Influence of CaCO ₃ polymorphs from mussel shell and limestone on the hydration of low‐carbon cements

Abstract

Calcium carbonate is widely used as a low‐carbon component in cementitious binders, but its effect is commonly discussed in terms of dilution, filler effects and nucleation. The potential influence of CaCO ₃ polymorphs remains less understood. This study compares calcite‐dominant limestone powder (LSP) and aragonite‐rich mussel shell powder (MSP) as partial replacements for Portland cement. Cement pastes containing 5–20 wt.% LSP or MSP were prepared to investigate the effects of CaCO ₃ polymorphs on hydration kinetics, phase assemblage, mechanical performance and microstructure. Both carbonate powders accelerated early hydration by providing additional nucleation sites. Compared with LSP, MSP showed a stronger hydration‐promoting effect at moderate replacement levels, which is attributed to the metastable aragonite structure and its enhanced interaction with aluminate phases. The use of MSP promoted the formation of C–S–H and carbonate‐bearing AFm phases, leading to comparable or improved early strength at replacement levels not exceeding 10 wt.%. At 20 wt.% replacement, dilution became more pronounced and showed limited strength development. These results indicate that mussel shell powder can be regarded as a waste‐derived substitute from marine of limestone, which is a carbonate source with polymorph‐dependent reactivity for low‐carbon cement pastes.