Effect of curing conditions on freeze-thaw resistance of geopolymer mortars containing various calcium resources

This study focused on the effects of curing conditions (standard and steam curing) on the freeze–thaw resistance of geopolymer mortars containing Class C fly ash, Class F fly ash, slag, and Ca(OH)2. Geopolymer mortar specimens were prepared with water-to-Class C fly ash ratios of 0.35 and 0.40, and CaO contents of 45% and 100% with different compositions. The specimens were subjected to freeze–thaw cycles until the mass loss exceeded 5% or the relative dynamic elastic modulus was below 60%. The reaction hydrates and pore structures were analyzed using scanning electron microscopy with energy-dispersive spectrometry and mercury intrusion porosimetry, respectively. In terms of mechanical performance, steam curing condition was better than standard curing condition for the early-age strength of geopolymer mortars. The compressive strength of the geopolymer mortars at 2 d and 28 d was increased 2.2–5.1 times and 0%-40%, respectively, under steam curing condition compared with standard curing condition. The experimental results indicated that Class C fly ash geopolymer mortar (CF35) under steam curing condition and Class F fly ash-slag geopolymer mortar with 100% CaO content (SF35C100) under standard curing condition exhibited remarkable freeze–thaw resistance with a freeze–thaw resistance grade of F300 at the lowest. Moreover, the water-to-ash ratio was critical for the freeze–thaw resistance under both standard and steam-curing conditions. The mass loss of specimens with water-to-ash ratios of 0.35 and 0.40 under standard curing condition were 4.67% and 8.79%, respectively, at 25 freeze–thaw cycles, whereas that of specimens with water-to-ash of 0.35 under steam curing condition was 3.03% at 300 freeze–thaw cycles. In addition, the freeze–thaw resistance of geopolymer specimens with different compositions had a direct relationship with the microstructure of the geopolymer mortars. Furthermore, the experimental results verified that a higher compressive strength did not necessarily indicate a better freeze–thaw resistance.