Understanding the strength evolution of alkali-activated slag pastes cured at subzero temperature

As a kind of low-carbon cementitious materials, alkali-activated slag (AAS) can make contributions to the construction in cold areas because of its low freezing point of alkaline activator solution. This paper investigates the evolution mechanism of the strength and microstructure of AAS binders cured at-10 degrees C. Subzero-cured AAS pastes activated with different molarities of NaOH solution were made comparisons with the specimens cured at 20 degrees C in terms of various properties including setting time, compressive strength, pore structure, and micro-structure. The results indicated that subzero temperature delayed the hydration process of AAS pastes, leading to prolonged setting times. The negative effects of subzero temperature were highlighted via the significantly decreased compressive strength, deteriorated pore structure, and porous microstructure. The phenomenon that subzero-cured AAS pastes exhibited lower compressive strength than ambient-cured specimens was attributed to the reduced reaction rate caused by subzero temperature, which influenced the formation position and compactness of hydration products. Interestingly, the compressive strength of subzero-cured AAS pastes still increased with the increase of NaOH molarity, which is believed to be associated with the fact that the increase in NaOH molarity caused the increase of hydration degree.