Investigation on the hydration of hemihydrate phosphogypsum after post treatment

Pretreatment is very important for the application of phosphogypsum (PG), but it is seldom adopted in practise due to high investment and costs. In order to promote the application of PG, post treatment of hemihydrate phosphogypsum (HPG) by addition of slaked lime (SL) was investigated. As an alkaline additive, SL can neutralize HPG. In addition, the interaction of HPG and SL can not possibly be avoided, because SL is generally presented in composite binder and it is also the hydration product of cement based materials which are widely used. Therefore, the hydration process, setting and hardening characters of HPG with different SL contents at a constant water/powder ratio were studied in detail. It was found that small amounts of SL can neutralize HPG but lead to super retardation and strength degradation due to the formation of calcium phosphate which would hinder the dissolution of HPG and alter the morphology of gypsum from needle-like to tiny crystal with little interlocking. Surprisingly, the addition of excessive SL regardless of pH would promote the rehydration of HPG and eliminate the influence of calcium phosphate contributing to the comparable strength and normal setting possibly due to the preferential precipitation mechanism of calcium phosphate. This important finding could be translated into specific design guidelines for post treatment. (C) 2019 Elsevier Ltd. All rights reserved.