Preparation Of Industrial Phosphogypsum-based α-hemihydrate Gypsum And Regulation Of Its Paste Properties

In China,natural gypsum resource is increasingly exhausted,but plenty of industrial phosphogypsum（PG）has been piling up,with an accumulated stock of more than 300million tons.If PG is not well disposed and utilized,it not only occupy plenty land,but also result in severe environment damage of soil,water and atmosphere.Rational utilization of PG produces high value-added products and realizes waste into treasure,which is an urgent problem to be solved.α-hemihydrate gypsum（α-HH）has been applied to ceramics,high-grade buildings,arts and crafts,medical and other industries with excellent biocompatibility and high mechanical strength,which is seen as the most profitable and promising cementitious material.In this paper,the optimum conditions forα-HH preparation in autoclave hydrothermal system and atmospheric alcohol-water system were studied.This work also estimated the regulatory mechanism ofα-HH crystal morphology by experiment and molecular dynamics（MD）simulation.And ultra-fine HH was prepared by steam jet mill,and the effects of ultra-fine HH,resortable latex powder and polycarboxylate superplasticizer on the properties ofα-HH slurry were discussed.The addition of Calcium oxide（CaO）can neutralize the residual acid in the PG and solidify the soluble impurities phosphorus（P）and fluorine（F）to remove the influence on the growth process ofα-HH crystal.In the autoclaved hydrothermal system,α-HH was prepared using CaO pretreated PG as the raw material.The optimum condition was pH of 4,maleic acid of0.10%,temperature of 140°C for 2.0 h with solid-liquid ratio of 4:5.When maleic acid（0.1%）was added,the obtained hexagonal prisms ofα-HH showed aspect ratio of 1.2,and the flexural and compressive strength of the hardenedα-HH could reach to 14.79 and 46.24MPa,respectively.The alcohol-water system is kinetically unfavorable to the formation ofα-HH,so the transition from PG toα-HH is difficult in glycerol-water solution.However,the addition of a small amount of NaCl as a phase transformation accelerator can significantly shorten the transition time.The optimum condition forα-HH prepared was pH of 4,NaCl of 5.0%,temperature of 105°C for 3.0 h with solid-liquid ratio of 4:5.The obtainedα-HH with high aspect ratio was calcined at 600°C,then modified by sodium silicate and stearic acid,finally replaced the RS4012 mineral fiber to prepare the sealing liner.The physical and chemical properties of the sealing liner are up to standard,but the transverse tensile strength is slightly lower.The crystal morphology ofα-HH determines its properties and applications.Modifier can effectively regulate the crystal morphology ofα-HH.Experimental analysis and MD simulation results showed that the electronegative modifier（such as maleic acid）could preferentially absorbed on top face that decreased the growth rate ofα-HH along the c-axis due to strong interaction with Ca²⁺ofα-HH top face,resulting in the morphology ofα-HH crystal transformed to hexagonal prisms.however,electropositive modifier(such as hexadecyl trimethyl ammonium bromide（CTAB）would conduct complexation with free SO₄^2-,which hindered crystal axial growth.resulting in the morphology ofα-HH crystal exhibited the acicular shape.PG could convert into ultra-fine HH by the steam jet mill,which was added intoα-HH to improve the mechanical properties ofα-HH.When the dosage of ultra-fine HH is 20%,the property ofα-HH is the best.Both the dispersible polymer powder and the polycarboxylate superplasticizer have a retarding effect onα-HH slurry.When the appropriate dispersible polymer powder was added,the mechanical properties ofα-HH are enhanced.However,the addition of the polycarboxylate superplasticizer decreased mechanical properties ofα-HH.