A mix design approach and evaluation of mechanical properties, shrinkage and non-destructive performance of fly ash-sugarcane bagasse ash geopolymer concrete

Environmental pressures associated with cement production and waste disposal are driving the development of geopolymer concrete incorporating fly ash (FA) and sugarcane bagasse ash (SCBA). This study developed a two-stage mix-design framework for FA-SCBA geopolymer concrete. In Stage 1, 60 mixtures were evaluated using SCBA content (0–20%), total aggregate-to-concrete ratio (Tagg/Tcon = 65–75%) and coarse-to-total aggregate ratio (Cagg/Tagg = 65–80%). Reducing Tagg/Tcon from 75% to 65% increased the average 28-day compressive strength from 29.65 to 47.46 MPa. At 10% SCBA, the average compressive strength reached 40.77 MPa, while Cagg/Tagg = 70% yielded 39.63 MPa. For Stage 2, the configuration achieving compressive strength above 30 MPa while minimising binder content was selected, with Tagg/Tcon and Cagg/Tagg fixed at 70%. Under these conditions, 15% SCBA achieved the highest compressive strength of 42.67 MPa, whereas 10% SCBA yielded the highest flexural and splitting tensile strengths of 4.92 and 4.23 MPa, respectively. The 90-day shrinkage decreased from 0.075% for the control to 0.023% at 15% SCBA. Compressive strength correlated strongly with ultrasonic pulse velocity and rebound number. FTIR analysis indicated a pronounced Si–O–T band, consistent with changes associated with geopolymerization. These findings provide a quantitative basis for FA-SCBA geopolymer concrete design in sustainable construction.