Recycling aluminosilicate-based solid wastes is imperative to realize the sustainable development of constructions. By using alkali activation technology, aluminosilicate-based solid wastes, such as furnace slag, fly ash, red mud, and most of the bio-ashes, can be turned into alternative binder materials to Portland cement to reduce the carbon footprint of the construction and maintenance activities of concrete structures. In this paper, the chemistry involved in the formation of alkali-activated materials (AAMs) and the influential factors of their properties are briefly reviewed. The commonly used methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TG), nuclear magnetic resonance spectroscopy (NMR), and X-ray pair distribution function technology, to characterize the microstructure of AAMs are introduced. Typical characterization results of AAMs are shown and the limitations of each method are discussed. The main challenges, such as shrinkage, creep, efflorescence, carbonation, alkali–silica reaction, and chloride ingress, to conquer for a wider application of AAMs are reviewed. It is shown that several performances of AAMs under certain circumstances seem to be less satisfactory than traditional portland cement systems. Existing strategies to improve these performances are reviewed, and recommendations for future studies are given.

Using solid waste to replace limestone filler in asphalt concrete can not only reduce the cost of road construction, but also improve the utilization rate of solid waste. In this study, PHC pile waste concrete (PPWC) was innovatively used to replace limestone filler in asphalt mixture and its effect on the physical and rheological properties of asphalt mastics was studied. Firstly, PPWC was ground into filler particles with a diameter less than 0.075 mm. The physical properties, particle characteristics and chemical composition of PPWC filler and limestone filler were compared. Asphalt mastics were prepared with different filler-asphalt volume ratios (20%, 30% and 40%) and the physical properties, high-temperature rheological properties and low-temperature cracking resistance of asphalt mastics were tested. The experimental results showed that the surface of PPWC filler is rougher and has lower density and smaller particle size than limestone filler. When the filler content is the same, PPWC filler asphalt mastics have lower penetration and ductility, higher softening point than limestone filler asphalt mastics, and the viscosity of PPWC filler asphalt mastics is more sensitive than limestone filler asphalt mastics. PPWC filler asphalt mastics demonstrated superior high-temperature stability, but poorer low-temperature cracking resistance compared to limestone filler asphalt mastics. In conclusion, PPWC fillers can be used to replace limestone fillers in asphalt mixtures. The finding of this study will provide a new solution for the construction of eco-friendly roads.

Fiber-reinforced cemented paste backfill (FR-CPB) has attracted considerable attention in modern mining applications due to its superior mechanical properties and adaptability. Despite its potential, understanding its rheological behavior remains limited, largely because of the absence of quantitative methods for assessing fiber packing behavior within CPB. This study develops a rheology-based approach to determine the maximum packing fraction of polypropylene fibers in fresh CPB, revealing that shorter fibers (3 mm) achieve a maximum packing fraction of 0.661, significantly higher than longer fibers (12 mm) with 0.534. Building on these findings, a quantitative model for the static yield stress of FR-CPB was developed, showing that under a high fiber content (0.9%) and with longer fibers (12 mm), the yield stress reached 274.34 kPa, a 40% increase compared to shorter fibers. Additionally, the study modeled the time-dependent evolution of yield stress, achieving a prediction accuracy with a correlation coefficient of 0.92. These advancements enable the optimization of FR-CPB composition, which can reduce material usage, enhance pipeline transport efficiency, and improve backfill stability in underground voids. By minimizing the risk of structural failure and optimizing resource allocation, this research provides a theoretical foundation for safer and more cost-effective mining operations.

Calcium-sodium aluminosilicate hydrate (C-(N)-A-S–H) gels are the primary phases in alkali-activated slag (AAS) systems and govern paste-scale microstructural evolution. Conventional models assume cross-linked and non-cross-linked C-(N)-A-S–H gels with Ca/(Si + Al) > 0.67 and Al/Si < 0.25. However, several studies have reported Al-rich gels (Al/Si > 0.25) in both AAS and in alkali-activated blends with lower calcium content, whose nanostructure and chemistry remain insufficiently resolved. In this study, four amorphous Al-rich C-(N)-A-S–H gels spanning Al/Si = 0.31–0.57 and Ca/Si = 0.56–1.04 were synthesized to enable two orthogonal comparisons: (i) varying Al/Si at constant Ca/Si to isolate Al-for-Si substitution effects, and (ii) varying Ca/Si at constant Al/Si to probe calcium role on network connectivity. Comprehensive characterization revealed the coexistence of local structural environments characteristic of C-(N)-A-S–H/C-A-S–H, (N,C)-A-S–H, and N-A-S–H-like gel domains without the formation of crystalline secondary phases. ²⁹ Si NMR revealed a progressive increase in Q ⁴ (mAl)-rich sites with increasing Al incorporation, while the relative proportion of the C-(N)-A-S–H/C-A-S–H environments decreased with decreasing Ca/Si ratio. ²⁷ Al NMR confirmed tetrahedral Al coordination, with chemical shifts varying with Ca/Si ratio, reflecting changes in interlayer charge-balancing. Octahedral Al was detected exclusively in gels with a higher Al/Si at 0.5, indicating the formation of more highly polymerized aluminosilicate environments under high-Al conditions. ²³ Na NMR revealed chemical shifts of − 5.81 to − 6.21 ppm, correlating strongly with the Na/Al ratio. Collectively, the findings demonstrate that gels with Al/Si > 0.25 deviate from classical tobermorite-like structures and exhibit increasingly polymerized and structurally heterogeneous aluminosilicate environments.

γ-dicalcium silicate (γ-C₂S) is a carbonatable binder with excellent carbonation reactivity. However, its extremely low hydration activity prevents the paste from setting and hardening properly, making it difficult to be directly cast and molded. This study introduces β-hemihydrate gypsum as an early-strength regulating agent, which utilizes its rapid hydration to form dihydrate gypsum, thereby imparting early-age strength to the composite system and enabling subsequent carbonation curing after demolding. The experimental investigation examined the effects of β-hemihydrate gypsum content on the fluidity, setting time, and carbonation performance of γ-C₂S paste. The results indicate that when the β-hemihydrate gypsum content is not less than 10%, the paste can obtain sufficient early-age strength to achieve smooth demolding. At a β-hemihydrate gypsum content of 10%, after pre-drying treatment, samples subjected to carbonation curing for 24 h under a CO₂ partial pressure of 0.3 MPa achieved a peak absolute dry compressive strength of 117.29 MPa with a softening coefficient of 0.92. Further increases in β-hemihydrate gypsum content lead to reductions in both strength and water resistance. Microstructural analysis reveals that at a gypsum content of 10%, the crystalline network of dihydrate gypsum interlocks and coexists with calcium carbonate and silica gel generated from γ-C₂S carbonation, forming a compact structure. In contrast, the framework formed by excessive gypsum before carbonation restricts the continuous interlocking and overall development of calcium carbonate products generated from γ-C₂S, resulting in deterioration of structural integrity and mechanical properties. This study provides theoretical foundation and technical support for the engineering application of γ-C₂S-based carbon mineralization materials.