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.

Automated summary

This study investigated the impact of curing conditions on the freeze-thaw resistance of geopolymer mortars. The results showed that steam curing led to better early-age strength and improved freeze-thaw resistance. The water-to-ash ratio was found to play a critical role in the freeze-thaw resistance under different curing conditions.