Effect of Mixing Water Composition on Cement Mortar Durability in XA3 Environment: Correlation and ANOVA Analysis

The use of alternative water sources in construction, especially in regions with limited freshwater availability, makes the influence of mixing water composition on the durability of cement mortars a critical issue, particularly under aggressive conditions such as ammonium exposure (XA3). A clear difference in material behavior was observed before and after exposure to an aggressive aqueous environment, highlighting the importance of durability assessment under realistic service conditions. Cement mortar specimens prepared with tap water, distilled water, and modified waters containing Cl−, Ca2+, SO42−, and PO43− ions were tested. The experimental program included flexural and compressive strength, water absorption, and residual properties after exposure to an NH4Cl solution. Statistical analysis was performed using one-way ANOVA, correlation analysis, a heatmap, and PCA. Compressive strength varied within a narrow range (33.85–47.24 MPa), while flexural strength showed larger differences (5.21–10.40 MPa). After exposure, residual flexural strength decreased to 1.16–5.87 MPa and compressive strength to 23.92–37.68 MPa. The most severe degradation was observed for sulfate- and chloride-modified waters. Correlation analysis revealed weak dependence between flexural and compressive strength. ANOVA confirmed a significant influence of water composition (p < 0.05), with the strongest effect observed for residual compressive strength (η2 = 0.81). The results demonstrate that mixing water composition is a key factor controlling durability in an XA3 environment. Compressive strength alone is not a reliable durability indicator. Durability is governed primarily by transport properties and microstructure. A multi-parameter approach is required for an accurate durability assessment.