Determination of calcium diffusion coefficients as an estimator of the long-term dissolution potential of phosphogypsum : cement : lime composites

Phosphogypsum (PG), a solid waste byproduct of phosphoric acid production, was stabilized with lime and cement to produce composites for aquatic applications. Fifteen PG:cement:lime compositions were fabricated and subjected to a 28 day dynamic leaching test (salinity = 20 parts per thousand) to determine calcium diffusion coefficients used to estimate the composites' long-term dissolution potentials. A one-dimensional diffusion model was used to calculate diffusion coefficients, which were then regressed against PG, cement, and lime percentages to determine the optimal composition for saltwater applications. The diffusion coefficients of the composites ranged from 10(-4) to 10(-7) (cm(2).day(-1)). Both cement and lime contribute negatively to the diffusion coefficient; however, the interactions between cement and PG and between lime and PG contribute positively. Scanning electron microscope (SEM) observations of the composites used in the dynamic leaching test showed that ruptures developed in all combinations except the 70%:30%:0% PG:cement:lime composites. The development of the ruptures contributed to the observed diffusion coefficient increase from 10(-7) to 10(-5)-10(-4) (cm(2).day(-1)). SEM and microprobe results showed that the formation of ettringite was responsible for rupture development. The formation of a CaCO3 coating and a stable composite matrix were responsible for maintenance of physical integrity.