The cement industry is one of the most CO₂-intensive industrial sectors, and reducing clinker consumption through mineral substitution is an effective strategy for developing low-carbon cementitious materials. Calcium carbonate (CaCO₃) is widely used as a clinker substitute because of its abundant resources, low cost, and compatibility with cement production. In addition to the thermodynamically stable calcite, CaCO₃ also exists in metastable polymorphs such as vaterite and aragonite, which differ in crystal structure, particle morphology, specific surface area, and dissolution behavior. These differences may produce distinct effects on the rheology, hydration, pore structure, and mechanical properties of cement pastes. This study aims to clarify how different CaCO₃ polymorphs regulate the coupled rheology-hydration-structure-strength relationship of cement paste.

Ordinary Portland cement pastes containing 15% (mass fraction) calcite, vaterite, or aragonite were prepared at a constant water-to-binder ratio of 0.42, with pure cement paste as the reference. Commercial limestone powder was used as the calcite source, while vaterite and aragonite were synthesized using a double-decomposition method. Rheological properties were characterized using a rotational rheometer, and the dynamic yield stress was determined by fitting the descending branch of the flow curve with the Herschel-Bulkley model. Early hydration behavior was evaluated by isothermal calorimetry. Flexural and compressive strengths were measured at 1, 3, 7 d, and 28 d. Hydration products and phase evolution were examined by X-ray diffraction and thermogravimetry, and the pore structure was characterized by mercury intrusion porosimetry.

The three CaCO₃ polymorphs exhibited distinct physicochemical characteristics and consequently exerted different effects on cement paste performance. Calcite mainly consisted of irregular blocky particles and mainly acted through dilution and filling, leading to reductions in both static and dynamic yield stress and only limited promotion of hydration and strength development. Vaterite, composed of loosely aggregated fine particles with relatively high surface activity, increased the dynamic yield stress, accelerated early hydration most effectively, promoted the formation of portlandite, chemically bound water, and carboaluminate phases, and refined the pore structure, and was therefore more beneficial for compressive strength development. Aragonite, with a needle-like morphology, reduced the dynamic yield stress but markedly increased the apparent viscosity and thixotropic index, which indicated a stronger effect on structural build-up and rebuilding. It also improved pore structure and showed the highest compressive strength at 1-7 d as well as the highest flexural strength and flexural-to-compressive strength ratio at 28 d. These results indicated that the effects of CaCO₃ polymorphs could not be attributed to a single parameter such as particle size, but were jointly governed by morphology, specific surface area, and reactivity.

Different CaCO₃ polymorphs significantly affect the rheology, hydration, and strength development of cement pastes. Calcite mainly shows weak dilution, filling, and nucleation effects, resulting in reduced yield stress and limited strength compensation. Vaterite markedly promotes early hydration and sustained product accumulation, leading to higher dynamic yield stress, increased portlandite and chemically bound water contents, greater formation of carboaluminate phases, refined pore structure, and improved early compressive strength. Aragonite, owing to its needle-like morphology, strongly enhances static yield stress, apparent viscosity, and thixotropic recovery, while also refining pore structure and achieving the highest flexural strength and flexural-to-compressive strength ratio. Compared with calcite, metastable CaCO₃ polymorphs are more effective in regulating the coupled “rheology-hydration-strength” behavior of cement pastes. Vaterite is more suitable for enhancing hydration and promoting compressive strength development, whereas aragonite is more advantageous for rheological control, flexural performance, and overall mechanical properties.