Impact of sulfate and nitrate incorporation on potassium- and sodium-based geopolymers: geopolymerization and materials properties

One of the most promising applications of geopolymers is their use as a waste encapsulating matrix. These binders are indeed compatible with aqueous waste streams and capable of activating several chemical and physical immobilization mechanisms for a wide range of inorganic waste species. Several works have investigated the immobilization of cations, mainly heavy metals or radioactive wastes, but very few studies have taken counterions, namely anions, into account. This work is an experimental investigation of the impact of anions with different valences on the material properties in regard to the requirements of an industrial process at ambient or slightly elevated temperature, including the setting time, maximum achievable compressive strength, or resistance to leaching. The modifications caused by the introduction of monovalent and divalent anions, such as sulfate and nitrate, are also evaluated in term of mineralogy, porosity, and microstructure. Their immobilization appears to be related to the progress of the geopolymerization reaction. Moreover, depending on the alkali ions used in the activation solution, the anionic species considered may also enhance the precipitation of some zeolites.