Effect of Sodium Hydroxide Molarity and Nano-Alumina on Mechanical and Durability Properties of Alkali-Activated Mortar

This study evaluates the performance of slag-based alkali-activated mortar (AAM) under ambient curing, focusing on the effects of 2% nano-alumina, sodium hydroxide (NaOH) molarity (12, 14, and 16 M), and sodium silicate-to-sodium hydroxide (S.S/S.H) ratios (1.5 and 2.5). Fresh and mechanical properties, including flowability, compressive strength, and flexural strength, were investigated. Short-term chemical-exposure resistance was examined by measuring water absorption and sorptivity, and by assessing the compressive and flexural strengths of AAM specimens cured for 28 days and subsequently exposed for 28 days (56 days total specimen age) to 5% sulfuric acid, 5% magnesium sulfate, and 3.5% sodium chloride solutions. General linear model analysis of variance (GLM-ANOVA; α = 0.05) was applied to the strength data, with NaOH molarity, S.S/S.H ratio, and nano-alumina content as factors, and exposure condition included as a fourth factor for post-exposure compressive strength. The results indicate that flowability varied markedly with NaOH molarity, S.S/S.H ratio, and nano-alumina addition, while strength depended mainly on interactions among these factors (GLM-ANOVA). For fresh-state and unexposed mechanical performance, the mix with 12 M NaOH, S.S/S.H = 2.5, and nano-alumina addition achieved the best balance of workability and compressive strength. Flexural strength increased with NaOH molarity up to an optimum at 12 M, beyond which a decline was observed at 14–16 M, while increasing consistently with curing time up to 56 days. GLM-ANOVA showed significant NaOH × nano-alumina interactions for compressive strength at 28 and 56 days (p = 0.002 and 0.030, respectively), significant three-factor interactions for flexural strength (p = 0.004 and 0.022, respectively), and exposure condition as the largest contributor (32.2%) to post-exposure compressive strength. For short-term chemical-exposure resistance, mixes with 2% nano-alumina at 12–16 M and an alkali ratio of 1.5 retained superior residual compressive and flexural strengths after 28 days of exposure to sulfuric acid, magnesium sulfate, and sodium chloride, compared with their counterparts having a higher S.S/S.H ratio. Overall, no single mixture provided the best balance across all the properties investigated.