Effect of carbonation in supercritical CO₂ on the properties of hardened cement paste of different alkalinity

It is well-known that the strength of cementitious building materials is increased by carbonation. This process was accelerated using supercritical carbon dioxide scCO₂ at 150 bar. Hardened Portland cement pastes with different alkali contents and w/c ratios of 0.5 and 0.6 were treated for four hours in scCO₂ after preliminary storage at different relative humidities. Strengths were measured and the microstructure investigated by X-ray diffraction, scanning electron microscopy and mercury intrusion porosimetry. Carbonation in scCO₂ produces a substantial increase in strength which is favoured by low moisture contents and low-alkali cements. The retarding effect of alkalis on the carbonation reaction was confirmed by thermodynamic calculations.