Experimental evidence for the acceleration of slag hydration in blended cements by the addition of CaCl2

Abstract Cements with high substitution rates of ground granulated blast furnace slags (GGBS) have the potential to significantly lower CO2 emissions of concrete, but their early age strength is often below those of traditional OPC cements. One way of mitigating this drawback is to use accelerating admixtures. In this study, the effect of CaCl2 additions on the hydration of blended cements was investigated by measuring compressive strength, porosity, heat release and propagation of ultrasound in blends containing 70 wt% of GGBS. The onset of formation of aluminate phases was monitored using in-situ XRD. The effect of CaCl2 on slag hydration was isolated by replacing GGBS by an inert quartz filler. Results showed that compressive strength values at one, two and seven days were increased by 50% by the CaCl2 addition. The increases in compressive strength corresponded to a reduction in pore space. GGBS hydration contributed to the heat development, structuration and compressive strength of the blended cements from 15 h. The addition of CaCl2 led to an earlier onset of the GGBS reaction, at around 10 h, and increased the rate of GGBS hydration during the first seven days. The time of onset of the GGBS contribution was also the moment when AFm precipitation started. In CaCl2-containing blends, Cl was incorporated in AFm.