Development of eco-hydrophobic composite concrete: a multi-scale investigation of strength, durability, and sustainability metrics

Hydrophobic concretes using silane, siloxane, or fatty acid-based admixtures provide water repellency but often impair hydration efficiency and mechanical strength. Conversely, pozzolanic blends with SCMs enhance microstructural densification yet remain hydrophilic, creating a critical gap for multifunctional admixtures that deliver both hydrophobicity and pozzolanic reactivity. This study investigates M40 grade concrete incorporating Symbio-Pozzolanic Composite Hydrophobic Powder (SPCHP), synthesized from fly ash, silica fume, metakaolin, and zinc stearate, as a fine aggregate replacement (10–30%). The optimum mix (SPCHP20) achieved a 28-day compressive strength of 45.2 MPa, only 2.1% below control, while reducing water absorption and sorptivity by 75% and chloride permeability by 56.6%. Acid and sulfate resistance improved significantly. Microstructural analysis confirmed dense C–S–H gels (Ca/Si = 1.71) with a water contact angle of 116.3°. Life-cycle assessment showed 18.5% lower global warming and 21.3% lower acidification potential, validating SPCHP’s dual hydrophobic–pozzolanic functionality.