Impact of vaterite integration on the hydration, microstructure and performance of calcined clay cements.

Vaterite Calcined Clay Cement (VC3) is a novel binder system in which the conventional calcite used in Limestone Calcined Clay Cement (LC3) is replaced with vaterite, a metastable polymorph of calcium carbonate characterized by its higher reactivity and spherical particle morphology. Although initial studies suggest potential benefits, the effects of vaterite on hydration kinetics, fresh-state behavior, and long-term mechanical performance remain insufficiently understood. To address this, eight mortar formulations were developed with 0-15% of vaterite content and 40%-50% Ordinary Portland Cement (OPC) content. Results showed that, in VC3 specimens, vaterite enhanced fresh-state workability and prolonged the induction period of hydration while maintaining long-term reactivity. Although early-age compressive strength was reduced, VC3 mortars exhibited significant strength development over time. At 91 days, the mortar incorporating 15% vaterite and 50% OPC showed a marked increase in strength, reaching 129% of the compressive strength of the LC3 reference mortar. Even formulations with only 40-45% OPC achieved 110-124% of the reference strength. Analysis of the phase assemblage and microstructural analyses confirmed sustained pozzolanic activity, with ongoing portlandite (CH) consumption and the formation of ettringite and carboaluminate phases. These results position VC3 as a promising low-clinker alternative, combining improved fresh-state performance with competitive long-term strength and enhanced sustainability.