Changes in mineral composition, growth of calcite crystal, and promotion of physico-chemical properties induced by carbonation of β-C2S

Larnite (beta-C2S) is a low-calcium mineral with a low rate of early hydration which has a high CO2 sequestration potential. Synthetic beta-C2S was exposed to pure CO2 for gas-solid carbonation for varied time periods to investigate changes in mineral composition, calcite crystal growth, and the development of physico-chemical properties. Results show that the ultimate degree of carbonation reached 61.5% and the ultimate CO2 sequestration capacity was 314.7 g/kg raw beta-C2S. High-resolution transmission electron microscopy, C-13 magic angle spinning nuclear magnetic resonance spectroscopy, the Rietveld method, and thermogravimetric analysis confirmed that amorphous and/or poorly-crystalline calcium carbonates were produced during carbonation. Well-crystalline calcite with a single crystal structure was formed, having a crystallite size of 107 nm and an average particle size of 1.8 mu m after 168 h of carbonation. The compact stack and the strong mechanical bond between well-crystalline calcite particles were observed in scanning electron microscopy images, and they are thought to contribute to a strong micro level force, thereby leading to the good macro level performance of carbonation products which reached 127 MPa after 168 h of carbonation. This study contributes to a more insightful understanding of the vital role of calcite in promoting the mechanical and physico-chemical properties of beta-C2S-containing wastes and cements produced by carbonation. A further investigation into the micro level mechanical properties of calcite is required to understand the relation between carbonation and increased strength.