Morphological characteristics of calcium carbonate crystallization in CO 2 pre-cured aerated concrete.

Early-stage CO 2 curing technology for alkaline construction materials (such as cement concrete) has gained increasing interest owing to the advantages of material properties improvement and high potential of CO 2 sinking. Less attention, however, has been paid to morphological characteristics of CaCO 3 in carbonated cement concrete. The crystal structure and micromorphology of CaCO 3 in an early-age aerated concrete (AC) cured under CO 2 gas pressures of 0.1, 1, and 2 bar were investigated. The fabricated AC has a high CO 2 sorption capacity (∼35 g CO 2 per 100 g cement in a 100 mm cube). The morphological characteristics of CaCO 3 were statistically analyzed in terms of long-axis length ( b ), short-axis length ( a ), and aspect ratio ( K = b / a ). As CO 2 pressure increases, b is almost unchanged from 0.8-1.8 μm, a decreases from 0.7 to 0.4 μm, and, consequently, K increases from 1.3 to 2.5. The different CaCO 3 crystal morphologies in AC are ascribed to the CO 2 pressure-associated crystal transformation processes: low gas pressure induces a symmetric CaCO 3 growth, while high gas pressure causes a faster calcite growth at the crystal tip ends. The findings would deepen the understanding of CaCO 3 crystal formation under different CO 2 curing pressures for tuning the microstructure of CO 2 -cured cement concrete.