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.