Abstract

The current study explores the effects of alkaline activator composition and curing regimes on the properties of geopolymer mortars synthesized primarily from pumice dust. Three sodium silicate-to-sodium hydroxide (SS/SH) ratios (2.5, 2.0, and 1.5) and two NaOH molarities (10M and 12M) were the variables to design six mixtures. Specimens were treated under three curing regimes: C1 (80 °C for 2 days), C2 (80 °C for 3 days), and C3 (hybrid regime: 80 °C for 2 days followed by 160 °C for 1 day). The six mixtures were evaluated based on flowability, compressive strength, dry density, water absorption, and visual efflorescence. Results revealed that flowability increased with the decrease in the SS/SH ratio and molarity. The increase in the SS/SH ratio increased compressive strength. The highest compressive strength was recorded in curing (C1) at SS/SH of 2.5 and the molarity of 10, indicating that higher molarity does not always lead to higher compressive strength. Prolonged curing duration decreased compressive strength. Hybrid curing (C3) caused 12M mixes to have the highest strength, unlike the curing regime (C1), where 10M mixes were the highest. Moreover, mixes with SS/SH of 1.5 and 2 at 12M achieved their highest strength values in the hybrid regime (C3). The decrease in the SS/SH ratio and molarity decreased the density and increased the absorption. The extended curing regime (C2) or the hybrid regime (C3) increased density and reduced absorption, but did not necessarily increase compressive strength in most cases. The decrease in the SS/SH ratio increased efflorescence, but the extension of curing duration mitigated it. The highest recorded compressive strength of 37.2 MPa was achieved at 10 M and SS/SH = 2.5 under curing 1 (C1), accompanied by a water absorption of 12.7%.

Key words: Pumice dust, geopolymer mortar, sodium silicate/sodium hydroxide ratio, molarity, and curing temperature