Impact of ultrafine fly ash on mechanical properties of recycled concrete aggregate: A factorial design study

The increasing generation of construction and demolition waste has led to resource depletion and waste accumulation. This research evaluates the mechanical properties of concrete formulated with recycled concrete aggregates (RCA) and ultrafine fly ash (UFFA) as partial substitutes for natural aggregates and Portland cement. Replacement levels ranged from 0% to 100% for RCA and 0% to 15% for UFFA. A 4² factorial design and statistical tools including ANOVA and regression were employed to analyze compressive, flexural, and tensile strengths alongside workability. While RCA incorporation generally reduced strength due to its higher porosity, the inclusion of UFFA improved all measured properties. The mix with 15% UFFA and 66% RCA showed optimal split tensile strength (4.75 MPa), while maximum compressive strength (49 MPa) occurred with 15% UFFA and 0% RCA. The blending of 7.5% UFFA and 50% RCA achieved a balance of sustainability and strength. The statistical results identified UFFA as the dominant factor influencing mechanical performance and workability, underscoring its efficacy in developing environmentally responsible structural concrete.