Impacts of variations in calcium carbonate on sulfate optimization and performance of LC3-type cementitious systems

Limestone calcined clay cement (LC 3 ) is a low-clinker binder that leverages the synergy of limestone filler and calcined clay to achieve comparable performance as conventional Portland cement. The socio-economic benefits of LC 3 binder can be further enhanced by utilizing the calcium carbonate (CaCO 3 ) from biogenic sources or carbon mineralization of calcium-rich wastes. In this work, five types of CaCO 3 fillers, comprising limestone, waste oyster shell, and three synthetic CaCO 3 polymorphs produced by wet carbonation of cement suspension, were examined and compared regarding their influence on optimal sulfate content, phase assemblage, pore structure, and strength development of LC 3 -type binders. The results showed that oyster shell performed comparably to limestone, whereas synthesized calcite required more sulfate than other fillers to reach sulfate balance. Consistent with their lower crystallinity and greater CO 3 2- release in alkaline media, the three synthesized polymorphs promoted earlier and greater monocarboaluminate (Mc) precipitation, accompanied by a refined pore structure and improved compressive strength in hardened LC 3 containing synthesized CaCO 3 . These results demonstrate the materials-level viability of biogenic and CO 2 -mineralized CaCO 3 as low-carbon replacements for limestone under matched fineness and sulfate-optimized conditions, providing a potential route for sustainable LC 3 formulation and production.