Utilization of granite sludge in the production of low carbon cement composites after coupled mechanical and CO2 activation (CMCA)

Granite sludge (GS), as an industrial byproduct, has caused serious pollution and waste of resources. To reduce the environmental impact and achieve resource utilization, this study adopted two mechanochemical treatment methods, mechanical activation and coupled mechanical and CO₂ activation (CMCA), to treat GS to prepare a new low-carbon cement. After CMCA treatment, the compressive strength increased significantly in both early and late stages, reaching 5.5 MPa at 1 d and 38.19 MPa at 28 d. After the addition of sodium silicate, the compressive strength reached 5.87 MPa and 39.76 MPa at 1 d and 28 d, respectively. This performance improvement is attributed to synergistic physical and chemical activation. Physically, the treatment refines the particles to the 1–10 μm range and increases the specific surface area, thereby providing a large number of nucleation sites to accelerate early hydration. Chemically, the CMCA process generates highly active metastable calcium carbonate. The metastable calcium carbonate generated has high reactivity, and its significant nucleation effect promotes the overall hydration process. These highly active particle surfaces act as effective chemical nucleation sites, accelerating the formation of C-S-H gel. At the same time, the metastable calcium carbonate also directly participates in the reaction, reacting with the aluminate in the cement to generate additional reinforcing phases such as Mc and Hc. After adding sodium silicate, the generated silica gel has pozzolanic activity, which can not only undergo secondary hydration reaction with CH to generate more C-S-H, but also accelerate the overall hydration reaction and further improve the compressive strength. This method shows broad application prospects in industrial solid waste utilization and low-carbon cement production and has significant environmental benefits and resource utilization potential.