Durability Performance of Alkali-Activated Natural Pozzolan and Limestone Powder Mortar in Sulfate Environments

The pressing need for sustainable construction materials has identified alkali-activated materials (AAMs) as eco-friendly alternatives to conventional Portland cement. This study explores the synergistic performance of alkaline-activated natural pozzolan and limestone powder (AANL) blends against sulfate attack, evaluating mortar specimens immersed in sodium sulfate, magnesium sulfate, and a combined sulfate solution over 12 months. The samples were synthesized using natural pozzolan (NP) and limestone powder (LSP) in three distinct binder combinations to evaluate the influence of varying precursor ratios on the material’s performance, as follows: NP: LSP = 40:60 (AN40L60), 50:50 (AN50L50), and 60:40 (AN60L40). At the same time, the alkaline activators of 10 M NaOH(aq) and Na₂SiO3(aq) were combined in a ratio of 1:1 and cured at 75 °C. The research examines the weight variations of the samples, their residual compressive strength, and microstructural characteristics under exposure to magnesium sulfate, sodium sulfate, and a combined sulfate solution. In terms of weight change, samples exposed to Na2SO4 gained weight slightly, with AN40L60 recording the highest gain (3.2%) due to the ingress of sulfate ions and pore filling. Under MgSO4, AN60L40 had the lowest weight gain (29%), while AN40L60 reached 54%. In mixed sulfate, AN60L40 showed negligible weight gain (0.11%); whereas, AN50L50 and AN40L60 gained 2.43% and 1.81%, respectively. Compressive strength retention after one year indicated that mixes with higher NP content fared better. AN60L40 exhibited the highest residual strength across all solutions—16.12 MPa in Na2SO4, 12.5 MPa in MgSO4, and 19.45 MPa in the mixed solution. Conversely, AN40L60 showed the highest strength degradation, losing 47.22%, 58.11%, and 55.89%, respectively. SEM-EDS and FTIR analyses confirm that LSP’s vulnerability to sulfate attack diminishes with increased NP incorporation, highlighting a synergistic interaction that mitigates degradation and retains structural integrity. The combination of 60% NP and 40% LSP demonstrated superior resistance to all sulfate environments, as evidenced by visual durability, minimized weight gain, and retained compressive strength. This study highlights the potential of tailored NP-LSP combinations in developing durable and sustainable AAMs, paving the way for innovative solutions in sulfate-prone environments, while reducing environmental impact and promoting economic efficiency.