Effect of curing regimes on fly ash geopolymer mortars: a statistical and regression analysis toward sustainable development

The construction industry’s heavy reliance on ordinary Portland cement (OPC) is associated with substantial energy consumption, CO2 emissions, and potable water demand, impeding progress toward UN Sustainable Development Goals (SDGs) 9, 11, 12, and 13. Class F fly ash-based geopolymers present a promising alternative; however, their performance is critically sensitive to curing conditions. This study systematically examines the influence of oven curing and steam curing on the mechanical and transport properties of Class F fly ash-based geopolymer mortar, benchmarked against a companion OPC mortar of identical binder-to-sand ratio. Geopolymer mixtures with a 1:1 binder-to-sand ratio and a sodium hydroxide–sodium silicate activator system were prepared. One batch was subjected to oven curing at 60 °C for 24 h, another to steam curing in a self-fabricated arrangement at the same nominal temperature, while the OPC mortar was water-cured under standard conditions. Compressive strength, bulk density, water absorption, and failure patterns were evaluated at 7 and 28 d. One-way and two-way ANOVA with Tukey’s Honestly Significant Difference (HSD) post-hoc tests, together with multivariate linear and nonlinear regression modelling, were employed to explore treatment effects and empirical property relationships. The oven-cured geopolymer (GP-OV) developed the highest early-age strength (30.52 MPa at 7 days), exceeding OPC by 20.4% and steam-cured geopolymer (GP-ST) by 22.9%. At 28 d, the difference between GP-OV (32.41 MPa) and OPC (33.25 MPa) was not statistically significant (Tukey HSD p = 0.081); however, this non-significance must not be interpreted as statistical equivalence, particularly given the small number of specimens and the large observed effect size (Cohen’s d = − 1.98). GP-ST (28.03 MPa) was consistently inferior ( p < 0.001). Two-way ANOVA indicated large main effects of curing regime (η² = 0.412) and age (η² = 0.422) on compressive strength, with a significant interaction (η² = 0.147, p < 0.001). Multivariate regression yielded an empirical correlation for compressive strength (fc = 52.12 − 0.0076ρ − 1.024Wab + 0.203t, R² = 0.920, adjusted R² = 0.903), which, given the small within-study dataset, is interpreted as an empirical correlation rather than a validated predictive model. At 28 d, a strong inverse linear correlation was observed between strength and water absorption (fc = 39.46 − 0.737Wab, R² = 0.970, adjusted R² = 0.966); because this relationship is based on only three independent material groups, it is presented as an empirical correlation within the investigated systems and not as a general predictive or non-destructive quality-control tool. Bulk density followed the order OPC > GP-OV > GP-ST, while water absorption exhibited the inverse trend, with GP-ST showing a marked 90.9% higher absorption than OPC at 28 d. Failure mode analysis revealed brittle, sudden fracture in OPC cubes versus gradual, edge-spalling failure in geopolymer specimens, a qualitatively different failure morphology; no quantitative fracture or toughness measurements were performed, so no inference on fracture energy or damage tolerance is drawn. The strength–density–absorption trends collectively indicate that, for the investigated mix design, oven curing at 60 °C provides a more favourable balance between mechanical performance and transport resistance than steam curing. Because a single fly ash source and one activator composition were investigated, the findings are indicative rather than universal. The results support the technical feasibility of oven-cured Class F fly ash geopolymer mortars as potentially lower-carbon alternatives for precast and controlled-curing applications - an environmental benefit that is anticipated on the basis of published life-cycle data rather than demonstrated by a life-cycle assessment in this study - thereby prospectively advancing SDG 9 (Industry, Innovation and Infrastructure), SDG 11 (Sustainable Cities and Communities), SDG 12 (Responsible Consumption and Production), and SDG 13 (Climate Action).