Formulation and optimization of a phosphogypsum – fly ash – lime composite for road construction: A statistical mixture design approach

•Extreme vertices design is adopted to represent the experimental space.•The suitability of composites is investigated based on UCS and W/D durability tests.•The effect of each mixture component and their interactions are established.•Fly ash acts on UCS as a filler in the short term and as a reactant in the long term.•A negative impact of excess lime on strength and durability is highlighted. This paper investigates the valorization of phosphogypsum and fly ash by-products as a road construction material through lime stabilization. To this end, an extreme vertices design was adopted to model the compaction characteristics, mechanical performance at different curing periods and durability upon wetting/drying (W/D) cycles of the mixture. Statistical analysis of the obtained regression models showed that the measured responses decrease with phosphogypsum addition while fly ash and lime cause them to increase. The increasing effect of fly ash addition was especially noticeable on compressive strengths at early ages (7–28 days), whereas at longer curing periods (90–360 days), these were mainly governed by its interaction with lime following the activation of the pozzolanic reaction where phosphogypsum acted as a catalyst to improve fly ash reactivity. However, a negative impact of excess lime beyond a threshold has been noted. The compressive strength at 28 days combined with weight loss upon W/D cycles were used as optimization responses to detect regions of interest, within the design space, where requirements are met for a subbase utilization.