Eco-efficient symbio-pozzolanic hydrophobic cementitious binders for sustainable and durable infrastructure.

This study presents the development of an internally hydrophobic cementitious binder through the incorporation of a Symbio-Pozzolanic Composite Hydrophobic Powder (SPCHP), synthesized from fly ash, silica fume, metakaolin, and zinc stearate. SPCHP was introduced as a partial cement replacement at levels ranging from 5% to 40%, and its effects were systematically evaluated in terms of fresh properties, mechanical performance, durability, microstructural behaviour, and cost efficiency. The mix containing 25 wt% SPCHP (HP25) exhibited the most balanced performance, achieving 73% of the compressive strength of the control mortar while imparting strong hydrophobicity (water contact angle of 114°) and reducing water absorption to 4.2%. Enhanced resistance against chloride ingress, sulphate and acid attack, along with improved ultrasonic pulse velocity, confirmed the role of SPCHP in densifying the microstructure and reducing transport properties. Life-cycle benefits were demonstrated through lower CO₂ emissions and favourable cost-to-strength ratios. These findings establish SPCHP as a sustainable material innovation for prolonging the service life of cement-based structures in aggressive environments, with potential for scalable engineering applications.