Influence mechanism of alkaline environment on the hydration of hemihydrate phosphogypsum: A comparative study of various alkali

• The influence of various alkali on hydration and microstructure of HPGP was investigated. • Two opposite effects of alkali resulted by neutralization effect and ion strength effect on hydration of HPGP were observed. • Gypsum crystal morphology was significantly affected in alkaline environment except of excessive lime. • The influence effect of alkaline environment is related to its alkalinity and the impurity content in HPG. Although lime neutralization is widely recommended for raw phosphogypsum pretreatment, the influence mechanism of alkaline environment on hydration of hemihydrate phosphogypsum (HPG) is not clear. In the present paper, effect of alkaline environment created by different kinds of alkali including Mg(OH) 2 , NH 3 ·H 2 O, NaOH and Ca(OH) 2 with variable contents on hydration and microstructure of HPG were investigated. Abnormal setting and strength degradation were observed in the low-alkaline environment, while the setting time was normalized and crystal size developed in the high-alkaline environment regardless of the alkali category. Interestingly, the strength of HPG in the alkali environment only increased with excessive Ca(OH) 2 . It is verified that the formation of Ca 3 (PO 4 ) 2 from neutralization reaction in the low-alkaline environment served as retarder, and the excessive alkali beyond the threshold value of neutralization reaction played as accelerator in the high-alkaline environment, while only excessive lime particles promoted the development of gypsum crystal at c axis probably due to its absorption of Ca 3 (PO 4 ) 2 . This finding would promote the application of HPG in an alkaline environment and thus improve the utilization rate of phosphogypsum.