Phase assemblage, microstructure and shrinkage of cement paste during carbonation at different relative humidities

Abstract

This study investigates the effect of relative humidity (RH) on the carbonation of hydration products. The samples of ordinary Portland cement (OPC) and OPC/blast furnace slag paste were exposed to accelerated carbonation under 3% CO2 concentration at different RHs from 11% to 85%. Thermogravimetric/differential thermal analysis and X-ray diffraction/Rietveld analysis were performed to quantify the amount of portlandite and calcium carbonate polymorphs. The Ca/Si ratio of the carbonated C-S-H phase was determined using a phase equilibrium calculation. To evaluate the microstructural changes in carbonated sample, water vapor sorption and nitrogen sorption measurements were conducted before and after carbonation. Results show that the carbonation shrinkage progressively increased at intermediate RHs (43% and 66% RH). The relation between carbonation shrinkage and the Ca/Si ratio of C-S-H agrees with the change of C-S-H mean chain length. There were significant changes in the carbonation rates of C-S-H and portlandite at different RHs. The coarsening of the meso scale pore likely derives from the polymerization and aggregation of C-S-H due to decalcification. This paper is an extended and enhanced version of an earlier study.