Development and characterization of H 2 O 2 -aerated concrete: effect of NaOH and curing regime on porosity and mechanical performance

Highlights

• NaOH (4 mol/L) controlled the aeration expansion and enhanced fly ash reactivity through the OH⁻ mechanism.
• The mix B2M3 (ii) of ordinary Portland cement (OPC):fly ash (70:30) achieves 946 kg/m³ and 9.42 MPa, thereby satisfying IS 2185 Part 3 for classification under Grade 1.
• Autoclave curing at 16 bar pressure promotes the development of tobermorite, supported by X-ray diffraction (XRD), Fourier transform infrared (FT-IR), and field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM-EDX) analyses.
• Multiscale X-ray computed tomography (CT), mercury intrusion porosimetry (MIP), and theoretical porosity analysis revealed a three-tier pore hierarchy.

1 Introduction

Aerated concrete is a lightweight porous material produced by introducing air into the cement paste through chemicals. Due to its less dense nature, it has good thermal insulation and is easy to handle in construction. Thus, it has become a developing construction material in India. Because of its lightweight characteristics, aerated concrete is widely used by manufacturers for producing autoclave aerated concrete (AAC) blocks, in renovation works, and interlocking insulation wall systems (Liu et al., 2020). AAC attracts builders due to its superior thermal properties (Narayanan and Ramamurthy, 2000a), reduced dead load (Jitchaiyaphum et al., 2013), and its lower environmental impact (Pehlivanlı et al., 2015). AAC is used mainly for its thermal insulation properties (Bonakdar et al., 2013) and its acoustical properties (La et al., 2006). Kumar et al. (2023) suggested that AAC can be used in both load-bearing and non-load-bearing wall panels. Aerated concrete is produced by blending calcareous and siliceous materials with gypsum, water, and an aerating agent, followed by casting the slurry into molds and curing it in an autoclave using pressurized steam (He et al., 2019; Qu and Zhao, 2017). Siliceous material plays huge role in strength gain, to accommodate various ash utilized, such as bagasse ash combined with rice husk ash (Kunchariyakun et al., 2018), approximately 15% ground granulated blast furnace slag (GGBS) (Bernard et al., 2022), approximately 30% basic oxygen furnace (BOF) slag (Jiang et al., 2022), or approximately 20% palm oil fuel ash (POFA) (Lim et al., 2013) in aerated concrete production and accounting for approximately 50%–70% ash by mass (Ye et al., 2024). The industrial by-product fly ash alone is utilized for about 30%–50% along with OPC.

AAC has a cellular porous structure that is attained by gas forming agent/aerating agent such as aluminum powder, aluminum paste, hydrogen peroxide (H₂O₂), and carbon dioxide (Shi et al., 2025). Its lightweight characteristics are attained by entrapping air within the concrete using metallic powders such as aluminum and zinc powder, or chemicals such as liquid hydrogen peroxide (Narayanan and Ramamurthy, 2000b). The porous medium and the pore stability are primarily governed by the liquid (water)-to-binder ratio and the dosage of aerating agent (Chen et al., 2021). The compressive strength-influencing agents, which are tobermorite, ettringite, and anhydrite (Liu et al., 2023; Wu et al., 2021; Luo et al., 2023), are generated in aerated concrete by a high-pressure environment. It is stated that hydration products react further if the curing is done in an autoclave, resulting in significantly improved performance (Qu and Zhao, 2017). During autoclave curing, C-S-H undergoes transformation and dissolution in an alkaline environment to form crystalline-phase tobermorite, following Equations 1, 2 (Sun et al., 2023).Autoclave curing:

C − S − H → α − C 2 SH + S i 4 + → C 5 S 6 H 5 Partly from C ‐ S ‐ H gel . (1)

S i 4 + + C a 2 + + O H − → C 5 S 6 H 5 p a r t l y f r o m a l k a l i n e e n v i r o n m e n t . (2)

The mechanical strength is governed by the geometry and dimensions of pores, the age of the specimen at the time of testing, the properties of the raw materials, and the type of curing applied (Isu et al., 1995). Heat curing in an accelerated curing tank shows better mechanical strength than humidity chamber curing and hot air oven curing (Anand et al., 2024a). According to Cai et al. (2020), the microwave precuring technology was introduced to the slurry to control the temperature during expansion to improve pore stability and also to reduce the precuring time. Luo et al. (2025) reported that autoclave curing highly influences the pore structure and strength; increasing the curing time in an autoclave from 1 h to 10 h increases the compressive strength by 127% due to the formation of fibroid tobermorite crystals. In AAC production, fly ash is widely used as a silica source (Narayanan and Ramamurthy, 2000a). It is the best binder combination in AAC production (Serhat Baspinar et al., 2014). Fly ash is rich in silica and alumina and contains amorphous glassy phases that are activated in an alkaline environment (Provis and Van Deventer, 2009). Sodium hydroxide helps to rupture the glassy surface of fly ash particles, promoting alkaline activation of aluminosilicate phases. Figure 1 shows the rupture of the fly ash surface in an NaOH alkaline environment (Chalee et al., 2021). It is hypothesized that NaOH performs a dual role in this system: mediating H₂O₂ decomposition and promoting fly ash activation, as represented in Equations 3–5.

NaOH with H 2 O 2 , H 2 O 2 + O H − → H O 2 − + H 2 O , (3)

2 H O 2 − → O 2 + 2 O H − . (4)

NaOH with fly ash,

C a O H 2 + S i O 2 f r o m f l y a s h , r e l e a s e d b y N a O H → C − S − H gel . (5)

Many studies have been conducted on aerated concrete systems employing NaOH as an alkaline activator. Most of these investigations have focused on geopolymer matrices or alkali-activated slag. This indicates that our understanding of the behavior of hybrid OPC–fly ash (FA) binder systems under H₂O₂ aeration is limited. Furthermore, the specific role of NaOH as an alkaline environment creator in governing H₂O₂ decomposition kinetics, bubble stability, and pore structure development within an OPC–FA cementitious matrix has not been reported in the open literature. The present study addresses this identified research gap by investigating the fresh state and hardened state performance of aerated concrete, with NaOH incorporated as a reactivity agent at two dosage levels. The fresh slurry was tested for aeration rate analysis and slurry spread measurements; thereafter, the hardened state performance was evaluated in terms of dry density and compressive strength across multiple curing regimes: oven curing, autoclave curing, and precuring followed by autoclave curing to establish the strength development. To elucidate the underlying pore structure and microstructural characteristics governing mechanical performance, a comprehensive multiscale characterization encompassing Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), X-ray computed tomography (X-CT), mercury intrusion porosimetry (MIP), and field emission scanning electron microscopy with energy-dispersive X-ray spectroscopy (FESEM-EDX) was conducted on the best-performing mix. The novelty of this study lies in establishing NaOH as a true decomposition catalyst rather than an alkali activator for controlled H₂O₂ aeration and providing microstructural evidence linking catalyzed aeration behavior. The compressive strength of the optimized mix, determined using 50 mm cube specimens, is compared with the minimum strength specified for Grade 1 autoclaved aerated concrete masonry blocks in IS 2185 (Part 3): 1984. This comparison serves as an indicative assessment and highlights the potential viability of the proposed binder for building applications. Figure 2 shows the overall framework incorporated in this study.

2 Materials and methods

2.1 Materials

Ordinary Portland cement of Grade 53 with the specific gravity of 3.12 was used as the primary binder. The specific gravity was determined by the Le Chatelier flask method in accordance with IS 4031 (Part 11). Siliceous fly ash, sourced from the Ennore thermal power plant, Chennai, Tamil Nadu, India, was used as the siliceous source as a replacement for OPC. The specific gravity of fly ash is 2.24, which was determined by using a pycnometer as per IS 2720 (Part 3). The chemical composition of fly ash was determined by X-ray fluorescence (XRF) analysis and is presented in Table 1. Based on XRF results, the combined SiO₂ + Al₂O₃ + Fe₂O₃ content of the fly ash was 69.66%, and CaO was 7.23%, satisfying the requirements of IS 3812 (Part 1): 2013 Grade 2 type of fly ash. Particle size distribution of fly ash was determined by dynamic light scattering (Litesizer DLS 500, Anton Paar), yielding a hydrodynamic diameter of 2,432 nm with a single dominant peak at 1,463.6 nm (polydispersity index: 15.5%). Sodium hydroxide in the form of white solid flakes with a purity of 96.5% (laboratory grade) was used as a reactive agent and to accelerate the decomposition of H₂O₂ in an alkaline environment. Hydrogen peroxide in the form of a solution at 30% (v/v) concentration, supplied by Isochem Laboratories, was used as a chemical aerating agent. Potable water free from suspended impurities with the water-to-binder ratio of 0.4 was maintained constant in all mix combinations. Figure 2 illustrates the functional role of each constituent material and the multiscale characterization adopted in this study.

TABLE 1. Oxide composition of fly ash.

Trace elements in fly ash include ZrO₂ (400.2 ppm), BaO (355.8 ppm), Cr₂O₃ (165.9 ppm), ZnO (116.8 ppm), NiO (84.2 ppm), and CuO (67.3 ppm), determined by XRF analysis.

2.2 Preparation of samples

Initially, three different mixes were prepared: M1 (non-aerated control mix), M2 (H₂O₂ without NaOH), and M3 (H₂O₂ with NaOH), each with three binder combinations representing different OPC:FA ratios. Based on the optimum curing condition obtained from M2 and M3, two additional binders, B4 and B5, were added to the mixes M1 and M3 to further investigate the influence of FA and OPC content on aeration behavior and microstructural performance. The mix design and curing regimes adopted in this study are presented in Figure 3.

Because NaOH is exothermic in nature, the solution of 4 mol/L was prepared by dissolving flakes in water (160 g NaOH flakes + 840 g water) 24 h before the experiment. The next day, before mixing, to ensure uniform mixing (Pratap et al., 2023), the NaOH solution was swirled in a mechanical stirrer. Initially, fly ash, sodium hydroxide solution, and half the amount of water were mixed in a mortar mixer. Mixing was continued for 10 min to allow leaching of Si⁺ and Al⁺ ions present in fly ash (Rattanasak and Chindaprasirt, 2009). Subsequently, cement and the remaining water were added to the mixture and mixed for 120 s at moderate speed to obtain a homogeneous paste.

After that, H₂O₂ was added, then the mixture was mixed for 60 s at low speed. The total water requirement for each mix was calculated based on the target w/b ratio as expressed in Equation 6:

W t o t a l = w b X B . (6)

Because H₂O₂ (30% v/v) and NaOH solution (4 mol/L) both contained water in the mix, the water added separately was determined by deducting the water content of each solution from the total requirements, as provided in Equation 7:where W_final is the final mass of added water, B is the total binder mass (g), mH₂O₂ is the mass of H₂O₂ liquid (g), which has 70% water content, and mNaOH is the mass of the 4 mol/L NaOH solution with 84% water by mass. This ensures the w/b ratio of 0.40 is maintained consistently across all mix combinations.

W F i n a l = W t o t a l − m H 2 O 2 X 0.70 − m N a O H X 0.84 , (7)

2.3 Casting and curing

Fresh slurry for each mix combination was cast into cubical molds of 50 mm × 50 mm × 50 mm to prepare the specimen for the study. The slurry was filled up to 65% of the mold for free aeration (Şahin et al., 2018). The filled molds were left undisturbed at room temperature. For the first two curing regimes, the slurry was allowed to attain initial stiffness, after which the expanded top layer was trimmed using cutting wire of 0.29 mm diameter to ensure a uniform surface. In the third method, the slurry in the mold was precured in an oven at 40 °C for 2.5 h. Subsequently, the expanded layer was removed following the same procedure described above. The specimens were then maintained under ambient conditions and demolded after 24 h of casting. Three curing regimes were adopted for the study: (i) oven curing at 60 °C for 24 h, (ii) autoclave curing for 8 h at 16 bar pressure, and (iii) precuring + autoclave, slurry cured in an oven and specimen cured in an autoclave.

2.4 Testing methods

Figure 4 presents (a) a schematic workflow and (b) corresponding photographic documentation of the experimental procedure adopted in this study. The process spans four stages: materials and mixing, sample preparation, curing, and characterization.

2.4.1 Aeration rate measurement

The volumetric expansion behavior was monitored and measured using a transparent measuring jar and a stopwatch. The measuring jar was 70 mm diameter × 350 mm height, with a total volume of 1,000 mL, graduated at 10 mL intervals. Once the fresh slurry was prepared in the mixer, it was immediately poured into a 1,000 mL measuring jar to measure the expansion vs. time. The slurry was prepared for the fixed binder mass of 500 gm, proportioned in accordance with the mix design provided in Table 2. The rise was measured at regular intervals of each 10 mL increase in the jar scale. The total observation period was continued until no further rise was detected. The test was carried out at room temperature and repeated for all aerated mixes M2, M3 (i), and M3 (ii) to compare the effect of H₂O₂ dosage and NaOH on expansion behavior, as presented in Figure 5A.

TABLE 2. Mix details of aerated slurry.

The mini slump cone test was conducted for the well-aerated samples to determine their spread by following the procedure as mentioned by Pasupathy et al. (2022) to find the spread diameter of the slurry. The slurry was filled in a mini slump cone with a top diameter of 50 mm, a bottom diameter of 100 mm, and a height of 150 mm. The cone was lifted vertically, allowing the slurry to spread freely. The final spread diameter was measured when the spread stopped completely.

2.4.2 Density and compressive strength

The cast 50 mm cubes were allowed to aerate, and the top expanded layer was trimmed to make an even surface after the expansion stabilized for testing. Dry density and compressive strength were determined on hardened specimens across three curing regimes. Measurements were recorded both before and after curing to understand the density evolution resulting from the adopted curing methods. Compressive strength was determined on 50 mm cube specimens using a compression testing machine of 450 kN capacity and a loading rate of 0.3 kN/s. The measured strength was evaluated and verified with the minimum requirement prescribed in IS 2185 (Part 3): 1984 for autoclaved aerated concrete. Aerated concrete has numerous porous structures, and hence, conventional water curing is not suitable. Three different curing methods were selected based on the recommendations from the literature. Wang et al. (2023) describe that elevated temperature curing improves the compressive strength of fly ash-containing concrete at both early and later ages. Tawfi et al. (2022) declare that autoclave curing produces a superior density and strength combination at 10 bar pressure for 8 h. According to Saad et al. (2024), in reinforced aerated concrete, the long-term autoclaving process strongly influences reinforcement strength and the stiffness of the matrix. In this study, oven curing was carried out at 60 °C for 24 h, and autoclave curing was performed at a pressure of 16 bar for 8 h. In the third method, the slurry in the mold was cured in a hot air oven at 40 °C for 2.5 h, followed by ambient curing. Specimens were then demolded and autoclaved under the same conditions stated above.

2.4.3 Microstructural study

Microstructural characterization was carried out on hardened specimens selected on the basis of their compressive strength performance, testing from surface-level observation to nano-scale pore analysis. Surface pore shape, pore distribution, and surface texture were examined using an optical stereo microscope on the surface of the specimen. Functional group identification and characterization of hydration and pozzolanic reaction products were performed by Fourier transform infrared spectroscopy (FT-IR) on powdered specimens for the B2M2 and B2M3 mixes (ii). Spectra were collected using an IRAffinity-1 with the attenuated total reflectance (ATR) accessory, over a wavenumber range of 4,000–400 cm⁻¹ at a resolution of 4 cm⁻¹ with 32 scans per spectrum. The crystalline phase under different curing mixes was identified by X-ray diffraction (XRD) using a Bruker D8 QUEST. To study the internal pore structure based on 3D tomographic images, the cubes were examined using X-ray micro-computed tomography (X-ray CT) with a Bruker Skyscan 1273. The scanned images were reconstructed and analyzed using CTVox and CTAn software to determine pore size distribution and pore volume fraction. Mercury intrusion porosimetry (MIP) was conducted using an Anton Paar Poremaster 60, which measures total open porosity, mean pore diameter, and specific surface area. The intrusion data were determined using the Washburn equation, assuming a contact angle of 140° and a mercury surface tension of 480 mN/m. Elemental composition and microstructural features were characterized by FESEM-EDX using an FEI QUANTA 250 FEG instrument. Specimens were examined after gold sputter coating to prevent charge accumulation. Images were acquired at multiple magnifications to examine the reactivity of fly ash particles, C-S-H gel formation, and tobermorite needle development, and the Ca/Si molar ratio of the reaction products was determined from EDX point analysis.

3 Results and discussion

3.1 Aeration behavior

The volumetric expansion behavior of the fresh slurry produced using cylindrical measuring jars is presented in Figure 5. An increase in tH₂O₂ and NaOH shows a slower initial rise and prolonged expansion duration. For the binder B2, the time to reach expansion increased from 12.3 min to 17 min, which demonstrated that NaOH extended the aeration significantly. The pore formation in the aerated system was controlled by the flowability and viscosity of the mixture (Vaou and Panias, 2010). Thus, the flowability was confirmed according to Nambiar and Ramamurthy (2007), which states that a spread between 150 mm and 200 mm is ideal for flowability from the pump. Spread tests were conducted for aerated samples and satisfied the spread requirement. Figure 6 shows the spread test for mix B3M3 (i), and Table 3 shows the spread diameter of all aerated mixes. The presence of fly ash in the binder system, which contributed reactive Fe₂O₃ and TiO₂ components, was essential for sustaining the NaOH and H₂O₂ cycle through Fenton-type decomposition, consistent with the mechanism reported by Şahin et al. (2018).

TABLE 3. Spread diameter of aerated mixes.

Although the aeration rate was high without NaOH, the bubbles were not stable. Among all mix combinations, B2M3 (ii), comprising 3% H₂O₂ and 2% NaOH, demonstrated the optimal balance between expansion magnitude and aeration duration, with gradual gas evolution contributing to stable pore formation. The effect of binder composition and the effect of NaOH are presented in Figure 7a on expansion magnitude and in Figure 7b on time taken to stabilize. It shows that, among the M2 mixes, the binder B2 has the highest volumetric expansion of 80 mL, followed by B4 (60 mL), B3 (50 mL), B5 (40 mL), and B1 (10 mL). This indicates that an intermediate fly ash replacement of 30% (B2) provides a favorable alkaline environment. In binder B1, the OPC confirms the consumption of available alkalinity through early hydration reactions (Labianca et al., 2022), leaving insufficient hydroxide ions for H₂O₂ decomposition. For M3(i) mixes, the highest expansion measured is 70 mL. This result indicates that the addition of NaOH reduces the expansion. This supports the hypothesis that the higher Fe₂O₃ content available in the 50% fly ash binder system may provide Fenton-type catalytic synergy with NaOH (Li et al., 2022). Owing to the minimum reaction observed in M3 (i), this mix series was omitted from the additional binder ratios (B4 and B5). The increased dosage of NaOH in M3 (ii) showed that a higher dosage of H₂O₂ and NaOH increases the final gas through expansion. The presence of NaOH appears to increase the decomposition rate, contributing to more gradual and sustained O₂ evolution than mixes without NaOH (M3 (i) and M3 (ii)), producing more controlled O₂ evolution than uncatalyzed systems. Figure 7b shows the time taken to reach the expansion plateau. The M2 mix reached stabilization in the shortest time across all binders (240–720 s), indicating rapid expansion in the short period, which shows uncontrolled gas evolution. For M3 (i), it shows a similar parallel relationship, but the expansion is less, and hence, the density will be increased. The line drops abruptly to zero at B4 and B5, and hence, these binder compositions are not considered in the present study. In contrast, M3 (ii) required the longest stabilization time (840–1,080 s), confirming that the NaOH at 3% dosage extends the aeration significantly. The line shows that the peak at B3 (1,080 s) then reduced to B5 (950 s), while the corresponding expansions of 50 mL and 45 mL are lower than B2M3 (ii), which has 55 mL of expansion at 1,020 s. This confirms that B2M3 (ii) represents the optimal combination to achieve the highest expansion volume among the mixes containing NaOH.

3.2 Mechanical properties

The dry density of all mix combinations was found by measuring their mass and volume after demolding and curing. The dimension of the demolded specimens was measured using digital vernier calipers to find the volume. The density and the compressive strength are the main properties considered for classifying engineering applications (Bonakdar et al., 2013) and grade classification in accordance with IS 2185 (Part 3), based on 50 mm cube specimens. As a preliminary validation, the M1 control mix (no aeration) density was set as a benchmark to compare the conventional 14-day water curing and 8-h autoclave curing. Table 4 shows the density and compressive strength across the mixes. Following that, Table 5 shows all investigated mixes reported as mean ± standard deviation of three replicate specimens per curing regime. Across the mix matrix, standard deviation remained comparatively low relative to the mean values, indicating consistent specimen preparation and curing behavior within each mix. Literature reports that autoclave curing enhances the strength by approximately 20%–35% relative to water curing (Chen et al., 2018). In the present study, a correlation of 80%–88% was observed between the two curing methods, confirming that autoclave curing is an effective and time-efficient alternative method.

TABLE 4. Mean density and compressive strength of all mixes.

* - Water curing; unmarked values represent for oven curing.

TABLE 5. Density and compressive strength with standard deviation (n = 3).

* - Water curing; unmarked values represent for oven curing.

For the aerated mixes, the density of the oven-cured and precured-and-autoclaved specimens were compared, and the results are presented in Figure 8. The dashed reference lines in Figures 8a,b show the range of density 751–1,000 kg/m³ and the compressive strength 6–7 MPa corresponding to IS 2185 (Part 3) highlighted boundary for Grade 1 (Bureau of Indian Standard, 1984), which was determined using a 50 mm cube specimen.

The size effect in lightweight concrete describes that smaller specimens yield higher measured strength due to the more localized crack band formation and poor crack distribution than the 150 mm cube specimen (Sim et al., 2013). Thus, the grade classification based on IS 2185 (Part 3) is presented for comparative assessment. The conservative interpretation and the results are treated as indicative of obtaining the minimum specified strength limits. Figure 8a shows that, among all the mix combinations, the following satisfied the density criterion: B2M2, B3M2, B4M2, B5M2, B2M3 (ii), B3M3 (ii), B4M3 (ii), and B5M3 (ii) under both oven/precuring and autoclave. However, when the compressive strength was simultaneously considered, only B2M2, B3M2, B4M3 (ii), and B5M3 (ii) fulfilled both requirements. It is noteworthy that in Figure 8b, B2M3 (ii) and B3M3 (ii) exhibited compressive strength exceeding 7 MPa, while the remaining mixes were within the specified range. Additionally, the mix B2M2 achieved the expected properties in the absence of NaOH, which must also be considered for further studies. Accordingly, the mixes were selected for detailed microstructural characterization. Table 6 presents the indicative comparison of density and compressive strength for the Grade 1 indicative mixes against IS 2185 (Part 3): 1984 classification, based on 50 mm cube specimens.

TABLE 6. Indicative comparison of density and compressive strength with IS 2185 (Part 3): 1984 Grade classification criteria.

3.3 Microstructural properties

3.3.1 Pore structure characterization

The pore formation mechanism in the aerated concrete system proceeds through three stages, as shown in Figure 9. It explains that, in stage one, H₂O₂ undergoes decomposition in the presence of 4 mol/L NaOH, releasing O₂ and water according to Equation 8. The rate of decomposition depends on the concentration and temperature (Şahin et al., 2018).

2 H 2 O 2 → 2 H 2 O + O 2 , (8)

H 2 O 2 → H 2 O + 1 2 O 2 . (9)

In Equation 9, the decomposition of 1 g of H₂O₂ produces 0.47 gm of O₂, corresponding to a gas volume of approximately 329 mL (Xie et al., 2024). Simultaneously, NaOH activates the glassy surface of fly ash particles, initiating reactions. In stage two, the nucleated bubbles expand outward under internal O₂ gas pressure, while the surrounding matrix develops early stiffness through initial hydration of cement, generating inward matrix resistance that regulates bubble growth, and grains start reacting with the liquid. The balance between gas pressure and matrix resistance determines the final pore size and distribution. In stage three, the continued hydration and autoclaved curing progressively harden the matrix and promote the formation of tobermorite crystals that grow as needles from the pore walls. The experimental evidence supporting the proposed three-stage mechanism is presented through X-ray CT analysis, MIP, and FESEM-EDX microstructural characterization.

3.3.2 XRD analysis

Multiphase Rietveld-type refinement was performed using Match! software (Cu-Kα radiation, 2θ = 20°–50°) on the selected mixes B2 and B3 to identify crystalline phases developed after curing. The quality of refinement was evaluated using the weighted profile R-factor (Rwp). Figure 10A presents the pattern for mix B2, and Figure 10B presents the pattern for mix B3. Table 7 presents the phase details identified by XRD for different mixes. The selected mixes were prioritized for XRD analysis based on the superior mechanical performance. The autoclave mixes exhibited diffraction peaks indicative of tobermorite-related phases together with quartz and calcite, whereas the oven-cured mix was dominated by residual quartz and calcite. The persistence of quartz indicates the presence of unreacted crystalline silica, while the development of C-S-H-related phases in autoclave specimens is consistent with the improved compressive strength observed under hydrothermal curing conditions (Matsui et al., 2011).

TABLE 7. Phase details identified by XRD.

3.3.3 FTIR analysis

The broad band near 3400 cm⁻¹ represents O-H stretching of B2M2, which is reduced in B2M3 (ii) due to improved kinetics because of the absorbed water molecules trapped inside the pores or on the surface (Ye et al., 2018). Figure 11 shows the spectra for different mixes. This indicates that the mix of B2M3 (ii) and B3M3 (ii) has fewer water molecules present inside. The bands present at 950 cm⁻¹, 875 cm⁻¹, and 480 cm⁻¹ showed sharp peaks for the B2M3 (ii) and B3M3 (ii) mixes. This can be attributed to the bending and stretching of cyclosilicate structures. The peak at 1,411 cm⁻¹ corresponds to CO₃ stretching vibration and also CO₃ asymmetric deformation (Morent et al., 2008). The consequence of the carbonation process and the silica and alumina structure formation confirm the alkaline activation in the NaOH-present mix. The CO₂ present in an ambient atmosphere penetrates into the hardened paste matrix and reacts with Ca(OH)₂ to form CaCO₃ as the primary carbonate product, as shown by the stretching at 1411 cm⁻¹.

3.3.4 X-ray tomography CT scanner

High-resolution X-ray CT tomography is widely used to analyze the pore structure of cementitious materials and provides information on internal defects through 3D anatomical images (Bernardes et al., 2015; Hong et al., 2019). The technique provides information about pore size, type of pores, and 3D distribution of pores (Yoon et al., 2020). As shown in Figure 12, 50 mm cube specimens were mounted on a plate that rotates 360° with a lower and an upper vertical position of 7,499.4 μm and 52,195.82 μm, respectively, and 3D images were captured at a grey threshold limit of 70 and 255.

Figure 13 shows the results of three cubes, B2M2, B2M3 (ii), and B3M3 (ii), that were selected to examine the volume of closed pores, volume of open pores, and total porosity percentage to confirm the uniform aeration and distribution of pores. Pores smaller than the X-ray CT resolution limit (<59 µm) cannot be resolved as distinct voids and therefore are not captured in this analysis. According to Qu and Zhao (2017), pores with a diameter of more than 60 µm are called macropores. Thus, the porosity percentage of macropores was found using Equation 10. Table 8 illustrates the porosity and connectivity attained from the result of scanning. The pore size distribution in Figure 14 shows that the NaOH-containing mix of B2M3 (ii) concentrates 19.66% of the total pore volume, with pores of diameter 120–360 µm. The narrowing of pore size distribution in the binder (B2) in the presence of NaOH is consistent with more uniform micropores. In Figure 13, the optical microscopy image of the surface of the cube is used to find the pore distribution on the surface, followed by pore segmentation shown in black and white. Black indicates the solid phase, and white indicates the pore distribution inside the matrix. The colors indicate various pore diameters, with red representing larger pores (1,200–2,300 µm), blue representing mid-range (600–1,200 µm), and green representing the smallest detected pores (1–600 µm). The mix B2M2 shows more solid phase but has high porosity, although unreacted fly ash particles were detected. The other two mixes have a lower porosity percentage with less solid phase, which indicates that the fly ash particles are well reacted in B2M3 (ii) and B3M3 (ii). Figure 15 shows the inner layer of the specimen, segregated to determine the uniformity of pore distribution throughout the cube.

TABLE 8. X-ray CT data analysis.

For mix B2M3 (ii), it is clear from the image that there is a smaller number of large pores and small pores that are well distributed. The value in the connectivity table is the dimensionless topological parameter, derived from the Euler characteristic, representing the degree of pore interconnection within the matrix (Koestel et al., 2020). Among the samples, the specimen with a connectivity of 954 exhibited a relatively higher degree of pore interconnection, whereas specimen B2M3 (ii) with a connectivity of 1926 showed predominantly isolated pores, indicating a more stable structure (Zheng et al., 2023). Although X-ray CT analysis provided 3D visualization of the macropore network and isolated porosity across all investigated mixes, its resolution has limited characterization of fine pore structure within the aerated matrix. Therefore, mix B2M3 (ii), which exhibited the most favorable pore characteristics among all mixes, was further examined using MIP to quantify the pore size distribution at the micro- and nanoscales (Ma, 2014).

T o t a l p o r o s i t y % = V o l u m e o f a l l d e t e c t e d p o r e s T o t a l v o l u m e o f s p e c i m e n × 100 . (10)

3.3.5 Mercury intrusion porosimetry (MIP)

MIP was conducted on the best mix B2M3 (ii) to characterize the open pore structure and pore size distribution of the hardened matrix. It characterizes the pores in the range of 3.2 nm–203.1 µm, found by forced intrusion of non-wetting mercury under increasing pressure to assess pore diameter through the Washburn equation (Zeng et al., 2020), assuming a contact angle of 140° and mercury surface tension of 480 erg/cm². The total mercury accessible open porosity of B2M3 (ii) was determined to be 16.79%, comprising interparticle and intraparticle porosity of 4.66% and 12.13%, respectively. Two assumptions are required to convert the MIP data into pore size distribution curves: pores are cylindrical, and pores are equally accessible to the intrusion of mercury (Diamond, 2000). Thus, it is evident that the identified porosity percentage is based on these assumptions. The median pore diameter (by volume) of 234.7 nm indicates that 50% of the total intruded volume corresponds to pores smaller than this size, while the mode pore throat diameter of 5.418 nm indicates that the most frequent pore entry size lies within the nanoscale gel pore range. The total specific surface area was measured as 45.43 m²/g, with a total intruded volume of 0.1679 cc/g.

The mean pore diameter was measured as 51.02 nm, positioning the mesopore (2–50 nm) and macropore (>50 nm) boundary as defined by IUPAC classification. The CT-measured microporosity (7.67%) and MIP-derived capillary porosity (16.79%) together illustrate the multiscale, bimodal pore structure characteristic of an aerated OPC–fly ash hybrid system (Song et al., 2015). Thus, the combination of total open porosity of 16.79% combined with a mean pore diameter of 51 nm is consistent with high-performance fly ash-modified aerated concrete cured under hydrothermal conditions (Song et al., 2015). It is noted that the theoretical porosity of 85.56% was determined by the MIP instrument using the formula, n = 1 − P * P s (Zhao et al., 2018), where P^* is the dry density of the specimen, and P^s is the density of solid materials. P^s solid particle density is measured using helium pycnometry, a built-in technique in the MIP instrument (ASTM, 2008). The higher theoretical porosity relative to MIP-measured open porosity reflects the inclusion of closed and gel-scale porosity inaccessible to mercury intrusion. Figure 16 shows the differential pore size distribution curve vs. pore diameter of B2M3 (ii). It represents the rate of mercury intrusion per unit of logarithmic pore diameter, and the peak indicates a comparatively uniform capillary pore distribution achieved through NaOH and H₂O₂ decomposition in B2M3 (ii). Table 9 illustrates the porosity of B2M3 (ii). Theoretical porosity (85.57%) reflects the inclusion of pores inaccessible to both X-ray CT and MIP, confirming the majority of the total pore system.

TABLE 9. Porosity correlation.

3.3.6 FESEM analysis

Figure 17 presents the FESEM micrograph of B2M2 and B2M3 (ii) specimens. According to He et al. (2019), pore morphology and material properties are primarily governed by the type of aerating agent, binder phase composition, and proportioning of reactive constituents. A significant number of visible pores were occupied by hydration products, indicating densification of the paste matrix (Sun et al., 2025). In Figure 15A for mix B2M2, at ×5,000 magnification, the microstructure reveals a heterogeneous matrix comprising unreacted fly ash particles. Spherical and angular particles are observed, indicating incomplete fly ash dissolution.

The appearance of flaky, plate-like structures indicates the formation of secondary reaction products during curing. Figure 15B, at ×20,000 magnification, reveals pores embedded within the gel matrix, indicating gas entrapment during aeration. Figure 15C, EDX analysis, shows that O, Ca, Si, and Al are the major constituents, consistent with compounds associated with strength development. A Ca/Si ratio of 1.56 was measured in the sectional area, indicating a calcium-rich gel composition under autoclave curing.

For mix B2M3 (ii), Figure 15D at ×5,000 magnification shows a ruptured fly ash particle surface. It is noted that NaOH-mediated activation increases fly ash reactivity in the alkaline environment. Figure 15E shows the pore wall with fine needle-like phases and other reaction products. The needle-like morphology is further supported by XRD analysis, which identified tobermorite in the mix. The EDX analysis shown in Figure 15F reveals that, in the particular zone, it is highly oxidized, and the Ca/Si ratio is 1.42. According to Chalee et al. (2021) and Anand et al. (2024b), C/S higher than 0.8 is optimal for forming needle-shaped tobermorite crystals.

Comparing the two specimens, B2M3 (ii) shows a higher proportion of oxide-forming elements than B2M2, consistent with more extensive fly ash activation in the presence of NaOH. Overall, the FESEM and EDX observations from the tested specimens are broadly consistent with a possible role for NaOH in enhancing fly ash reactivity, aligning with the aeration and XRD findings discussed above.

4 Research limitation

• The compressive strength reported in this study was determined using 50 mm cube specimens for comparative evaluation of the investigated mixes. Future work should include testing using the standard specimen dimension mentioned in IS 2185 (Part 3) to enable direct assessment of compliance with the standard.
• Under the examined circumstances, the created aerated concrete showed encouraging mechanical performance and density properties. However, before verifying its appropriateness for load-bearing wall applications, additional assessment of durability, dimensional stability, thermal performance, and full-scale block testing is necessary.
• The present study establishes the fundamental mix design and collects microstructural evidence for the requirement. Future investigation may extend this work to attain uniform and stable micropores through systematic variation of the molarity of NaOH and extended autoclave curing parameters.

5 Conclusion

This study proposed a method for producing AAC using NaOH as a catalytic agent to enhance fly ash reactivity, strength development, and pore characteristics. Based on the experimental results, the following conclusions are drawn:

• The incorporation of NaOH (4 mol/L) as a reactive agent for fly ash effectively controlled the rate of oxygen (O₂) evolution through the proposed hydroxide (OH⁻)-mediated mechanism, as indicated by aeration rate analysis. This result confirms more uniform and controlled aeration, with a reduced expansion rate of about 20%–33% compared to mixes without NaOH.
• FT-IR, XRD, and FESEM analyses indicated that hydroxide ions (OH⁻) from NaOH contribute to the dissolution of aluminosilicate phases in fly ash and the formation of calcium silicate hydrate (C-S-H) gel, with XRD indicating tobermorite in the B2M3 (ii) and B3M3 (ii) mixes under autoclave curing.
• Among the mix combinations investigated across five binder ratios and four mix series under three curing regimes, autoclave (hydrothermal) curing exhibited superior performance compared to oven curing. Precuring at 40 °C stiffened the slurry and facilitated the removal of the top layer, resulting in a uniform surface finish. Comparing curing regimes, the combination of precuring followed by autoclave curing improved handling, compressive strength, and dimensional stability relative to direct autoclave curing and oven curing alone.
• At a fixed H₂O₂ dosage, effective pore formation was observed in the B2M2 mix, achieving the desired density but with comparatively lower strength. However, under the investigated mix proportion and curing condition, the mixes containing 2% of NaOH and 3% of H₂O₂, B2M3 (ii) and B3M3 (ii), achieved better performance in both mechanical and microstructural properties. Mix B2M3 (ii) achieved a strength and dry density of approximately 9.42 MPa and 946 kg/m³, which is more than the target strength. This result satisfies the Grade 1 minimum strength requirement of IS 2185 (Part 3): 1984 for the corresponding density band.
• The findings of this study are consistent with the hypothesized dual role of NaOH as reflected in aeration kinetics, XRD phase development, and FESEM-EDX observations.
• MicroCT and MIP revealed an open, interconnected porosity of 16.79% and CT-resolved microporosity of 7.669% for B2M3 (ii), together indicating a hierarchically refined pore network: an evenly distributed macropore structure combined with a well-connected capillary network.
• The pore structure of B2M3 (ii) combines an evenly distributed macropore network (X-ray CT) with a well-connected network with a specific surface area of 45.43 m²/g (MIP), rather than porosity magnitude alone, underlies its favorable strength-to-density performance and is indicative of Grade 1 compliance with IS 2185 (Part 3): 1984.

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Footnotes

Edited by: Gorazd Žibret, Geological Survey Ljubljana, Slovenia

Reviewed by: Adithya Tantri, Manipal Academy of Higher Education, India

Bernard Kwame Mussey, Takoradi Technical University, Ghana