Preliminary investigations into the supercritical carbonation of cement pastes

Interactions between supercritical carbon dioxide (scCO2) and hydrated cement pastes, of various water/cement ratio, have been investigated. The carbonation process was greatly accelerated in the scCO2 compared to that in natural or CO2 enriched environments. The nature of the reactions was dependent on the amount of water present in the paste. Thus carbonation of samples dried prior to treatment resulted in the reaction of all the unhydrated C3S and C2S, but little conversion of calcium hydroxide to calcium carbonate. In contrast, carbonation of samples containing moisture resulted in the conversion of most of the calcium hydroxide whilst the amounts of C3S and C2S reacted increased as the water/cement ratio increased. During the carbonation treatment, the pore structure of the cement pastes was altered and substantial reductions in porosity were achieved. The process may be used to improve the durability of glass fibre reinforced cement by lowering the alkalinity and calcium hydroxide content of the matrix.