Indirect carbonation of hardened cement paste powder for high-purity vaterite formation: A route to high-performance carbonated cementitious materials

Construction and demolition waste (CDW) generates a vast amount of calcium-rich cementitious powders, yet these materials are currently underutilized—largely relegated to low-value applications that fail to exploit their inherent chemical potential. To address this challenge, this study develops an innovative strategy for converting hardened cement paste powder (HCPP) into high-value-added vaterite-type CaCO 3 (vaterite), aiming to simultaneously achieve CO 2 sequestration and the high-value utilization of waste cement materials. The research systematically investigates the effects of parameters on Ca 2 + leaching efficiency, and thoroughly reveals the synergistic regulatory mechanisms of CO 2 flow rate, liquid-to-solid (L/S) ratio, and crystal modifier on the phase composition, morphology, and particle size of the carbonation products. Under optimized conditions, the vaterite content reaches up to 96.69%, yielding spherical particles with uniform morphology. Furthermore, applying the obtained vaterite in a low-carbon cementitious material reveals that its reactivity is significantly higher than that of calcite. It participates earlier in the hydration reaction, promoting the early formation of a substantial amount of C-(A)-S-H gel, hemicarboaluminate (Hc) and monocarboaluminate (Mc) in the cementitious material system, thereby refining optimizing the pore structure and accelerating strength development. This study successfully establishes a functional conversion pathway from waste cement-based materials to high-performance CaCO 3 , providing important theoretical foundations and technical support for the recycling of construction waste and the development of low-carbon cementitious materials.