Effect of shrinkage-reducing admixtures on drying shrinkage, strength and sorptivity of ambient-cured geopolymer mortars

Geopolymer mortars are prone to high drying shrinkage under ambient curing conditions, which affects the early-age dimensional stability and potential long-term durability. Therefore, effective control of shrinkage is essential for widespread construction applications of ambient-cured geopolymer mortars. This study investigated the influence of commercially available shrinkage-reducing admixtures (SRAs) on the drying shrinkage, workability, strength and microstructure of ambient-cured geopolymer mortars. Fly ash (FA) and ground granulated blast furnace slag (GGBFS) were utilised as precursors, where glycol ether-based SRA (GSRA) at concentrations of 1%, 2.5% and 3.5% and polyether-based SRA (PSRA) at concentrations of 0.3%, 0.5% and 0.8% by weight of the binder were used to reduce drying shrinkage. The incorporation of SRA showed maximum reductions in drying shrinkage of 15.5% and 9.5% by 3.5% GSRA and 0.8% PSRA, respectively. The 90-day compressive strength of mortar increased by 9.6% and 3.4% for 3.5% GSRA and 0.8% PSRA, respectively. Furthermore, the initial water absorption rate declined with the increase of SRA dosage. Microstructural analyses revealed that both admixtures facilitated the development of calcium-aluminosilicate-hydrate (C-A-S-H) and sodium-aluminosilicate-hydrate (N-A-S-H) gels, characterised by the Si/Al and Ca/Al molar ratios of SRA-modified mixtures, indicating the formation of a more compact and highly polymerised gel network.