Curing by CO(2)is a way to utilize CO(2)to reduce greenhouse gas emissions. Placing early-age cement paste in a CO(2)chamber or pressure vessel accelerates its strength development. Cement carbonation is attributed to the quickened strength development, and CO(2)uptake can be quantitatively evaluated by measuring CO(2)gas pressure loss in the pressure vessel. A decrease in CO(2)gas pressure is observed with all cement pastes and mortar samples regardless of the mix proportion and the casting method; one method involves compacting a low water-to-cement ratio mix, and the other method comprises a normal mix consolidated in a mold. The efficiency of the CO(2)curing is superior when a 20% concentration of CO(2)gas is supplied at a relative humidity of 75%. CO(2)uptake in specimens with the same CO(2)curing condition is different for each specimen size. As the specimen scale is larger, the depth of carbonation is smaller. Incorporating colloidal silica enhances the carbonation as well as the hydration of cement, which results in contributing to the increase in the 28-day strength.