Effect of calcium oxide on mechanical properties and microstructure of alkali-activated slag composites at sub-zero temperature

Alkali-activated slag (AAS) has the potential to be applied in winter construction due to its continuous hydration at subzero temperature. However, to further enhance the mechanical properties of AAS as structure repair material in cold regions, calcium oxide (CaO) at proportions of 1 wt%, 2 wt%, 3 wt% and 4 wt% were incorporated as partial replacement of ground granulated blast furnace slag (GGBS) in AAS. An appropriate CaO content of 3 wt% was determined based on the setting time and mechanical testing, and the results showed that it increased the compressive and flexural strength of AAS by 40% and 48% at the age of 1 day and by 5% and 7% at the age of 28 days at a temperature of −10 °C. Moreover, semiadiabatic calorimetry, attenuated total reflection Fourier transform infrared spectroscopy, mercury intrusion porosity and scanning electron microscope measurements were carried out on typical samples. Experimental results showed that the hydration heat was enhanced and the generation ratio of C(-A)-S-H was increased by incorporation of CaO into AAS. Furthermore, the pore structure was refined and porosity was decreased for AAS with CaO incorporated. Finally, the proposed mechanism explanation of CaO to enhance the strength of AAS was determined by comparing the ATR-FTIR test results of 3 wt% CaO modified AAS sample and 3 wt% Ca(OH)₂ modified AAS sample cured at −10 °C for 1 day.