Improving early-age performance of alkali-activated slag paste backfill with calcium salts at low temperature

The hydration of alkali-activated slag (AAS) is highly sensitive to the curing temperature, especially at early ages. In low temperature environment, cemented paste backfill made of AAS (AAS-CPB) exhibits notably reduced early-age uniaxial compressive strength (UCS), attributable to the inhibitory effect of low temperature on slag hydration. This study aims to improve the performance of AAS-CPB at low temperatures by incorporating chemical additives. The results shows that calcium salts, specifically CaCl2, CaSO4, and Ca(COOCH3)2, can increase the early-age UCS of AAS-CPB by up to 1050%, depending on the specific anion involved. Overall, CaCl2 provides the greatest enhancement in the UCS. However, the presence of these salts result in strength degradation at later ages. A pronounced exponential relationship is evident between UCS and ultrasonic pulse velocity. The change of conductivity and moisture content are valuable indicators of hydration process at low temperatures. C(N)-A-S-H, hydrotalcite and portlandite are the primary hydration products. Anions play a decisive role in the morphology, precipitation and quantity of C(N)-A-S-H. The slag hydration degree and UCS basically exhibit a consistent trend.