Modification of cementitious building materials by treatment with CO2

Abstract

The aim of this study was to determine the effect of CO2 treatment conditions and cement composition (i.e. alkali and C3A content, cement fineness) on the microstructure, phases and the strength of hardened cement paste and mortar. The results show that CO2 treatment at elevated pressures produces significant changes in the microstructure of hardened cement paste. The decomposition of the hydrates and, to some extent, non-hydrated clinker minerals to form CaCO3 phases (mainly calcite) produces a dense microstructure and increases strength significantly. The strength increased with CO2 pressure and the duration of treatment. A further increase in strength occurred during the subsequent storage of carbonated specimens in water or in air at 20 °C and 65 %RH owing to the ongoing hydration of residual clinker minerals. Specimens treated with supercritical CO2 and then stored at 20 °C and 65 %RH reached a 28 d compressive strength which almost doubled the standard 28 d standard strength. The use of cements with a higher fineness (strength class) reduced the carbonation rate and therefore the gain in strength, owing to the lower porosity of the initial material made with these cements. The higher alkali contents of the cement led to a slower carbonation. The largest depth of carbonation and the highest increase in strength were obtained for cements with low C3A and alkali contents.