Optimization of oil shale ash-based cement composite incorporating a ternary chemical modifier

The high carbon footprint of Portland cement manufacturing motivates researchers toward finding suitable cement replacement. This study investigates the development and optimization of a high-volume oil shale ash (OSA) cement composite using ternary chemical additives composed of sodium sulfate (2-8%), cysteine methyl ester (0.2-1.0%), and silica gel powder (1-5%). The ordinary Portland cement (OPC) was replaced by a 50% OSA. The clinker demand was reduced, achieving compressive strength up to 33.66 MPa, meeting structural requirements. An optimization statistical model was used to determine the optimal formulation, which was then evaluated through fresh, mechanical, water transportation, and durability tests. Among the 15 experimental mixtures, 28-day compressive strengths ranged from about 18-26 MPa, and 90-day strengths ranged from 21 to 34 MPa, showing a steady increase in strength due to improved pozzolanic activity, with the 90-day compressive strength exceeding 33.5 MPa. Physicochemical characterization of the paste was performed to gain insight into the hydration mechanism. The performance enhancement observed in the optimization mix is attributed to a synergistic chemical-activation mechanism in which sulfate-induced ettringite formation, sodium-driven dissolution of reactive OSA phases, and chelation-stabilized crystallization, supplemented by silica-gel-based pozzolanic refinement, collectively densify the microstructure and promote sustained strength development.