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

• Large increase in compressive strength of hardened cement paste and mortar.
• Significant changes in porosity and structure of hardened cement paste.
• Unfavourable reduction in carbonation rate and strength due to alkalis.

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.