Fly ash replacement and sand-to-binder ratio shape trade-offs among workability, strength, and stiffness in joint-filling mortar

Introduction Joint-filling mortars must retain placement mobility while developing adequate load-bearing capacity without excessive stiffness. Methods A 5 × 3 full-grid experiment was conducted using 40%–60% fly ash (FA) replacement, sand-to-binder ratio (S/B) of 0.35–0.55, a fixed water-to-binder ratio (W/B) of 0.30, and polypropylene (PP) fiber at 2.0 vol.% of total mortar volume. Strength and 28-d compressive secant modulus were evaluated using nested empirical regression models with leave-one-mixture-out cross-validation, while a targeted three-block randomized complete block design (RCBD) with a fixed high-range water reducer (HRWR) dosage was used to validate the principal FA and S/B response gradients and to support three-objective Pareto screening. Results Across the full grid, increasing FA from 40% to 60% reduced mean 28-d compressive strength and secant modulus by 10.28 MPa and 5.0 GPa, respectively. At FA = 60% under fixed HRWR, increasing S/B from 0.45 to 0.50 reduced free spread by 7.17 mm while increasing 28-d compressive strength by 5.20 MPa, with no resolved modulus change; the subsequent increase to 0.55 reduced free spread by 20.50 mm, decreased 28-d compressive strength by 2.53 MPa, and increased modulus by 1.92 GPa. Strength development remained age dependent, with the 60% FA group showing the largest ratio of group-mean 28-d to 7-d compressive strengths in the full-grid dataset. Pareto screening of the five fixed-HRWR validation mixtures identified M9, M10, M11, and M12 as non-dominated alternatives. Discussion Within the tested material and processing window, equal S/B increments therefore did not retain the same marginal response, and the non-dominated mixtures represent application-dependent trade-off options among workability, strength, and stiffness.