Efflorescence and strength effects of different ambient regimes of curing fly ash – based geopolymer concrete

This study investigated the effects of different ambient curing regimes, alkali activator composition, and ground granulated blast – furnace slag (GGBS) incorporation, upon the efflorescence deterioration of ambient – cured fly ash – based geopolymer concrete. Two blends of the binary activator comprising sodium silicate and sodium hydroxide solutions, made at different silicate moduli of SiO₂/Na₂O = 1.31 and 1.43, were used to prepare fly ash and fly ash – GGBS geopolymer concretes. The curing conditions employed comprised air – tight sealing, water – immersion, and open – air exposure of samples. Efflorescence deterioration was evaluated along with mechanical properties consisting of workability and compressive strength. The occurrence of efflorescence was assessed by conducting qualitative surface observations and quantitative mass measurements, along with analytical characterization of both the effloresced geopolymer concrete and the efflorescence salt that formed. Results showed that mixtures cured under sealed conditions consistently achieved higher compressive strengths and exhibited reduced salt crystallization, whereas air curing led to severe efflorescence deterioration. The lower silicate activator and/or higher GGBS content enhanced strength development, reduced efflorescence severity, and improved microstructural densification. Incorporation of 30–40 % GGBS effectively mitigated efflorescence formation under water – curing of geopolymer concretes. • Sealed – curing effectively mitigated efflorescence due to moisture retention. • Water – curing also suppressed surface efflorescence due to leaching of alkalis. • Air – curing led to most severe efflorescence due to a highly porous matrix. • Slag or low silicate activator enhanced strength and reduced efflorescence under all curing regimes.