Assessing Ternary SCM Concrete for Civil Engineering Applications: Performance, Cost Effectiveness, and Carbon Mitigation

This study investigated the performance of concrete incorporating fly ash (FA) and bentonite (B) as partial replacements for ordinary Portland cement (OPC). Ten mixtures were evaluated: one OPC control, five binary mixtures containing 10–30% FA, and four ternary mixtures with a fixed total replacement of 30%, comprising FA/B replacement percentages of 20/10, 15/15, 10/20, and 5/25 by total binder mass. The total binder content and nominal water-to-binder ratio were maintained at 430 kg/m3 and approximately 0.55, respectively. Fresh-state and binder-paste properties, compressive and splitting tensile strengths, hardened density, water absorption, hydrochloric-acid resistance, sorptivity, and ultrasonic pulse velocity were assessed. FA20B10 provided a favorable balance of mechanical and durability-related performance among the tested ternary mixtures, whereas FA30 achieved the highest overall 90-day compressive strength of 33.1 MPa. Water absorption decreased from approximately 7.6% for the control to 5.1% for FA20B10. The ternary mixtures also exhibited lower sorptivity and smaller strength losses following hydrochloric-acid exposure than the control. A provisional partial material-cost comparison was restricted to the control and binary mixtures and did not establish the cost-effectiveness of the ternary formulations. Environmental reporting was limited to a mass-based, OPC-only screening estimate using a fixed emission factor. Emissions associated with FA, B, other constituents, and the remaining life-cycle processes were excluded; therefore, the calculation does not establish the total concrete carbon footprint or a life-cycle CO2 reduction. The findings support further development of fly-ash–bentonite ternary concrete, subject to source-specific characterization and field-scale validation.