Performance of Pumice–Cement Kiln Dust Geopolymer Mortars: Influence of NaOH Molarity, Silicate Ratio, and Curing Regimes

The increasing need to reduce the environmental impact of ordinary Portland cement and to valorize natural and industrial by-products has encouraged the development of sustainable binder systems based on alkali-activated materials. This study investigated the performance of geopolymer mortars prepared with 95% pumice dust and 5% cement kiln dust (CKD) as precursor materials. The effects of the activator ratio (1.5, 2.0, and 2.5), NaOH molarity (10M and 12M NaOH), and curing regimes (CR 1: 48 hours at 80°C, CR 2: 72 hours at 80°C, and CR 3: 48 hours at 80°C followed by 24 hours at 160°C) were examined. Mechanical and durability-related properties, including compressive strength (CS), density, water absorption, and efflorescence, were evaluated at 28 days, in addition to flowability, which was assessed immediately after mixing. The results indicated that both increasing the molarity and increasing the activator ratio decreased flowability, and, in general, flowability at 12M was lower than at 10M. CS increased as the molarity increased. At 10M, the best performance was at an activator ratio of 2 under all curing regimes, while at 12M, the best performance was at an activator ratio of 2.5 under CR 1, and the best performance for the same molarity under CR 2 and CR 3 was at an activator ratio of 1.5. ANOVA analysis confirmed that molarity was the dominant factor influencing strength (p < 0.001) as the main effect, while curing regime and activator ratio had a significant impact primarily through their interactions. Density increased as the activator ratio, molarity, and curing duration increased; however, higher density did not necessarily lead to higher strength. Increasing the activator ratio, molarity, and curing duration reduced water absorption. Efflorescence was observed only under CR 1 and decreased as the activator ratio increased.