Effect Of Phosphorus And Fluorine Impurities In Phosphogypsum On The Properties Of α-hemihydrate Gypsum

Phosphogypsum is a bulk solid waste product from the dihydrous process of phosphoric acid production,with high emissions and large stockpiles,and low overall utilisation.The preparation of alpha hemihydrate gypsum from phosphogypsum is one of the main research directions for the resource utilization of phosphogypsum.Due to the influence of a small amount of harmful impurities in phosphogypsum,the quality of the prepared alpha hemihydrate gypsum is uneven.In this paper,αhemihydrate gypsum was prepared by semi-liquid method.By adding KH₂PO₄,Ca HPO₄·2H₂O and Na F into natural gypsum,the influence of impurities such as soluble phosphorus,eutectic phosphorus and soluble fluorine in phosphogypsum on the water consumption of standard consistency,setting time and mechanical properties ofαhemihydrate gypsum was simulated.Based on the microscopic analysis and molecular dynamics simulation,the action mechanism of phosphorus and fluorine impurities on the properties ofαhemihydrate gypsum was investigated.The main conclusions are as follows:（1）When the soluble phosphorus content increases from 0 to 0.6%,the content of water of crystallization content in the corresponding products changes little.The soluble phosphorus reacts with Ca²⁺to form Ca₃（PO₄）₂ attached to the crystal surface.With the increase of soluble phosphorus content,the crystal surface becomes rough and the crystal becomes elongated.At the same time,the early hydration rate ofαhemihydrate gypsum is accelerated,the hydration process is accelerated,the setting time is shortened,and the hardening strength ofαhemihydrate gypsum is reduced.When the soluble phosphorus content was 0.2%,the maximum temperature rise of hydration heat and its occurrence time were 75.3℃/41min,initial setting time/final setting time were 3min/4min,and the 2h flexural strength/absolute dry compressive strength were 5.23MPa/40.31MPa,respectively.（2）With the increase of eutectic phosphorus content,the content of water of crystallization content in corresponding products gradually decreased and then tended to be stable.The eutectic phosphorus is transferred in two steps during the whole process.Part of the eutectic phosphorus is dissolved during the preparation ofαhemihydrous gypsum,which forms insoluble substances with calcium ions,making the crystal surface ofαhemihydrous gypsum rough and elongated.The other part is dissolved in the hydration process ofαhemihydrous gypsum,delaying its early hydration rate,reducing the maximum temperature of hydration,inhibiting its hydration process,the setting time is significantly prolonged,reducing the hardening strength of α hemihydrous gypsum,especially 2h strength.When the eutectic phosphorus content is 0.1%,the maximum temperature rise of hydration heat and its occurrence time are 51.7℃/217 min,the initial setting time and final setting time are 41min/70 min,and the 2h folding resistance/absolute dry compressive strength are 2.98MPa/42.77 MPa,respectively.（3）When the soluble fluorine content increases from 0 to 1.2%,the water of crystallization content phase in the corresponding product decreases gradually.Soluble fluorine reacts with Ca2+ to form Ca F2,which is attached to the crystal planes of α hemihydrite along the a,b and c axes.With the increase of content,the crystal surface is wrapped,the crystal planes become coarser and the crystal becomes significantly smaller.At the same time,the early hydration rate of α hemihydrate gypsum is accelerated,the maximum temperature of hydration is increased,the hydration process is accelerated,the final setting time is shortened,and the strength of hardened α hemihydrate gypsum,especially the absolute dry compressive strength,is greatly reduced.When the content of soluble fluorine was 0.6%,the maximum temperature rise of hydration heat and its occurrence time were 74.77℃/21 min,the initial setting time/final setting time were 11min/15 min,and the 2h flexural strength/absolute dry compressive strength were 4.41MPa/13.83 MPa,respectively.（4）The influence of soluble phosphorus,eutectic phosphorus and soluble fluorine impurities on the mechanical properties of α hemihydrate gypsum was comprehensively analyzed.The adverse effect of soluble fluorine was the largest,followed by eutectic phosphorus and soluble phosphorus.When the contents of soluble phosphorus,eutectic phosphorus and soluble fluorine are controlled in the range of 0.2%,0.1% and 0.4% respectively,the above impurities have little adverse effects on the properties of α hemihydrate gypsum.