The early mechanical strength of high blending fly ash (FA) concrete in the hardening process is extremely low and greatly affected by curing temperatures, which seriously affects the construction speed and durability. To solve this problem, the influence of limestone powder (LS) as an activator on the enhancement of early mechanical strength and durability of high blending FA mortars cured under 5, 20, and 50 °C was investigated. The results reveal that LS can increase the early compressive strength of mortars with 30% FA cured under 5, 20, and 50 °C at 3 d by up to 21.0%, 18.2%, and 21.2%, respectively. Meanwhile, FA mortar with LS cured under 20 °C can achieve a maximum resistance to freeze–thaw cycles and chloride ion erosion, followed by under 50 °C and 5 °C. Under high curing temperatures of 20 °C and 50 °C, the nucleation effect of LS is more conducive to the cement hydration in FA mortar to produce more hydration products and the microchemical activity of LS can facilitate CaCO₃ from LS to react with calcium aluminate in cement to form calcium carboaluminate (Mc). However, a low curing temperature of 5 °C will significantly reduce the nucleation effect and microchemical activity of LS, causing LS to act more as a filler in FA mortar. LS as an activator can promote more hydration products in FA mortars and make the inner structure of FA mortar denser. LS can greatly reduce the total porosity of FA mortar cured under 20 °C and 50 °C.

Fly ash (FA) and limestone powder (LS) are both environmentally friendly substitutes for cement. The mechanical strength and durability of hydraulic concrete with FA and LS cured under low temperature deserve attention. In this study, the mechanical strength and durability properties of cement mortar based on FA and LS cured under a low temperature of 5 °C were investigated. The compressive strength and resistance to freeze–thaw cycles and chloride ions of mortar with FA and LS were evaluated. The results reveal that adding 5% LS can increase the compressive strength of mortar containing 30% FA by 14.31% at 3 d and 1.63% at 180 d. The addition of 5% LS can result in a 0.58% reduction in the mass loss rate and an increase of 2.98% in RDM compared to the mortar with 30% FA after 100 freeze–thaw cycles. Meanwhile, the addition of 5% LS results in a 5.68% reduction in the chloride diffusion coefficient compared to the mortar with 30% FA. However, for the mortar containing 60% FA, any proportion of LS cannot increase the compressive strength of FA mortar from 3 d to 180 d and will decrease the resistance to freeze–thaw cycles and chloride ions. For the mortar containing 30% FA, the addition of 5% LS can promote the hydration product of C-S-H and obtain the minimum total amount of large capillary pores and air pores. Adding 10% LS can result in the mortar with 30% FA obtaining the lowest unit CO₂ emission, and adding 5% LS can result in the mortar with 60% FA obtaining the lowest unit CO₂ emission.