Fabrication of vaterite blocks from a calcium hydroxide compact

Metastable vaterite blocks are expected to show high reactivity. However, they have not been reported to date. In this study, the feasibility of fabricating an artificial vaterite block was investigated by exposing a Ca(OH)₂ compact to CO₂ under several conditions. Under a 100% H₂O atmosphere, a calcite block was formed within three days. Under a 100% methanol atmosphere, no carbonation was observed for seven days. Under 90% methanol without atmospheric flow, a vaterite block containing 5 mass% of unreacted Ca(OH)₂ and 3 mass% of calcite was formed after 24 h. On day 7, it was transformed into a block with 70 mass% of calcite, 27 mass% of vaterite, and 3 mass% of unreacted Ca(OH)₂. By contrast, under 90% methanol with atmospheric flow to remove the accumulated H₂O in the compact, a pure CaCO₃ block of 98 mass% vaterite and 2 mass% calcite was formed on day 3. Therefore, the regulation of the amount of H₂O is important for fabricating vaterite blocks. Porosimetry measurements revealed that the calcite block fabricated under 90% methanol without atmospheric flow demonstrated a pore size profile closer to that of the vaterite block than that of the calcite block fabricated under 100% H₂O atmosphere because the vaterite with small spherical crystals was formed initially before its transformation to calcite.