An efficient approach for mitigation of efflorescence in fly ash-based geopolymer mortars under high-low humidity cycles

Abstract

The efflorescence of fly ash-based geopolymer (FG) is a significant factor of durability, which has been widely concerned in the application of civil engineering. Fly ash based-geopolymer mortars (FGMs) were fabricated by optimizing their water/cement (w/c) ratio from 0.6 to 0.4 as well as the addition of caltite solution (CS) in this paper. The characteristics and mechanism of efflorescence under the high-low humidity cycles were identified. These free cations in FGMs migrated in the direction of surface to form carbonate (Na2Ca(CO3)2·5H2O), resulting in efflorescence. The most effective way for FGMs to mitigate efflorescence from the obtained results was adding 1 wt% CS into the FGMs with the w/c ratio of 0.4. The filter membrane of alkaline stearate was formed by the reaction between CS and alkaline activator, and it adhered to the capillary to suppress migrating and leaching behavior of Na(H2O)n+. Efflorescence fraction was thus decreased from 92.16% to 2.82%, and the mechanical properties were simultaneously decreased by 23.17%. Adding 3 wt% CS into FGMs with the w/c ratio of 0.5 also mitigated efflorescence, the efflorescence fraction was decreased by 18.13%. With the dosage of CS increasing to 4 wt%, the efflorescence of FGMs with the w/c ratio of 0.6 could be accelerated. The results further confirmed that the trace addition of CS could obtain the optimal result for mitigating the efflorescence of the FGMs.