Cold Sintering of Calcite and Calcite–Silica Composites

ABSTRACT

Calcite and silica were selected as model systems for investigating the cold sintering of cementitious materials, mimicking negative‐carbon products formed during the mineral carbonation of silicate‐rich feedstocks. Dense calcite and calcite–silica composites were fabricated using NaOH solutions at 200°C under applied pressures of 375–500 MPa, reaching maximum relative densities of ∼95% and ∼92%, respectively. This study first examined how NaOH concentration and applied pressure influence the cold sintering behavior of calcite. In alkaline environments, calcite exhibited enhanced dissolution and reprecipitation, forming nanocrystalline calcite that facilitated cold sintering. Although the solubility of calcite is expected to decrease in alkaline solutions, enhanced cold sintering and grain growth were nevertheless observed and rationalized using a kinetic model that considers potentially enhanced dissolution and reprecipitation rates. During the cold sintering of calcite–silica composites, competitive dissolution between calcite and amorphous silica led to distinct microstructural evolution pathways. While a small amount of silica impeded cold sintering and generated inhomogeneous microstructures, effective densification of the composite was achieved at higher silica contents, resulting in more homogeneous microstructures. Partial crystallization of amorphous silica into α‐quartz was observed, generating nanoscale porosity that was offset by enhanced shrinkage under applied pressure, such that the overall relative density was not significantly affected. These findings provide a mechanistic basis for developing low‐temperature consolidation strategies for cementitious materials.