To promote industrial solid waste recycling and sustainable development, this study prepared alkali-activated recycled aggregate concrete (AARAC) using ground granulated blast-furnace slag and recycled coarse aggregate. The effects of alkali content and recycled aggregate replacement ratio on AARAC performance were examined. Results indicated that while recycled aggregate can impair concrete properties, an optimal alkali content of 6% significantly mitigates these effects, improving physical properties, mechanical stability, and compactness. Microstructural analysis showed enhanced hydration product formation and a denser interfacial transition zone at this alkali level. Entropy weight-TOPSIS evaluation confirmed that 6% alkali content provided the best overall performance with low sensitivity to aggregate variation. Additionally, carbon emission analysis revealed that AARAC reduced CO2 emissions by approximately 45% compared to conventional recycled concrete. These findings suggest that alkali-activated recycled aggregate systems offer a promising approach to developing low-carbon, sustainable construction materials.

Phosphate tailings were used as a partial substitute for cement in the preparation of foam concrete in this study to investigate its effect on the physical properties and pore structure of foam concrete. The results showed that Ca2+, Mg2+, and CO3 2- dissolved from phosphate tailings reacted with Al3+ in the aluminate cement to form MgAl2(OH)8 and Mg6Al2(CO)3(OH)16. The formation of this substance promoted cement hydration and shortened the setting time. The addition of phosphate tailings to foam concrete improved the pore structure, refining pore size, enhancing pore uniformity, and minimizing pore connectivity. When the phosphate tailings content was 10 wt.%, the foam concrete had fewer pores larger than 1000 μm in size and nearly no air holes on the pore wall, the compressive strength at 7 days of foam concrete reached a maximum value of 0.96 MPa, 2.3 times higher than the specimen without phosphate tailings.

To mitigate cement overuse and associated CO2 emissions, incorporation of supplementary cementitious materials (SCMs) offers a sustainable strategy for producing eco-friendly ultra-high-performance fiber-reinforced concrete (UHPFRC). This study investigates the effects and underlying mechanisms of replacement of ground granulated blast furnace slag (GGBS) and limestone powder (LP) on hydration, pore structure, mechanical properties, and durability of UHPFRC. Results show that GGBS significantly improves durability, with optimal performance at 30 wt%, attributed to enhanced secondary hydration and reduced pore connectivity. In contrast, LP offers strength benefits at 15 wt% but adversely affects chloride resistance due to poor fiber-matrix bonding. Correlation analysis indicates that chloride resistance in UHPFRC is primarily governed by pore structure and fiber-matrix interfacial transition zone refinement, wherein GGBS enhances both through secondary hydration, while LP tends to impair interfacial bonding at elevated replacement levels. The results offer a scientific basis for designing SCM-incorporated UHPFRC with enhanced durability and sustainability.

The demand for sustainable construction methodology is a need of the hour, as the material used in construction activities also greatly contributes to environmental emissions. Although conventional engineered cementitious composites (ECC) could greatly enhance the strength of composite materials by incorporating fibers, synthetic fibers, and cement production emits a great deal of carbon dioxide into the atmosphere. The work attempts to report a detailed thermal study on the performance of natural fiber-reinforced ultra-high toughness cementitious composites (UHTCC). Test parameters include temperature levels (300 °C, 600 °C and 900 °C), type of natural fiber (flax and hemp), and fiber volume fraction (Vf =2% and 3%). Moreover, a detailed microstructural analysis was performed using FESEM, x-ray diffraction (XRD), and surface roughness studies. Also, the effect of elevated temperatures on the porosity and pore size distribution of natural fiber UHTCC was evaluated using a MATLAB program based on the image greyscale method. Results from the mechanical characterization revealed a negligible strength loss of just 14.3% and 17.1% was observed with flax and hemp fiber composites, even when exposed to a high thermal gradient of 900 °C, attributable to the excellent fiber properties and particle packing of the UHTCC mix. Micro-structural and pore-size distribution analysis revealed that marginal changes occur in the composites if the temperature exceeds 600 °C, indicating that UHTCC are both strong and environmentally-friendly for practical applications.

Ultra-High Performance Concrete (UHPC) is prone to significant drying shrinkage due to its high binder content and low water-binder ratio. Microstructural refinement through particle packing can mitigate this issue. This study investigates the role of “Particle Packing Factor” (P PF) - a novel design metric, in controlling drying shrinkage across 19 UHPC formulations with varying binder contents (630–1575 kg/m3), incorporating fly ash (0–35%), slag (0–50%), and silica fume (0–25%). Porosity refinement led to 51% reduction in drying shrinkage at 28 days (990 µξ to 480 µξ) and a 42% reduction at 91 days (1032 µξ to 600 µξ). Strong correlations were observed between PPF and drying shrinkage (R 2 = 0.86–0.92) as well as P PF and open porosity (R 2 = 0.79–0.84), underscoring the role of particle packing in reducing shrinkage. These findings establish P PF as a powerful metric for optimizing UHPC formulations to minimize long-term shrinkage.

This study explores the use of calcined dolomite (CD) as a supplementary precursor and nano-silica (NS) as an additive in one-part sodium carbonate-activated slag-based mixes. Binary mixes incorporated up to 20% CD, while ternary mixes included 3% NS to enhance microstructure, strength, and shrinkage performance. Microstructural analysis was conducted using X-ray diffraction, scanning electron microscopy, and mercury intrusion porosimetry. Results showed that CD increased flowability and reduced setting time, whereas NS slightly decreased flow but significantly accelerated setting. The optimal compressive strength (55 MPa at 28 days) was achieved at 10% CD replacement. While NS improved strength and reduced shrinkage in slag-based mixes, it negatively impacted ternary mixes. Binary mixes exhibited lower shrinkage due to pore structure variations. Environmental assessments indicated that these mixes produce significantly lower CO₂ emissions than cement-based alternatives, highlighting their potential as a sustainable construction material.

To enhance the carbonation resistance of concrete, this study optimized CO2 mineralization parameters of steel slag using the Taguchi method, revealing a nonlinear relationship between carbon content and mineralized parameters. The optimal conditions (150 min, 2.1 MPa, 78.5 °C and 36.4% humidity) achieved a carbon fixation rate of 7.82%. A amount of carbonation products covering the surface of steel slag, significantly improving steel slag’s microstructure and chemical properties. Incorporating 10% CO2 mineralized steel slag reduced the carbonation depth of concrete to 7 mm, a 30% improvement compared to pure cement concrete (10 mm). The micro-nano mineralization products fill the pores and enhance the bonding between steel slag and hydration products, thereby reducing CO2 ingress into the concrete and maintaining a high pH level. These findings show an extensible way to recycle steel slag and improve the durability and sustainability of concrete.