Effects Of Impurities On The Physical Properties And Crystallization Properties Of ? High-strength Gypsum Prepared By Autoclaved Phosphogypsum

In addition to calcium sulfate dihydrate,phosphogypsum also contains soluble phosphorus,eutectic phosphorus,fluoride,organic matter and other impurities.The existence of impurities limits the efficient utilization of phosphogypsum.The preparation of phosphogypsum into high-strength?-hemihydrate gypsum by autoclavic method is an effective method for the resource utilization of phosphogypsum,but the autoclavic parameters and impurities in phosphogypsum will affect the quality of high-strength?-hemihydrate gypsum.Therefore,this paper conducted autoclaved phosphogypsum under different autoclaved parameters and different contents of soluble phosphorus and fluorine,to study the effects of autoclaved parameters and soluble phosphorus and fluorine impurities on the physical and chemical properties of high-strength?-hemihydrate gypsum.Firstly,using phosphogypsum as raw material,the effects of various autoclastic parameters on phase composition,relative crystallinity of?-hemihydrate gypsum crystal,microstructure and physical properties of the sample were investigated separately.The results show that when the autoclavic temperature is controlled in the range of 120?140?and the autoclavic time is more than 3 h,the dihydrate gypsum in phosphogypsum can be completely transformed into?-hemihydrate gypsum,and the crystal of?-hemihydrate gypsum in the sample has high relative crystallinity,complete microstructure and high hydration and hardening physical strength.If the autoclavic temperature is lower than 120?or the autoclavic time is less than 3 h,the transformation of dihydrate gypsum in phosphogypsum is incomplete,and if the autoclavic temperature is higher than 140?,the dehydration of?-hemihydrate gypsum will continue to form anhydrous gypsum.The liquid-solid ratio has little effect on the conversion rate of dihydrate gypsum in phosphogypsum,but it has great effect on the relative crystallinity and microstructure of?-hemihydrate gypsum crystal in the sample.When the liquid-solid ratio is controlled to 0.25,the products with high relative crystallinity,complete crystal microstructure and small aspect ratio can be obtained,and the liquid-solid ratio is too high or too low.It not only decreases the relative crystallinity and increases the aspect ratio of?-hemihydrate gypsum crystal,but also leads to incomplete crystal growth and rough crystal surface.The phase composition and the relative crystallinity of?-hemihydrate gypsum crystal are affected by the drying temperature.If the drying temperature is too low,the?-hemihydrate gypsum in the autoclavic sample will be transformed into dihydrate gypsum,while if the drying temperature is too high,the?-hemihydrate gypsum will be dehydrated into anhydrous gypsum.Under the condition that?-hemihydrate gypsum is not hydrated,the removal rate of attached water in autoclaved samples will affect the relative crystallinity of?-hemihydrate gypsum in autoclaved samples.If the removal rate of attached water is too fast,the relative crystallinity of?-hemihydrate gypsum crystal will decrease.Secondly,the saturated solubility of phosphogypsum was investigated by using the phosphogypsum which was washed with water to remove impurity.The results show that soluble phosphorus and soluble fluorine have a great effect on the solubility of washed phosphogypsum.A small amount of soluble phosphorus and fluorine can greatly improve the solubility of washed phosphogypsum.With the increase of the content of soluble phosphorus,the fluctuation range of its solubility decreases.When the content of soluble phosphorus is more than 0.8%,the solubility tends to be basically stable.When the soluble fluoride content is in the range of 0.05-0.2%,its solubility fluctuates in a small range and remains at about 10 g·L^-1.However,when the soluble fluoride content is greater than 0.3%,its solubility will increase sharply again,exceeding 17 g·L^-1.In solutions of soluble phosphorus or soluble fluorine,the solubility of washed phosphogypsum fluctuates very little with temperature.Finally,the effects of soluble phosphorus and fluorine impurity on phase composition,relative crystallinity,microstructure and physical properties of?-hemihydrate gypsum crystal in autoclavic gypsum samples were investigated by using fully washed phosphogypsum as raw material.The results show that soluble phosphorus can improve the conversion of dihydrate gypsum in phosphogypsum and the relative crystallinity of?-hemihydrate gypsum in autoclave samples.Small amount of soluble fluorine can decrease the conversion of dihydrate gypsum in phosphogypsum,but it can improve the relative crystallinity of?-hemihydrate gypsum in autoclavic sample.However,when the content of soluble fluorine is too much,the relative crystallinity of?-hemihydrate gypsum will be low.Without considering the addition of other impurities and transcrystallizer,a certain amount of soluble phosphorus in phosphogypsum is beneficial to improve the physical properties of hydride hardening body of?-hemihydrate gypsum in autoclastic sample,although it can also prolong the setting time of hydride hardening body,but it is more conducive to the practical application of?-hemihydrate gypsum.A trace amount of soluble fluorine(0?0.1%)in phosphogypsum can significantly improve the relative crystallinity of?-hemihydrate gypsum crystal.However,with the increase of soluble fluorine content,more insoluble Ca F₂ will be generated during the hydration process of?-hemihydrate gypsum,which leads to the shorten of setting time and the decrease of hardening strength,which is not conducive to practical application.