High-pressure molding and carbonation of cementitious materials

Bulk carbonation of cements can have several beneficial effects, including permeability and porosity reduction, increased compressive strength, and pH reduction. Using supercritical and near-critical CO2, we examined both in- situ molding processes and postsetting treatments of cement mixtures, inclu ding those with fly ash, cement slag, and reactive silica. Specimens were c haracterized by X-ray diffraction analysis, scanning electron microscopy, a nd thermogravimetric analysis (for carbonate content), pH by a contact meth od, and porosity measurements such as Na adsorption and water absorption. S urface carbonation was almost instantaneous for cured cements using supercr itical CO2, and rapid bulk carbonation of forms several millimeters thick c ould be effected using in-situ molding. Carbonation by supercritical CO2 fo rmed a dense layer of interlocking CaCO3 crystals in minutes. The best way to rapidly carbonate large cement forms was to harden them in a mold under CO2 pressure; these materials cured at an accelerated rate, were densified, and showed enhanced formation of crystalline calcite. In some cases this w as accomplished without significant loss of microporosity. The presence of different types of reinforcing fibers did not impede carbonation by this me thod.