Precursor-Dependent Performance of Low-Silicate-Modulus Industrial Byproduct-Based Geopolymers

This study examined 16 alkaline-activated paste formulations using granulated blast-furnace slag (GBFS) and fly ash from Çatalağzı (CAT), Kemerköy (KEM), and Kangal (KAN). Four nominal silicate moduli (0.2, 0.4, 0.6, and 0.8) were tested, with the water content of each precursor being adjusted using a Vicat-based consistency criterion. All specimens were heat-treated at 60 °C for 24 hours. The density, ultrasonic pulse velocity, and mechanical performance of the specimens were assessed in relation to the composition, fineness, and morphology of the precursors. The water-to-binder ratios ranged from 0.251–0.259 for CAT to 0.688–0.699 for KAN. GBFS achieved peak compressive and flexural strengths of 82.25 and 4.91 MPa, respectively, at a nominal modulus of 0.6 with a maximum ultrasonic pulse velocity of 3.65 km/s. CAT performed best at a modulus of 0.6, while KEM achieved its highest mechanical performance at a modulus of 0.4. KAN exhibited a loss-on-ignition of 9.60%, high water demand, bleeding, declining density, and low strength. Its density decreased by 8.45% between days 1 and 28, but this does not directly correlate with porosity. Differences in water content complicate strength comparisons across precursors, while changes in alkaline dosage limit the interpretation of modulus effects. Therefore, the best-performing formulations depend on the consistency and curing conditions used. These results highlight the importance of considering activator composition, precursor properties, and water demand together.