Hydration and Hardening of Composite Binder Containing Mechanically Activated Hemihydrate Phosphogypsum

Abstract

One of Lithuania´s most abundant technogenic inorganic wastes – extractive hemihydrate phosphogypsum (E-PG) was investigated for use in composite gypsum cement pozzolanic binding materials (GCP). E-PG, in such a state, as it is disposed of to stackpiles, cannot be used as a binding material, due to the acidic impurities present in it. Mechanical activation of E-PG and neutralization of acidic impurities by use of cement not only greatly increases E-PG strength, but also gives hydraulic properties. It is necessary to properly select the amount of pozzolanic component needed to avoid the formation of destructive minerals (ettringite, thaumasite) in the hardened GCP composition, i. e. the CaO concentration in the liquid phase of hydrating composition should not exceed after 5 days 1.1 g/l, after 7 days – 0.85 g/l. It was determined, that not only does the activity and mineral composition of pozzolanic component have an influence on CaO concentration, but also does mechanical activation. The neutralization in mixtures with opoka is preformed not only by Portland cement, but also by the finely dispersed carbonates (30 % – 35 %) present in opoka. While in mixtures with silica fume, the acidic impurities are neutralized only by the Portland cement minerals. It was found that mechanical E-PG activation greatly improves the hardened composition's strength and hydraulic properties.