Effect of Alkali Cation on Performance of Alkali-Activated Slag Mortar in Cold Environments

Alkali-activated slag (AAS) is a promising cementing material for winter construction due to its continuous hydration at sub-zero temperature. In order to obtain a higher mechanical strength of AAS mortar in winter construction, the most efficient alkali activator should be selected. Potassium silicate and sodium silicate as alkaline activators of AAS mortar possess a high strength and hydration rate at ordinary temperature. Potassium silicate and sodium silicate as alkali activators showed different properties because of different alkali cations, so the effect of alkali cation on the performance of AAS mortar was studied at sub-zero temperature. The mechanical properties of potassium-silicate-activated AAS (K-AAS) mortar and sodium-silicate-activated AAS (N-AAS) mortar were thoroughly compared at the ambient temperatures of −10 °C, 0 °C, and 20 °C. The compressive and flexural strength of K-AAS mortar was increased by 130.4% and 72.3% at the age of 1 day and increased by 49% and 33.7% at the age of 28 days at the ambient temperature of −10 °C compared with N-AAS mortar. In order to reveal the mechanism behind the influence of different alkali cations on the mechanical properties of AAS mortar, the hydration heat, hydration products, and pore structures of AAS were characterized. The hydration heat exhibited that the rate of heat release and the cumulative heat release of K-AAS are higher than N-AAS, indicating the higher reaction degree of K-AAS. DTG/TG and ATR-FTIR spectra showed that K-AAS generated more gel product C(-A)-S-H compared with N-AAS at the same curing temperature. The MIP results exhibited that the porosity of K-AAS was lower than N-AAS. Finally, the mechanism explanation of the effect of alkali cation on the performance of AAS mortar was proposed. To sum up, potassium silicate should be selected as an alkali activator of AAS in winter construction.