Mechanistic and RSM-based evaluation of concrete durability under organic and inorganic impurities in mixing water

The increasing use of non-potable water for concrete mixing requires understanding how dissolved constituents influence durability and performance. This study evaluates biodegradable and non-biodegradable organics, and carbonate, bicarbonate, sulphate, and chloride ions in relation to international codal limits. Compressive and flexural strength, porosity, water absorption, pore solution pH, SEM/EDX microstructure, and RSM modelling were examined up to 365 days. The control mix reached 60.2 MPa with stable alkalinity (pH 12.7). Biodegradable organics beyond 75 mg/L reduced strength by up to 45%, whereas non-biodegradable organics up to 200 mg/L caused < 5% reduction; balanced combinations produced the highest strengths. Inorganic ions showed threshold deterioration: carbonates/bicarbonates near 1000 mg/L maintained 51–57 MPa but increased porosity to 11–12%, while sulphates ≥ 500 mg/L and chlorides ≥ 2000 mg/L caused strength losses up to 18 MPa, porosity up to 18%, and pH approaching 10. Strength reduction correlated inversely with pH and directly with porosity, supported by SEM evidence of ettringite formation and gel decalcification confirmed by EDX. RSM and pH–porosity mapping demonstrated that deterioration zones coincide with codal limits, establishing a mechanistic link between impurity type, pore chemistry, pore structure, and durability of concrete made with alternative water sources.