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, including those with fly ash, cement slag, and reactive silica. Specimens were characterized by X-ray diffraction analysis, scanning electron microscopy, and thermogravimetric analysis (for carbonate content), pH by a contact method, and porosity measurements such as Na adsorption and water absorption. Surface carbonation was almost instantaneous for cured cements using supercritical CO2, and rapid bulk carbonation of forms several millimeters thick could be effected using in-situ molding. Carbonation by supercritical CO2 formed 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 was accomplished without significant loss of microporosity. The presence of different types of reinforcing fibers did not impede carbonation by this method.