Multi-objective optimisation of unfired clay bricks stabilised with date-palm waste and palm-frond ash for low-carbon hot-arid construction

Unfired clay bricks reinforced with regional bio-waste offer a credible low-carbon alternative to fired masonry, but small-sample, single-objective campaigns have so far prevented engineers from extracting statistically defensible mix recipes. This study presents a validated experimental–computational framework that closes that gap. A rotatable three-factor central composite design (CCD) was deployed across fifteen unique mixes of UAE coastal clay reinforced with Phoenix dactylifera frond fibre (0–5 wt%), ordinary Portland cement (0–12 wt%) and palm-frond ash (0–8 wt%). A two-batch design was used: a primary batch (N₁ = 45) for mechanical, thermal and hygric analyses, and a supplementary durability batch (N₂ = 135) providing triplicate replication for 20-cycle wet–dry, 14-day salt-spray (ASTM B117) and 30-cycle hot-humid protocols (total 180 specimens). The full quadratic response surface passed the lack-of-fit F-test (p = 0.20 for compressive strength; p = 0.41 for thermal conductivity) with R² of 0.985 and 0.991. Five surrogate learners were benchmarked under recipe-level Leave-One-Mix-Out cross-validation; Gaussian Process Regression with a Matérn 5/2 kernel emerged as the best performer. NSGA-II Pareto optimisation produced three engineerable optima — thermal (P-A), balanced (P-B) and structural (P-C) — verified within ±6% of predicted values. The joint fibre–binder interaction, the efflorescence suppression at C ≥ 5 wt% and the durability gains of the hybrid mixes are consistent with a mineral–biogenic synergy hypothesis in which cement- and ash-derived reaction products reinforce the fibre/matrix interfacial transition zone; direct microstructural verification of this mechanism is defined as follow-up work. A transient ISO 13786 simulation shows the P-A optimum delivers a 45% reduction in annual cooling demand, and a cradle-to-gate life-cycle assessment shows an 89% reduction in embodied CO₂ relative to Gulf-kiln fired clay.