The Influence of CO2 Curing on the Mechanical Performance and the Corresponding Chloride Ion Resistance of Alkali-Activated Compound Mineral Admixtures

In this paper, the mechanical properties (the flexural strength, compressive strength and the drying shrinkage rate) of CO2-cured alkali-activated compound mineral admixtures (blast furnace slag powder (BFS) and fly ash (FA)) are investigated. In addition, the corresponding chloride ion mobility coefficient is measured. Additionally, the freeze–thaw cycles with an NaCl concentration of 3% is studied. Thermogravimetric analysis and scanning electron microscopy are applied in analyzing the mechanical properties. The curing ages of the alkali-activated compound mineral admixtures are 1 day, 3 days and 28 days. Results show that the mechanical strengths are decreased by the addition of FA and increased by the increasing curing age and CO2 curing. The maximum reducing rates of flexural and compressive strengths by FA are 47.6% and 42.3%. Meanwhile, the corresponding increasing rates by CO2 curing are 26.5% and 23.1%, respectively. The improving effect of alkali-activated BFS by CO2 curing is higher than that of FA. Furthermore, the drying shrinkage rate is increased by the increasing dosages of BFS, the increasing curing ages and CO2 curing. Additionally, CO2 curing and the increasing dosage of BFS leads to decreasing the chloride ion mobility coefficient. Finally, CO2 curing and the addition of BFS can effectively improve the resistance of NaCl freeze–thaw cycles. The compactness of the hydration products is improved by the addition of BFS and the roughness of hydration products is increased by CO2 curing.