Response surface evaluation of wheat straw-reinforced fly ash-GGBS geopolymer bricks for mechanical and thermal performance

The use of agricultural residues in alkali-activated composites offers a practical route for waste valorisation, although its environmental benefit requires a complete system-level assessment. This study evaluates treated wheat straw in fly ash-GGBS geopolymer bricks using a 27-run Box–Behnken design. Wheat straw content (0.50–2.00%), fibre length (10–30 mm), NaOH concentration (8–12 M), and GGBS replacement (20–40%) were examined against eight mechanical, physical, thermal, and ultrasonic responses. Measured compressive, flexural, and splitting tensile strengths ranged from 30.2 to 43.3 MPa, 4.74–6.16 MPa, and 2.57–3.31 MPa, respectively. DOE1 (1.25% wheat straw, 20 mm fibre length, 12 M NaOH, and 40% GGBS) produced the highest measured 28-day compressive strength and is identified throughout as the best-performing experimental condition. An equal-weight desirability analysis predicted a compromise at 1.21% wheat straw, 18.4 mm fibre length, 12 M NaOH, and 40% GGBS, with a composite desirability of 0.816. Because this predicted combination was not independently tested, it is presented as a model-derived result within the investigated factor space. All eight quadratic models were significant (p < 0.001); the thermal-conductivity model is used as a screening model because its lack-of-fit was significant. SEM, XRD, FTIR, and TGA/DTA provide qualitative, mutually consistent evidence of differences in matrix morphology, phase features, bonding environments, and mass-loss behaviour, without constituting direct quantitative confirmation of reaction extent or interface chemistry.