Synergistic improvement of sulfate durability in hybrid geopolymer mortars incorporating blast furnace slag, waste andesite powder, and Portland cement

This experimental study involved the production of a total of seven mortar mixtures, including two geopolymer mixtures—one composed solely of blast furnace slag (SL) and the other solely of waste andesite powder (WAP)—two hybrid mortar mixtures combining SL with Portland cement (PC) and WAP with PC, and three ternary mixtures incorporating SL, WAP, and PC. To minimize energy consumption, thermal curing was not applied to the mixtures. Flexural and compressive strength tests were conducted on specimens at 7, 28, and 90 days. The mortars were exposed separately to 5 % Na₂SO₄ and 5 % MgSO₄ solutions for periods of 90 and 180 days. The relative strengths were assessed based on the initial strengths of the mortars. Among the single-component mixtures, the SL-based geopolymers exhibited a compressive strength of approximately 23.2 MPa at 28 days, while the WAP-based geopolymers achieved a strength of only 13.2 MPa. In the T3 mixture, which utilized equal proportions of SL, WAP, and PC, a compressive strength of approximately 34 MPa was attained at 28 days. While the sulfate resistance of the WAP-based geopolymer mortars was found to be poor, excellent sulfate resistance was demonstrated in all hybrid mixtures. X-ray diffraction (XRD) analyses indicated the formation of additional crystal phases following sulfate exposure. Scanning electron microscopy (SEM) observations revealed that sulfate exposure deteriorated the matrix of the WAP-based geopolymers, whereas the internal structures of the SL-based geopolymers and hybrid geopolymers exhibited no adverse effects. Furthermore, the demonstrated sulfate resistance of the hybrid mixtures indicates their viability for use in environments prone to sulfate attacks. The synergistic effects of combining materials in appropriate ratios effectively mitigate some of the drawbacks associated with their individual use, leading to improved performance and enhanced material properties. SL + WAP + PC ternary hybrid geopolymer mortars prove to be an effective strategy that improves environmental sustainability and economic efficiency by significantly reducing CO2 emissions, energy consumption and costs.