The objective of this ongoing work is to improve the physical properties of Portland cement pastes and Portland-composite cement pastes containing silica fume and/or fly ash through an accelerated carbonation process based on the use of SCCO2. Cement carbonation allows the preparation of cementitious composites reinforced with low-cost alkali-intolerant products and the use of these materials in solidification/stabilization of hazardous waste. In the supercritical treatment, results suggested that the carbonation rate of the portlandite phase slowed down after the first reaction stage and was overtaken by the reaction rate of the CSH and ettringite phases. Samples carbonated under the supercritical conditions developed a higher volume of gel pores than those obtained by natural carbonation. The microstructural analysis was completed by analyzing the water permeability of a cement paste/sand composite (mortar) after supercritical carbonation. Obtained values showed that supercritical carbonation reduced water permeability significantly.