Precursor-Dependent Performance of FA-, GBFS-, MK- and WBP-Based Geopolymer Mortars: Effects of NaOH Molarity and Thermal Curing on Strength, Transport Properties and Cost Efficiency

This study investigated the effects of precursor type, NaOH molarity, and thermal curing temperature on the performance of geopolymer mortars produced using fly ash (FA), ground granulated blast-furnace slag (GBFS), metakaolin (MK), and waste brick powder (WBP). Mortars were activated using 12 M and 16 M NaOH solutions at a constant Na2SiO3/NaOH ratio and thermally cured at 60 and 90 °C for 24 h. Physical, mechanical, transport, microstructural, and cost-performance properties were evaluated. The results demonstrated that the optimum activation conditions strongly depended on precursor type. MK-based mortars cured at 16 M–90 °C exhibited the best overall performance, achieving the lowest apparent porosity (6.1%) and water absorption (5.4%), and the highest oven-dry density (2194 kg/m3), compressive strength (25.8 MPa), and flexural strength (3.43 MPa). These mortars also exhibited the lowest capillary water absorption (1.88 kg/m2), the highest electrical resistivity (248.00 kΩ·cm), and the lowest charge passed (177 C), indicating enhanced pore refinement and chloride-ion penetrability. In contrast, GBFS performed better under milder activation conditions, whereas WBP showed lower performance due to its coarser, more crystalline structure. SEM/EDS analyses confirmed that the formation of dense aluminosilicate gel governed matrix quality and overall performance. Overall, MK activated at 16 M and cured at 90 °C provided the most favorable balance between technical performance and cost efficiency.