Morphological characteristics of calcium carbonate crystallization in CO₂ pre-cured aerated concrete

Abstract

Early-stage CO₂ 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 CO₂ sinking. Less attention, however, has been paid to morphological characteristics of CaCO₃ in carbonated cement concrete. The crystal structure and micromorphology of CaCO₃ in an early-age aerated concrete (AC) cured under CO₂ gas pressures of 0.1, 1, and 2 bar were investigated. The fabricated AC has a high CO₂ sorption capacity (∼35 g CO₂ per 100 g cement in a 100 mm cube). The morphological characteristics of CaCO₃ were statistically analyzed in terms of long-axis length (<i>b</i>), short-axis length (<i>a</i>), and aspect ratio (<i>K</i> = <i>b</i>/<i>a</i>). As CO₂ pressure increases, <i>b</i> is almost unchanged from 0.8-1.8 μm, <i>a</i> decreases from 0.7 to 0.4 μm, and, consequently, <i>K</i> increases from 1.3 to 2.5. The different CaCO₃ crystal morphologies in AC are ascribed to the CO₂ pressure-associated crystal transformation processes: low gas pressure induces a symmetric CaCO₃ growth, while high gas pressure causes a faster calcite growth at the crystal tip ends. The findings would deepen the understanding of CaCO₃ crystal formation under different CO₂ curing pressures for tuning the microstructure of CO₂-cured cement concrete.