Effect of Gypsum Content on CSAB Cement-Based Immobilization of Se and SO₄ from Industrial Filter Sludge and Sodium–Selenium Salts

Abstract

Release of different heavy metals from various industries is a significant environmental hazard worldwide. In order to reduce the threat from these chemicals, different treatment methods are needed to make them harmless. This study approaches the problem by examining the effect of gypsum content for immobilization of selenium oxyanions selenite and selenate, and sulfate, with calcium sulfoaluminate belite (CSAB) cement-based ettringite binder systems. The study has two experimental sections. In the first section, an industrial filter sludge with high concentrations of selenium and sulfate is immobilized with CSAB and varying amounts of gypsum. The immobilization is efficient and reducing the gypsum amount in the hydrating mixture enhances the incorporation of both selenium and sulfate. The binding occurs through encapsulation and incorporation into ettringite and other hydrated phases. In the second section, two reagent grade sodium–selenium salts, sodium selenite and sodium selenate, are mixed with CSAB and varying amounts of gypsum. The immobilization is not effective and ettringite is not formed in large quantities, most likely due to high sodium content in the reacting system.