Research on the interface-strengthening mechanism of iron tailings in cement-stabilized macadam containing recycled aggregates

This study examines the micromechanical properties of recycled aggregate cement-stabilized macadam (CSM) with iron tailings sand (ITS) and recycled concrete aggregate (RCA). Microhardness analysis shows interfacial heterogeneity driven by unreacted particles and hydrated phases. Optimal ITS content (50%) increases hardness through micro-aggregate filling and pozzolanic effects, while >50% ITS reduces hardness due to retarded geopolymerization. 3D elastic modulus mapping reveals distinct aggregate ITZ matrix stratification, with ITZ as a low modulus band. Incorporating 50% ITS reduces ITZ width by 35%, confirming microstructural densification. The micromechanical findings suggest optimal ITS and RCA thresholds that may guide sustainable CSM design, although direct macroscopic validation is needed in future work.

The construction industry generates more than 1.8 billion tons of concrete waste annually globally (Wang et al., 2018), driving urgent demand for recycling recycled concrete aggregates (RCA) in infrastructure projects. Cement-stabilized macadam (CSM), a key material for road bases, offers a promising application for RCA utilization. However, RCA’s inherent weakness lies in its porous adhered mortar, which creates a defective interfacial transition zone (ITZ) when bonded with new cement paste (Lee and Choi, 2013). This ITZ, typically 20–50 μm wide, exhibits higher porosity, lower hardness, and reduced elastic modulus than the bulk matrix, leading to premature cracking and strength degradation in CSM (Wang et al., 2019; Yang et al., 2023; Nian et al., 2024).

Current methods to strengthen RCA interfaces encompass three primary strategies: 1) Physical strengthening typically employs organic polymers, high-molecular treatment agents (Wang et al., 2017; Ismail and Ramli, 2013; Shi et al., 2016), etc., to strengthen the old mortar adhering to the aggregates or uses methods such as mechanical grinding and ultrasonic cleaning (Bru et al., 2014; Akbarnezhad et al., 2011; Katz, 2004) to separate the aggregates from the old mortar, thereby reducing the crushing value and water absorption rate of the recycled aggregates. Physical strengthening is widely adopted by domestic and foreign recycled aggregate processing plants due to its simple process and short aging time, but this method is prone to cause damage to recycled aggregates. 2) Chemical strengthening methods typically employ chemical reagents, such as strong acids, that readily react with calcium hydroxide (Tam et al., 2007; Al-Bayati et al., 2016; Kou and Poon, 2010) to treat recycled aggregates. Although this can effectively remove old mortar, it introduces a large number of acid radical ions. 3) Microbial enhancement mainly involves the mineralization reaction of different bacterial strains in the pores of recycled aggregates, generating carbonate precipitates that fill the micropores of old mortar to improve the performance of recycled aggregates (Qiu et al., 2014). Qiu et al. (2014) treated recycled aggregates with microbial carbonate precipitation, which reduced the water absorption rate of the recycled aggregates to some extent. However, the microbial activity was difficult to guarantee, and it posed certain hazards to concrete.

In contrast, iron tailings sand (ITS), an underutilized mining residue with annual outputs exceeding 1.2 billion tons in China, presents a synergistic waste-valorization opportunity. ITS uniquely addresses RCA’s interfacial deficiencies through three inherent mechanisms (Fang et al., 2025; Xu et al., 2020; Yang et al., 2025): First, its granular morphology enables physical blockage of RCA surface voids. Second, its high reactive silica/alumina content undergoes pozzolanic reactions with portlandite to precipitate secondary C-S-H gels, thereby strengthening interfacial cohesion. Third, iron oxide components participate in ferrite-phase hydration, forming nanoscale Fe-substituted hydrates [e.g., C 4 (A,F)H 13 ] that densify the ITZ nanostructure. Crucially, ITS requires zero additional processing and directly replaces natural sand in CSM formulations, simultaneously reducing virgin material costs while diverting mine waste from ecologically hazardous landfills. This dual functionality, with both interface enhancement and sustainable resource conservation, positions ITS as a technically viable and economically scalable solution for recycled aggregate applications. Despite established applications of ITS in conventional concrete, its efficacy in enhancing the ITZ of CSM remains unexplored, particularly regarding nanoscale mechanical evolution. Current research relies predominantly on macroscopic tests, which fail to directly quantify the ITZ properties critical for durability prediction.

This study employs a multi-scale characterization method to comprehensively evaluate the interfacial strengthening effect of ITS in the cement-stabilized crushed stone of recycled aggregates. Through microhardness spectroscopy and nanoindentation-derived elastic modulus mapping, the potential strengthening mechanism was clarified, particularly focusing on the influence of ITS content on ITZ nanomechanics.

Materials

Cement

Portland cement (Type P.O 42.5) was employed throughout this study. Its physical properties are summarized in Table 1.

Aggregates

Coarse natural aggregate (NA, 5–25 mm) and medium sand (fine NA) constituted the primary aggregates. Key properties of NA were characterized per Chinese Standard JGJ 52-2006, while recycled coarse aggregate (RA) followed GB/T 25177-2010 (Table 2). Iron tailings (Xi’an, Shaanxi) were processed to 0.075–0.6 mm particles through controlled grinding (Figure 1).

ITS

The ITS used in this study was sourced from a typical iron ore mine in Xi’an, Shaanxi Province, and the as-received material was mechanically sieved into three particle size fractions. Figure 1 presents graphs of these fractions, showing that the particles are predominantly angular to sub-angular with rough surface textures and visible attached fines. The finest fraction possesses the highest specific surface area, which is expected to increase both physical filling and pozzolanic reactivity, whereas the coarsest fraction exhibits more irregular contours and fewer surface fines, and the intermediate fraction displays a transitional morphology.

Mix proportions

Prior studies demonstrate that increased cement content increases strength but reduces shrinkage resistance in cement-stabilized recycled materials (Bendimerad et al., 2016; Zhang et al., 2023; Jahanshahi and Ghanizadeh, 2024). To balance minimal shrinkage and strength compliance, a constant 5% cement content was adopted based on Zhang and Wang (2011). Experimental variables included: 50% RCA replacement and ITS substitutions of 0%, 25%, 50%, and 75%. The water-to-cement ratio for each mix was determined from the optimum moisture content obtained through standard compaction tests in accordance with JTG 3441-2024. Specimens were fabricated by static compaction into cylindrical molds (Φ150 mm × 150 mm) and cured under standard conditions (20 °C ± 2 °C, RH ≥ 95%) until the designated testing ages. For each mix proportion, three replicate specimens were prepared.

Test methods

Microhardness test

The microhardness of the CSM specimens was measured using an HX-1000 T microhardness tester at ×400 magnification. Tests employed a 500 g load with a 5 s dwell time and 10 μm step size, following the specimen preparation and testing methodology of Zhou and Duan (2016). Specimen processing involved the following: Sectioning 40 mm × 40 mm × 160 mm specimens cured to target ages into 40 mm × 40 mm × 10 mm slices.

Drying slices at 50 °C for 48 h.

Sequential grinding with 320 CW, 800 CW, and 1500 CW grit silicon carbide paper.

Final polishing using a metallographic polisher to achieve an optical-grade surface while minimizing damage.

Three interface types were tested: old aggregate (LG)–old mortar (LJ), old aggregate (LG)–new mortar (XJ), and old mortar (LJ)–new mortar (XJ). For each of the three interface types, linear scanning was performed perpendicular to the interface direction, starting from −50 μm (inside the old aggregate) to +140 μm (into the new paste), with a step size of 10 μm. Each scan line contained approximately 15 indentation points, and three parallel scan lines were tested per specimen, with no fewer than 20 valid indentation points per interface type. The results were averaged across the three scan lines.

Nanoindentation test

Nanoindentation testing was performed using a Bruker Hysitron TI 980 instrument equipped with a Berkovich diamond indenter (65.27° centerline-to-face angle) to characterize the elastic modulus of recycled concrete materials. Displacement-controlled loading cycles consisting of 5 s loading, 2 s holding, and 5 s unloading phases were implemented (Figure 2), with data analysis following Equation 1: where E r is the elastic modulus of recycled concrete (GPa); ν = 0.2 represents the concrete’s Poisson ratio (Li et al., 2012; Li, 2008); E is the indenter’s elastic modulus (GPa); ν i = 0.07 is the indenter’s Poisson ratio; and E i = 1140 Gpa is the indenter’s elastic modulus.

Specimen preparation began with sectioning using a precision saw (2800 rpm), followed by 12 h immersion in anhydrous ethanol to terminate hydration. Samples were subsequently oven-dried at 50 °C until mass stabilization. Sequential surface processing involved grinding with silicon carbide papers and multi-stage polishing using diamond suspensions on a semi-automatic polisher. Final specimens (1.5 × 1.5 × 0.5 cm 3) underwent ultrasonic cleaning (10 min in deionized water), followed by 12 h oven-drying at 50 °C and desiccated storage prior to testing.

Spatial mapping of interface transition zones (ITZs) employed a 20 μm lateral and 20 μm longitudinal spacing between indentation points (Figure 3).

Interfacial microhardness of recycled water-stabilized crushed stone

Figure 4 presents optical micrographs (400× magnification) of microhardness indentations on 0% ITS, 25% ITS, and 50% ITS specimens. The indentation area variability under identical loading conditions reveals microhardness heterogeneity across the ITS matrix. This heterogeneity primarily arises from localized phase differences: residual ITS grains exhibit higher hardness (manifested as smaller indentations), while hydrated phases show lower hardness (larger indentations). Notably, the 50% ITS sample displays systematically smaller indentations than the 25% ITS sample, indicating increased microhardness with increasing iron tailings sand content.

Figure 5 delineates the microhardness profiles across three distinct interfaces in the CSM with varying ITS contents: old aggregate–old mortar (LG–LJ), old aggregate–new mortar (LG–XJ), and old mortar–new mortar (LJ–XJ). Significant dispersion in indentation data reflects the inherent heterogeneity of the geopolymer paste matrix, primarily attributed to the stochastic spatial distribution of geopolymeric reaction products and localized phase contrasts induced by unreacted ITS particles. Crucially, robust statistical trends emerge upon sufficient sampling, revealing that the LG–XJ interface exhibits the highest sensitivity to ITS content. This phenomenon is explained by two factors: the pre-established cohesion of the LG–LJ interface limits its susceptibility, while the LJ–XJ interface benefits from the intrinsic chemical compatibility between homogeneous phases.

At moderate ITS additions (≤50%), a pronounced interfacial densification occurs, manifested by a 35% reduction in interface width. It is postulated that this microstructural refinement may be attributed to the physical filling effect of fine ITS particles and potentially enhanced reactions if the ITS exhibits pozzolanic reactivity; however, direct evidence for new hydration product formation (e.g., via X-ray diffraction (XRD) or SEM–EDS) is not available in the present study and awaits future investigation. Conversely, excessive ITS triggers progressive interfacial degradation: hardness declines while interface width expands. A possible explanation is that the inherently low reactivity of ITS may suppress reaction kinetics and reduce the formation of hydration products, thereby increasing interfacial porosity.

Interfacial elastic modulus of recycled water-stabilized crushed stone

Figure 6 presents a three-dimensional mapping surface of the elastic modulus at the LG–XJ interface, revealing pronounced gradients between aggregate particles and the cement-stabilized matrix. Significant surface undulations within the matrix region reflect inherent modulus heterogeneity. For quantitative comparative analysis, this 3D representation was converted to elastic modulus contour maps plotted against distance from aggregate surfaces (Figure 7).

The contour visualization demonstrates distinct stratification among aggregate, ITZ, and matrix phases, evidenced by sharp chromatic contrasts. The ITZ manifests as a narrow, low-modulus band (<40 μm width) adjacent to aggregates, which is a characteristic signature of interfacial weakening. Notably, 50% ITS incorporation reduced ITZ width by 35%, confirming microstructural densification. Localized modulus anomalies were attributed to fine sand particles (<75 μm) and micropores within the matrix. These features induce indentation size effects during nano-probing, explaining the observed data dispersion.

This study systematically elucidates the interfacial micromechanical enhancement of recycled aggregate CSM through ITS modification. Key conclusions are drawn as follows: Nanoindentation mapping reveals that 50% ITS incorporation reduces the width of the low modulus ITZ band by 35%, confirming microstructural densification. The ITZ’s distinct stratification (<40 μm low modulus zone) correlates with localized anomalies from fine sand particles (<75 μm) and micropores.

The micromechanical degradation mechanisms diminished microhardness beyond material thresholds (ITS > 50%), and ITZ widening may imply potential deterioration in macroscopic performance, but this implication requires direct validation through future macro-mechanical testing. This suggests that interfacial engineering could serve as a predictor of bulk performance, provided that cross-scale correlations are established.

To balance mechanical performance and resource efficiency, CSM formulations should restrict RCA to ≤ 50% and utilize 50% ITS as a reactive filler. These thresholds mitigate interfacial defects while leveraging waste materials, advancing the circular economy in construction.

The optimal ITS content (50%) identified in this study is material-specific and may vary with RCA source. However, the applicability of the underlying mechanism that governs how moderate ITS densifies the ITZ while an excessive dosage causes degradation is universal, and the multi-scale characterization methodology is reliable for screening optimal dosages across different RCA systems.