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Abstract

A method is disclosed for rapidly carbonating large cement structures, by forming and hardening cement in a mold under high carbon dioxide density, such as supercritical or near-supercritical conditions. The method is more reliable, efficient, and effective than are post-molding treatments with high-pressure CO.sub.2. Cements molded in the presence of high-pressure CO.sub.2 are significantly denser than otherwise comparable cements having no CO.sub.2 treatment, and are also significantly denser than otherwise comparable cements treated with CO.sub.2 after hardening. Bulk carbonation of cementitious materials produces several beneficial effects, including reducing permeability of the cement, increasing its compressive strength, and reducing its pH. These effects are produced rapidly, and extend throughout the bulk of the cement--they are not limited to a surface layer, as are prior methods of post-hardening CO.sub.2 treatment. The method may be used with any cement or concrete composition, including those made with waste products such as fly ash or cement slag. Surface carbonation is almost instantaneous, and bulk carbonation deep into a form is rapid. By combining molding, curing, and carbonation into a single step, carbon dioxide is better distributed throughout the entire specimen or form, producing a uniform product.

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