Effect of pre-hydration age on phase assemblage, microstructure and compressive strength of CO2 cured cement mortar

•CO2 curing accelerates the reaction of clinker phases, especially at early age.•Pre-hydration promotes the formation of CaCO3 in decomposition mode I.•Calcite is the main crystalline phase, and no aragonite is found.•Pre-hydration refines the pore structure and gets lower pore connectivity. This study investigated the effect of pre-hydration age on phase assemblages, microstructure, and strength development of CO2 cured cement mortars. The results indicated that CO2 curing accelerated the reaction of clinker phases, especially at early age. In addition, CO2 curing promoted the formation of CaCO3, especially the CaCO3 in decomposition mode II. The content in decomposition mode I and the total content of CaCO3 increased with pre-hydration duration. Calcite was the main crystalline phase, and no aragonite was found. Compared with the reference, the pore structure parameters and pore connectivity of cement mortars with pre-hydration were improved, especially the sample with pre-hydration for 6 h; and the compressive strength was obviously increased, especially at early age. However, the 28 d compressive strength did not increase continuously with the pre-hydration duration.