Mechanistic influence of water chemistry on cement hydration through acidity–alkalinity interactions governing strength and durability

The chemical composition of mixing water plays a critical yet often overlooked role in governing cement hydration, microstructure formation, and long-term durability. This study systematically investigates the influence of water chemistry covering acidic (S2), bicarbonate (S3), hydroxide (S4), and saline (S5) waters against potable control (S1) on the performance of ordinary Portland cement-based mortar and concrete. Experimental evaluation included fresh properties, mechanical strength, pore solution pH evolution, transport properties and microstructural characterization using SEM–EDS. Results revealed that water chemistry significantly altered hydration kinetics and matrix densification. Acidic water (S2) delayed setting and caused strength reductions exceeding 30% relative to control, whereas hydroxide-rich water (S4) accelerated hydration, yielding the highest 90-day compressive (47 MPa) and flexural (6 MPa) strengths. Pore solution analysis confirmed stable alkalinity for S4 and S1 (pH > 12.3 at 365 days), moderate decline for S3 (12.5 → 12.0), and severe depletion for S2 and S5 (11.9 → 11.1 and 12.1 → 11.4). Correspondingly, carbonation depth increased from 5 mm (S1) to 12 mm (S2), while S4 remained lowest (< 3 mm). Hydroxide alkalinity produced the densest microstructure (porosity < 7%, absorption 2.3%), whereas acidic and saline systems exceeded 12% porosity and 5% absorption. Chloride profiling further confirmed that S4 limited chloride ingress (< 0.1% by cement mass at 30 mm depth) compared with S5, which surpassed corrosion thresholds (> 0.45%). SEM–EDS observations validated these trends: hydroxide-rich mixes exhibited compact C–S–H morphology, while acidic and saline waters induced microcracking, carbonate precipitation, and salt crystallization. Findings confirm that pH alone cannot define water suitability for concrete. Instead, the combined effects of mineral acidity, CO₂ acidity, and hydroxide/bicarbonate alkalinity govern hydration efficiency, pore stability, and long-term durability providing essential criteria for using non-potable or recycled waters in sustainable construction.