Compressive strength

Figure 5 shows the evolution of the compressive strength of the mortar mixes at 7 days and 28 days. As expected, compressive strength of all experimental mixtures was lower than that of the control mixture. This reduction can be attributed to the combined effects of MSWBA replacement and the associated differences in binder content, w/b ratio, particle packing, density, and aggregate morphology. The relatively limited early-age strength development may also be consistent with the slower reactivity reported for some MSWBA-containing cementitious systems. Previous studies have associated this behavior with the heterogeneous composition and relatively limited reactivity of certain ash constituents compared with Portland cement (Faleschini et al., 2021; Malaiškienė, Spudulis and Stonys, 2023; Wong et al., 2025). However, the contribution of MSWBA reactivity to the strength development of the present mixtures was not directly investigated through pozzolanic-activity testing or mineralogical and microstructural characterization. Therefore, the lower early-age strength should not be attributed to MSWBA reactivity alone, but rather interpreted as the combined response of the mixture composition and the physical characteristics of the MSWBA.

However, the strength reduction was less pronounced for the experimental series T4, T7, and T8 than for the others. The highest binder contents in the test series (475.0–524.9 kg/m 3) were observed for these mixtures. This was associated with higher binder contents and may have contributed to a denser cementitious matrix and better binding capacity, as reflected in the measured mechanical performance (Table 2). The addition of silica fume (SF) in these mixtures is very important due to the ultra-fine particle size and high amorphous silica content. These characteristics are commonly associated with micro-filling and pozzolanic effects in cementitious systems, which may contribute to matrix refinement and improved mechanical performance. However, hydration products, pore structure, and ITZ characteristics were not directly characterized in the present study; therefore, these mechanisms are considered possible explanations based on previously reported behavior rather than experimentally verified processes in the present mixtures.

The enhanced performance of T4 and T8 is also attributed to the particle size distribution of the MSWBA, and the sieve analysis indicates that these mixtures have a well-graded distribution. This may contribute to more efficient void filling, potentially improving packing and mechanical interlocking (Deng et al., 2025; Duan et al., 2025).

On the other hand, T7 with a high binder content of 524.9 kg/m 3 had a slight limitation in packing efficiency due to its very high MSWBA replacement level (90%), further emphasizing that aggregate gradation, particle morphology, and filler proportion are important factors in addition to total binder content and binder reactivity. In addition, the use of a controlled w/b ratio together with a constant superplasticizer dosage of 4% was intended to maintain adequate cohesiveness and particle dispersion, although differences in water demand among the mixtures may still have influenced their strength development. These results suggest that the effective combination of silica fume and refined aggregate gradation can help bridge the performance gap between waste-modified mortars and traditional control mixes, a phenomenon also documented in extensive research on industrial by-products (Mousa, 2023). Previous studies have reported continued strength development in some MSWBA-containing cementitious systems at later curing ages; however, longer curing ages were not investigated in the present study. Therefore, no conclusion regarding strength development beyond 28 days can be drawn from the present results.

It should be emphasized that the present study did not include direct characterization of pozzolanic activity, hydration products, pore structure, or interfacial microstructure. Accordingly, interpretations involving MSWBA reactivity, C–S–H development, pore refinement, or ITZ modification are based on mechanisms reported in previous studies and are presented as possible explanations for the observed performance trends rather than as experimentally confirmed mechanisms of the present mixtures.

Flexural strength

The flexural strength results at 7 days were generally lower than those of the control mixture, presented in Figure 6, (6.9 MPa), which may be related to the combined effects of MSWBA replacement, binder content, w/b ratio, particle packing, and the physical characteristics of MSWBA. The relatively limited early-age development may also be consistent with the slower reactivity reported for some MSWBA-containing cementitious systems. Mixtures T8 (5.9 MPa), T4 (5.3 MPa) and T7 (5.3 MPa) showed the highest flexural performance, notably. The better performance is attributed to their optimized binder contents (475.0–524.9 kg/m 3) and higher silica fume (SF) content.

The ultrafine SF particles may act as effective micro-fillers, contributing to matrix densification and pore refinement and potentially improving the quality of the interfacial transition zone (ITZ). These effects may reduce internal discontinuities associated with the irregular morphology of MSWBA and contribute to improved resistance to crack propagation under tensile stress. However, these mechanisms were not directly verified in the present study because no microstructural characterization was performed. Similar beneficial effects of optimized binder combinations and fine supplementary materials have been reported in other blended cementitious systems. Hasim et al. (2022) reported that synergistic binder combinations can improve the performance of mortars incorporating high volumes of bottom ash, while Abdullah et al. (2022) highlighted the influence of bottom ash grading on the structural density of the cementitious matrix. More recently, Mishra et al. (2026) reported that multi-scale binder modification incorporating ultrafine fly ash and nano-silica promoted hydration and the development of dense C–S–H/C–A–S–H phases, contributing to improved mechanical performance. Similarly, Mishra et al. (2025b) reported that the incorporation of GGBS and metakaolin enhanced secondary hydration and microstructural densification in blended mortar systems. These studies provide supporting evidence for the potential role of fine supplementary materials and optimized binder systems; however, their findings were obtained for different blended cementitious systems and therefore should not be interpreted as direct microstructural evidence for the present MSWBA–SF mixtures.

In contrast, mixtures with lower binder contents (T1, T2 and T3) showed a clear “dilution effect” where the combination of fewer reactive cementitious phases and increased porosity (due to higher w/b ratios needed for workability) may have contributed to lower structural cohesion and reduced flexural strength. The lower binder availability and higher w/b ratios may have resulted in a less dense cementitious matrix and reduced mechanical performance. Although a lower concentration of C-S-H gel could contribute to this behavior, the C-S-H content was not directly measured in the present study; therefore, this mechanism is considered a proposed explanation rather than an experimentally verified result. The results of the T4, T7 and T8 series indicate that an appropriate combination of binder content, SF incorporation, and MSWBA aggregate replacement can improve the early-age flexural performance of the mortars. However, the present experimental results demonstrate the observed strength trends, while the underlying microstructural mechanisms remain proposed based on mechanisms reported for other blended cementitious systems.

Ultrasonic pulse velocity (UPV)

The Ultrasonic Pulse Velocity (UPV) test at 7 days was used as a non-destructive indicator of the internal structural characteristics and homogeneity of the mortar matrix. The control mixture showed the highest velocity (3,759 m/s), as presented in Figure 7. The experimental mixtures generally exhibited lower UPV values, which may be associated with the porous and irregular morphology of the municipal solid waste bottom ash (MSWBA) particles and their influence on the internal continuity of the mortar matrix. The performance-leading mixtures, T7 (3,523 m/s), T8 (3,482 m/s), and T4 (3,359 m/s), had relatively high binder contents (475.0–524.9 kg/m 3) together with silica fume (SF) incorporation. These mixture characteristics may have contributed to improved particle packing and matrix continuity, as reflected in their relatively higher UPV values. The ultrafine SF particles may also contribute to improved particle packing and reduced inter-particle voids, which can favor ultrasonic wave transmission. Such behavior is consistent with previous studies of waste-containing cementitious systems reporting associations between UPV and the internal characteristics of cementitious matrices (Wu et al., 2024). However, these possible effects were not directly verified through pore-structure or microstructural characterization in the present study.

On the other hand, mixtures with high ash replacement levels and relatively low binder contents, such as T3 (3,008 m/s), showed substantially lower UPV values. This behavior is consistent with less favorable internal structural characteristics and lower mechanical performance associated with these mixture proportions. These results are in general agreement with the compressive and flexural strength results presented in Sections 3.4, 3.5 and support the observed differences in mechanical performance among the investigated mixtures. Overall, the UPV results indicate differences in the internal characteristics of the investigated mixtures, with higher values generally observed for mixtures incorporating higher binder contents and SF. However, UPV should be interpreted as an indirect indicator of internal material characteristics rather than as direct evidence of pore refinement, matrix densification, or microstructural modification, since no direct microstructural characterization was performed in the present study.

Water absorption

The 28-day water absorption test was used as an indicator of water uptake and the physical characteristics of the mortar matrix. The results show that the addition of Municipal Solid Waste Bottom Ash (MSWBA) tends to increase water abosrption but this can be systematically controlled by an optimized mix design. As seen from Figure 8, the control mixture presented the minimum absorption (4.8%) and the performance of experimental mixtures was very sensitive to the balance between the binder content, SF supplementation and w/b ratio. In particular, the experimental series T7 (5.6%), T4 (6.8%) and T9 (6.8%) were characterized by the lowest water absorption among the MSWBA-modified mixtures. This confirms the need to use higher binder contents (from 475.0 to 524.9 kg/m 3) together with a controlled w/b ratio (0.55) to overcome the porous nature of MSWBA. This approach is consistent with many studies that have shown that a low w/b ratio is one of the primary means of reducing the capillary water content, thereby improving the pore structure and reducing water uptake (Yusslee and Beskhyroun, 2023). In these mixtures, the ultrafine SF particles may have contributed to improved matrix packing and reduced water absorption through their micro-filling and pozzolanic effects, as reported for SF-containing cementitious systems. These effects may be associated with pore refinement and changes in hydration-product development. However, because pore structure and hydration products were not directly characterized in the present study, these mechanisms are considered possible explanations rather than experimentally verified processes. On the other hand, the higher absorptions in T1 (8.4%) and T2 (8.7%) are indicative of the detrimental “dilution effect” associated with the use of low binder contents, high ash replacement levels and high w/b ratios, which leave the matrix vulnerable to fluid ingress. These results indicate that an optimized binder-to-ash-to-water combination can reduce water absorption in MSWBA-modified mortars; however, further durability testing is required to determine whether these mixtures provide adequate long-term durability for structural applications.

Correlation analysis

To evaluate the relationships among the measured properties, simple linear regression analyses were performed using the 10 mixture-level observations (Control and T1–T9). For each correlation, the regression equation, coefficient of determination (R 2), and p-value were calculated to assess the strength and statistical significance of the observed relationships. Given the limited number of mixture-level observations and the simultaneous variation of several mix-design parameters, these correlations are considered exploratory and specific to the mixtures investigated rather than universally applicable predictive models.

Compressive Strength and Unit Weight

The relationship between unit weight and 28-day compressive strength is presented in Figure 9. The fitted regression was f c = 0.00784UW+3.451, with R 2 = 0.095 and p = 0.387. Although a positive slope was obtained, the relationship was not statistically significant. This indicates that unit weight alone did not adequately explain the variation in compressive strength among the investigated mixtures. Previous studies have reported that matrix density and unit weight can be associated with mechanical performance in cementitious materials (Fajardo et al., 2025; Wang et al., 2023). However, the present regression results indicate that this relationship was not sufficiently strong or statistically significant for the MSWBA-modified mixtures investigated here. The observed behavior highlights the combined influence of binder content, MSWBA content, SF incorporation, and w/b ratio.

Compressive strength and pulse velocity

Figure 10 shows the relationship between 7-day UPV and 7-day compressive strength. The fitted regression was f c = 0.00742UPV - 9.646, with R 2 = 0.415 and p = 0.0445. The statistically significant positive relationship indicates that higher UPV values were generally associated with higher compressive strength within the investigated mixtures. However, the moderate R^2 value indicates that UPV alone explained only part of the observed variation in compressive strength. The observed positive association is consistent with the general relationship between ultrasonic pulse velocity and the internal compactness and continuity of cementitious matrices reported in previous research (Ezzedine El Dandachy et al., 2024). In the present study, mixtures T4, T7, and T8 exhibited relatively high UPV and compressive strength values, whereas T1–T3 showed comparatively lower values. This behavior may be associated with differences in matrix compactness and internal pore structure resulting from the combined effects of MSWBA replacement, SF incorporation, binder content, and w/b ratio. However, these microstructural interpretations were not directly verified by microstructural characterization in the present study. Therefore, the regression relationship should be regarded as an empirical association specific to the investigated mixtures rather than a universally applicable predictive model.

Unit weight and water absorption

The relationship between unit weight and 28-day water absorption is shown in Figure 11. The fitted regression was WA = −0.003996UW + 15.299, with R 2 = 0.132 and p = 0.302. The negative slope is consistent with the expected tendency for denser matrices to exhibit lower water absorption; however, the relationship was not statistically significant. Therefore, unit weight alone cannot be considered a reliable predictor of water absorption for the mixtures investigated. The observed variation is likely influenced by the combined effects of MSWBA replacement, binder content, SF incorporation, and w/b ratio. A general tendency toward lower water absorption at higher unit weights can be observed in Figure 11, although the relatively low R 2 value indicates considerable scatter among the investigated mixtures. Mixtures T4, T7, and T8 exhibited relatively high unit weights together with comparatively lower water absorption, whereas T1, T2, and T3 generally showed lower unit weights and higher water absorption. This behavior may be associated with differences in matrix compactness and pore structure resulting from the combined effects of MSWBA replacement, SF incorporation, binder content, and w/b ratio. The potential micro-filling contribution of SF may also have contributed to matrix densification; however, this mechanism was not directly verified through microstructural characterization in the present study. Similarly, the higher water absorption observed in some mixtures with greater MSWBA contents may be related to the porous nature and water-retention characteristics of MSWBA.

Overall, the results suggest that unit weight may provide a useful qualitative indication of differences in water absorption among some of the investigated mixtures, but the weak and statistically non-significant regression relationship demonstrates that it should not be used alone to predict water absorption. Further investigation using a larger dataset and direct characterization of pore structure would be required to establish a robust predictive relationship.

Flexural strength (7 days) and compressive strength (28 days)

Figure 12 presents the relationship between 7-day flexural strength and 28-day compressive strength of the investigated mortars. The fitted regression was f c = 2.880f flex + 4.765, with R 2 = 0.711 and p = 0.0022. The strong and statistically significant positive relationship indicates that mixtures exhibiting higher 7-day flexural strength generally also developed higher 28-day compressive strength. The relatively high R 2 value indicates that 71.1% of the observed variation in 28-day compressive strength was explained by the linear relationship with 7-day flexural strength within the present dataset. The observed association may be physically related to the fact that both flexural and compressive strength are influenced by the overall integrity, particle packing, and bonding of the mortar matrix. However, the use of 7-day flexural strength to infer 28-day compressive strength should be interpreted cautiously because strength development may differ between these two mechanical properties and may be affected by the combined effects of binder content, SF incorporation, MSWBA replacement, and w/b ratio. Mixtures T4 and T8 exhibited relatively high values of both flexural and compressive strength, whereas mixtures containing a higher effective proportion of porous MSWBA generally showed lower values of both properties. These observations provide a possible physical basis for the observed association, but the underlying mechanisms were not directly verified through microstructural characterization.

Despite the statistically significant relationship, the analysis was based on only 10 mixture-level observations, with several mix-design parameters varying simultaneously. Therefore, the regression should be interpreted as an empirical association within the investigated mixtures rather than as a universally applicable predictive model. Further validation using a larger and independently designed dataset would be required to determine whether 7-day flexural strength can be used as a robust predictor of 28-day compressive strength in MSWBA-based mortars.

Contour analysis: bottom ash/MSWBA series

A two-dimensional contour analysis study was used to investigate the combined effect of water to binder (w/b) ratio and incorporation of municipal solid waste bottom ash (MSWBA) on the compressive strength behavior of the modified mortars. The experimental results were used to construct contour maps to illustrate the interaction of the mixture parameters and the resulting strength distribution (Figure 13). The contour maps were generated from the available experimental observations within the investigated ranges of w/b ratio and MSWBA replacement and represent an interpolated visualization of the experimental strength distribution rather than a separately fitted response-surface regression model. Therefore, the contour surfaces were not associated with an independent model equation, goodness-of-fit statistic, or validation dataset. They are intended to illustrate the observed experimental trends and favorable strength regions rather than to provide a statistically validated predictive model. Because the number of experimental observations is limited, the interpolated regions between measured points should be interpreted with caution, particularly away from the experimental observations.

The strength response in the studied range of water binder ratios (0.53–0.66) showed a negative association between water content and compressive strength with the correlation coefficient of r = −0.72. The observed trend suggests a negative effect of the increase of the effective water content on the matrix densification and mechanical properties, especially in the case of blends with porous MSWBA particles. However, this correlation represents an association within the present experimental dataset and does not demonstrate that w/b ratio alone controls compressive strength.

The area of maximum compressive strength was concentrated around the lower limit of the water to binder ratio investigated (around 0.53–0.55). The mixtures T4 (18.54 MPa) and T8 (18.30 MPa) were in this favorable experimental range, which suggests that a w/b ratio of approximately 0.53–0.55 was associated with favorable compressive-strength performance in the investigated mixtures. This range should therefore be regarded as a favorable experimental range rather than a statistically established optimum. It was observed that replacement levels of MSWBA around 75% level can be achieved effectively by adequate binder content, addition of silica fume and controlled water availability, which may contribute to improved matrix packing and strength development.

The observed strength distribution should also be interpreted in relation to the simultaneous variation of binder content, SF incorporation, and MSWBA replacement among the mixtures. In particular, the higher-strength mixtures located within the lower w/b range also incorporated relatively higher binder contents and/or SF, which may have contributed to improved particle packing, matrix densification, and strength development. Therefore, the present contour analysis does not allow the individual contribution of w/b ratio to be separated from the effects of binder content, SF, and MSWBA replacement. Accordingly, w/b ratio should be considered an important mixture parameter within the investigated range, rather than conclusively identified as the predominant controlling factor.

On the other hand, the low strength zone was observed at high w/b ratios, particularly at w/b = 0.66. In this area, the mixtures T1, T2 and T3 presented a relatively low compressive strength, ranging from 12.8 to 14.7 MPa. The decrease in strength may be associated with the higher water availability required at these mixture proportions, which can be consistent with increased capillary porosity and reduced matrix continuity reported for cementitious systems with higher w/b ratios. A similar behaviour was reported for cementitious systems containing porous aggregates where higher water to binder ratios increase the permeability and decrease mechanical strength.

The contour maps demonstrating the steep iso-strength gradients indicate that changes in w/b ratio were associated with substantial variations in compressive strength within the investigated mixture range; however, these variations occurred concurrently with changes in binder content, SF incorporation, and MSWBA replacement. Appropriate mixture optimization and control of these interacting parameters can greatly alleviate the challenges arising from the porous architecture, uneven morphology and high absorption capacity of MSWBA particles regarding matrix density. Lowering the w/b ratio was associated with higher strength in the investigated mixtures, a trend that is commonly related to reduced capillary porosity and improved matrix continuity in cementitious systems. Overall, the contour analysis provides a graphical representation of the experimental strength distribution and indicates that lower w/b ratios, particularly within approximately 0.53–0.55, were associated with more favorable compressive-strength performance in the investigated mixtures. However, the contour surfaces should not be interpreted as a statistically validated optimization model or as evidence of a universal optimum. A larger and independently designed dataset, together with a formally fitted response-surface model and independent validation, would be required to establish the relative contributions of w/b ratio, binder content, SF incorporation, and MSWBA replacement and to determine a statistically robust optimum.

Life cycle assessment of MSWBA-based mortars

The embodied carbon of the analyzed mortar mixtures was significantly influenced by the combined effect of constituent-material production, the specified transportation of natural fine aggregate, and progressively substituting natural fine aggregate with municipal solid waste bottom ash (MSWBA). The control mixture contained 1,512 kg m −3 of natural fine aggregate and no MSWBA and served as the reference mixture. Its embodied carbon footprint of 380.76 kg CO 2 e m −3 was used as the baseline for comparison. The calculated embodied carbon values, natural sand reductions, and percentage changes relative to the control mixture are summarized in Table 4. Ordinary Portland Cement (OPC) represents the primary emission driver at 0.8300 kg CO 2 e/kg based on the localized Lebanese cement-production inventory reported by MoE/UNDP/GEF (2015), reflecting the substantial emissions associated with clinker production and thermal energy requirements. The emission factor for silica fume (SF) was taken as 0.000310 kg CO 2 e kg −1 based on Wang et al. (2022), which reported this value for the material-stage CO 2 emissions associated with silica fume. This emission factor was applied consistently to the SF content of each mixture. Given the very low emission factor applied, the contribution of SF to the total calculated embodied carbon is negligible, and OPC remains the dominant contributor to the overall carbon footprint.

The cut-off allocation method applied assumed that MSWBA was a secondary material from the waste stream and that no upstream environmental burden was allocated to its use in the mortar. This is a common approach in life-cycle assessment studies of recycled and waste-derived products, where the environmental impacts of the original waste generation are attributed to the original waste management system and not to the subsequent use (ecoinvent Association, 2025). Therefore, the substitution of natural fine aggregate with MSWBA reduced the quantified contribution of natural aggregate production and the specified transportation of natural fine aggregate. Thus, six of the nine modified mixtures had lower embodied carbon values compared to the control mixture, which demonstrates the ecological potential of MSWBA valorization as a substitute for natural fine aggregate under the defined inventory and allocation assumptions.

A critical boundary consideration in evaluating MSWBA valorization is the environmental burden associated with material pretreatment. Prior to incorporation into the mortar matrix, raw MSWBA underwent a 4-day aqueous leaching protocol, mechanical agitation, liquid-solid separation, and thermal oven drying. These laboratory-scale processing steps were not explicitly quantified in the baseline LCA because corresponding energy, water, equipment, and process inventories were not included in the adopted LCI. Nevertheless, these pretreatment operations may introduce additional energy and water requirements when implemented at industrial scale and could therefore reduce the net environmental benefit associated with MSWBA utilization. Accordingly, the reported LCA results should be interpreted as representing the cradle-to-gate impacts captured by the defined constituent-material inventory and allocation assumptions rather than a full industrial-scale assessment of MSWBA pretreatment. This limitation does not alter the internal consistency of the baseline comparison because the same LCA boundary and allocation assumptions were applied to all formulations; however, inclusion of industrial-scale MSWBA pretreatment could change the magnitude of the environmental benefits.

To examine the influence of the MSWBA allocation assumption, a sensitivity analysis was performed using alternative non-zero emission factors for the secondary material. Scenario A represents the baseline cut-off allocation with 0.000 kg CO 2 e kg −1, while Scenarios B and C represent hypothetical MSWBA burdens of 0.025 and 0.050 kg CO 2 e kg −1, respectively. These scenarios were considered to evaluate the sensitivity of the comparative results to the allocation assumption rather than to represent measured industrial emission factors. For T3, which contained 968.96 kg MSWBA m −3, applying the 0.025 kg CO 2 e kg −1 factor would add approximately 24.22 kg CO 2 e m −3 to the baseline result, increasing its embodied carbon from 300.51 to approximately 324.73 kg CO 2 e m −3. This corresponds to an approximately 14.70% reduction relative to the control mixture. Under the more conservative 0.050 kg CO 2 e kg −1 assumption, the embodied carbon of T3 would increase to approximately 348.96 kg CO 2 e m −3, corresponding to an approximately 8.35% reduction relative to the control. These results indicate that the environmental advantage of T3 remains under the assumed non-zero allocation scenarios, although the magnitude of the reduction decreases as additional burdens are assigned to MSWBA.

The ecological benefits were most prominent in the low-binder mixtures (T1–T3) where increasing rates of sand substitution resulted in a steady reduction in embodied carbon. Mixture T1 has 10.46% lower embodied carbon compared to the control mixture and reduces the use of natural sand by 63.26%. Mixture T2 had 14.99% lower embodied carbon than the control, together with a 76.72% reduction in natural sand use. T3 was the eco-friendliest, with a 90.54% reduction in natural sand and the lowest embodied carbon footprint of 300.51 kg CO 2 e m -3, which is a reduction of 21.07% compared to the reference mixture. The results indicate that large reductions in carbon emissions can be achieved through extensive substitution of aggregates from quarries, provided that additional demand for higher-carbon constituent materials is kept to a minimum. Similar works have demonstrated that the use of waste-derived aggregates in cementitious composites can reduce the ecological footprint associated with the excavation of natural aggregates and the promotion of circular economy practices in the construction sector (Cho et al., 2020; Chen et al., 2025).

Transportation had a particularly great effect in the particular Lebanese context used for this research. Transportation emissions are a significant component of the environmental impact of aggregates. Natural aggregates are frequently transported from inland quarries to mortar production facilities in coastal areas. Thus, the substitution of natural sand by locally available MSWBA reduces the quantified impacts associated with natural aggregate production and the specified transportation of natural fine aggregate. Previous life-cycle analyses have indicated that transportation distances and regional supply chain characteristics can significantly influence the embodied carbon of cementitious materials, particularly in regions where aggregates must be transported over long distances (Pacheco et al., 2022; Zhu et al., 2023). This was the case for the intermediate blends (T4–T6) where the carbon reductions obtained from aggregate replacement partially offset the higher emissions due to higher binder contents. For example, with a natural sand reduction of 64.61%, the T6 mixture resulted in a 6.27% reduction in embodied carbon, whereas the T4 and T5 mixtures achieved reductions of 3.01% and 2.28%, respectively.

On the other hand, the high-binder mixtures (T7–T9) showed higher embodied carbon values, but they achieved notable reductions in the use of natural sand. The mixture T7, with a 91.93% reduction in natural sand relative to the control mixture, resulted in an embodied carbon footprint of 420.78 kg CO2e m −3, which is a 10.50% increase compared with the control mixture. Similarly, T8 and T9 exceeded the baseline carbon footprint by 4.80% and 10.41%, respectively. These results demonstrate that a simple replacement of aggregates cannot reduce the environmental impact of cement-based materials when the cement content is considerably increased. The manufacture of Portland cement continues to be the largest contributor to embodied carbon in concrete and mortar, due to the energy intensive nature of the clinker production process, and the associated emissions of CO2. Therefore, the increase of the binder content to improve the mechanical performance can reduce some of the environmental benefits of the use of waste aggregates.

The LCA demonstrates that the environmental performance of mortars modified with MSWBA relies on the combined effects of aggregate substitution and binder formulation, rather than solely on the utilization of MSWBA. This aligns with broader research on binary and ternary blended cementitious systems, which confirms that optimizing supplementary industrial byproducts within the binder matrix is essential to achieving both structural performance and reduced embodied carbon (Mishra et al., 2025a).

The results indicate that when combined with moderate levels of binder, high volumes of MSWBA can significantly reduce embodied carbon, while high cement levels may offset the environmental benefits of aggregate substitution. Therefore, the design of sustainable MSWBA-based mortar should focus on optimizing the use of recycled materials and controlling the amount of high carbon cementitious components. Of the mixtures investigated, T3 achieved the best environmental performance by combining a very high replacement of natural sand with the lowest calculated embodied carbon footprint within the defined constituent-material inventory. The obtained results indicate the potential of MSWBA valorization as an effective strategy for the production of low-carbon cementitious materials, thus promoting the principles of circular economy in the field of construction (Chen et al., 2025; Cho et al., 2020).

This study showed the potential for developing sustainable mortar composites using high volume municipal solid waste bottom ash (MSWBA) as partial replacement of natural fine aggregate and silica fume (SF) as performance enhancing supplementary cementitious material. Experimental results confirmed that the aggregate framework altered significantly upon the substitution of 60%–90% of natural sand by treated MSWBA, by modifying particle size distribution, reducing density and increasing the sensitivity of mortar performance to mixture design parameters. Although MSWBA has inherent problems such as its porous structure, low specific gravity and irregular particle shape, the fresh and hardened properties were satisfactorily achieved by optimizing the binder content, water to binder ratio and the amount of silica fume.

The addition of silica fume was associated with improved measured mechanical and physical performance in selected MSWBA-containing mixtures. This improvement may be related to the fine particle size and micro-filling characteristics of SF, together with its established pozzolanic behavior in cementitious systems, which may contribute to improved particle packing and matrix refinement. However, these mechanisms were not directly characterized in the present study and should therefore be regarded as possible explanations rather than experimentally verified processes. The mixes T4, T7 and T8 exhibited the best mechanical and physical properties with the highest compressive strength, flexural strength, ultrasonic pulse velocity and lower water absorption than the other MSWBA-modified mortars. Correlation analysis identified statistically significant positive relationships between 7-day UPV and 7-day compressive strength and between 7-day flexural strength and 28-day compressive strength, whereas the relationships involving unit weight were weak and statistically non-significant. Moreover, the contour analysis indicated that lower w/b ratios, particularly within approximately 0.53–0.55, were associated with more favorable compressive-strength performance in the investigated mixtures. This range should be regarded as a favorable experimental range rather than a statistically established optimum.

The LCA showed that the use of MSWBA can provide environmental benefits in selected formulations through the reduction of natural aggregate consumption and associated production and transport impacts. Formulation T3 achieved the lowest cradle-to-gate embodied carbon footprint (300.52 kg CO 2 e/m 3), representing a 21.08% carbon reduction and a 90.54% reduction in natural sand consumption relative to the control mortar. However, this maximum carbon reduction is accompanied by a significant trade-off in mechanical performance, as T3 exhibited substantially lower compressive strength compared to the control mixture This outcome demonstrates that high-volume waste substitution without sufficient binder optimization may be more suitable for applications where lower mechanical strength is acceptable, whereas applications requiring higher load-bearing capacity necessitate optimized binder formulations. Among the investigated mixtures, T4 combined improved mechanical performance with a 3.01% reduction in embodied carbon relative to the control, whereas T8 exhibited strong mechanical and physical performance but resulted in a 4.80% increase in embodied carbon. Therefore, sustainable MSWBA mortar design requires a tailored approach that aligns aggregate substitution rates and binder intensity with specific target engineering applications.

Overall, the findings demonstrate that well-treated MSWBA combined with silica fume is a viable raw material for producing eco-friendly mortar mixtures with tailored physical and short-term mechanical properties. However, because this investigation focused primarily on 7- and 28-day laboratory performance metrics, claims regarding long-term durability and structural field deployment must be carefully qualified. To fully establish the practical feasibility of MSWBA-based mortars in actual construction, future research must evaluate long-term durability parameters, including chloride diffusion, freeze-thaw resistance, carbonation kinetics, heavy metal leaching stability, and time-dependent mechanical behavior.