Stabilization and solidification mechanism of Pb in phosphogypsum slag-based cementitious materials

This study involves the use of solid waste (GGBS and phosphogypsum) in combination with lime to prepare a phosphogypsum-slag based cementitious matrix (PS) for solidifying/stabilizing Pb (II). Effective Pb-stabilization was achieved regardless of using different contents of raw materials, the compositions of 60% GGBS, 35% phosphogypsum and 5% CaO had excellent compressive strength and low metal leaching concentration, and the mechanism of solidification/stabilization on heavy metals had been investigated. XRD patterns revealed that the hydration products of PS were ettringite (AFt), calcium silicate hydrate(C-S-H) and gypsum. Hydration heat test and TGA analysis showed that the hydration process of PS matrix was retarded after the addition of Pb, and the production (AFt and C-S-H) was reduced. FTIR, NMR and SEM-EDS analysis showed that there was no substitution of Pb to Si and Al, and PS solidified/stabilized Pb was mainly achieved by physical means. Such as the physical encapsulation and adsorption of Pb by ettringite and calcium silicate hydrate, the substitution of Pb to Ca ion, and the binding of Pb by covalent bond. PS matrix can stabilize Pb through the above ways. Lead phosphate precipitation may contribute to the reduction of metal leaching concentration.