Valorization of phosphogypsum as a road material: Stabilizing effect of fly ash and lime additives on strength and durability

Phosphogypsum is a by-product from the phosphoric acid industry following the wet process. Known for its low cost, this process generates large amounts of phosphogypsum, the management of which has so far remained problematic due to the impurities it contains including heavy metals, radionuclides and acidic residuals. It is above all, its acidic nature that hinders its use as a road material since it increases its solubility which in turn makes it sensitive to water variations. This paper presents an investigation on the stabilization of Moroccan phosphogypsum, through neutralizing acidity, while considering both techno-economic and environmental constraints. To this end, mixtures of phosphogypsum and fly ash were prepared then activated with varying quicklime additions (beyond Initial Consumption of Lime) to develop different combinations of phosphogypsum (40–80%), fly ash (20–60%) and lime (4–20%). These have been tested for their mechanical and mineralogical properties at different curing periods (7, 28, 90, 180 days) besides durability against Wetting/Drying cycles, in order to evaluate the stabilizing effect of fly ash and lime. Results from Unconfined Compressive Strength (UCS) tests showed a strength increase with fly ash and lime addition with increasing the curing time. The UCS at 180 days varied between 1 and 3 MPa for formulations containing 4% lime (ICL) while those with 12–20% lime reached 4–7 MPa (12 times higher than UCS at 7days). As for durability, these showed weight losses of up to 10% upon W/D cycles, a strength loss of up to 56% and high porosity compared to formulations with higher lime content which rather gained up to 264% in strength. The influence of curing time and mixture composition on the mineralogy of the prepared composites was assessed through XRD and TGA analyzes highlighting the formation of ettringite as main hydration product responsible for strength development.