Evolution of the microstructure of limestone-calcined clay cement pastes exposed to CO2

The aim of this study was to gain a deeper understanding of the carbonation resistance of binary and ternary calcined clay-blended cements, and to investigate how this resistance correlates with their microstructure and phase assemblage. A lower carbonation resistance was observed for calcined clay-containing cement pastes compared to OPC, which can be attributed to their lower contents of CaO reactive and portlandite. Keeping constant the clinker content of the blend, in limestone-calcined clay cements (LC 3 ) pastes, the increase of the amount of metakaolin increased their carbonation resistance. Additionally, the presence of limestone in the ternary blends, which promotes the formation of carboaluminate phases, appears to play a positive role in enhancing the carbonation resistance of the calcined clay-blended cements. After 6 months of exposure of the pastes to 1 wt% CO 2 , C-A-S-H formed in calcined clay blended cements suffered a substantial decomposition, leading to the formation of an aluminosilicate gel and a coarsening of the pore structure. In contrast, no significant decomposition of C-A-S-H was observed in the OPC pastes under the same conditions. Regarding the CaCO 3 polymorphism, calcite was the major polymorphism formed in all cases, although vaterite was also identified in both the binary and ternary calcined clay-blended cements.