Study On Preparation And Application Of Anhydrite Ⅱ From Phosphogypsum

PG is a kind of solid waste discharged from the wet-process phosphoric acid process.It is obtained by the reaction of phosphoric acid powder and sulfuric acid.Considering the economic and environmental problems,PG needs to be developed and utilized urgently.Type II anhydrite is one of the five phases of gypsum,including natural and artificial ones.In nature,it exists in anhydrite ore with dense structure,more impurities and less development and utilization.Type II anhydrite obtained by dehydration of gypsum or chemical gypsum is artificial type II anhydrite.Type II anhydrite can be used in building materials,agriculture and other industries with broad application prospects.The strength development and water resistance of type II anhydrite are better thanβHemihydrate gypsum is a high-quality building material.However,type II anhydrite sets and hardens slowly and can only be used after activation.The particle size of PG is reduced by grinding,the specific surface area is increased,the PG is heated evenly during the calcination process,and the activity of type II anhydrite is pre-activated.Type II anhydrite was prepared by calcining PG at high temperature,using activator to excite and adding additives to prepare type II anhydrite based self-leveling material with excellent performance.The effect of grinding on the particle size,specific surface area and hydration and hardening of type II anhydrite was studied.The results showed that grinding reduced the particle size of PG and increased the specific surface area,thus increasing the activity of type II anhydrite obtained by calcination.The best grinding time of PG is 30min,and the particle size of PG is 16.4μm.The specific surface area is 569.8 m2/kg,and the grinding efficiency is reduced after the grinding time exceeds 30 minutes,resulting in energy waste.The change of impurity content in PG during medium-temperature calcination was studied.The results showed that the soluble phosphorus,soluble fluorine and organic impurities were basically removed after the PG was calcined at medium temperature.The effect of calcination temperature on the hydration activity of type II anhydrite was studied,and the physical properties of type II anhydrite prepared at various calcination temperatures with the addition of activator were studied.The results show that the flexural strength and compressive strength of type II anhydrite calcined at 500℃are the highest without activator,and the flexural strength and compressive strength at 28 days are 4.29 MPa and 18.89 MPa;After adding 1%Na₂SO₄+10%lime composite activator,the type II anhydrite calcined at 750℃has the best performance,and the 28d compressive strength reaches 24.8 MPa.The effects of sulfate.building gypsum.lime.and cement on the setting and hardening of type II anhydrous gypsum were studied.The results indicate that the activator can effectively shorten the setting time of type II anhydrite.improve the flexural and compressive strength of the hardened body.and transform the hardened body from volume shrinkage to volume expansion.The activator has improved the water resistance of type II anhydrite to a certain extent.Among them.cement has the best improvement effect on water resistance.followed by lime.while sulfate and building gypsum have no significant improvement effect on water resistance.The mechanism of action of activator on type II anhydrite was studied using scanning electron microscopy.The results showed that the addition of activator increased the hydration rate of type II anhydrite,This greatly increases the crystal content in the hardened body and enhances the strength of the hardened body;In addition.the addition of cement makes ettringite.C-S-H gel and other new phases formed in the hardened body.The addition of lime makes the crystal arrangement compact and the pores reduced.which is the reason for the improvement of water resistance of the hardened body.The optimal combination of Na₂SO₄-lime-fly ash composite activator was determined through orthogonal experiments.After research.it was found that the composite cementitious material made by mixing 75%Type II anhydrite.15%lime.and 10%fly ash with 1%Na₂SO₄has high mechanical characteristics.The 28 day flexural and compressive strength reached 5.21 MPa and 38.26 MPa.respectively.and the softening coefficient reached 0.89.The optimum dosage of water-reducing agent,defoamer and water-retaining agent was studied by successive optimization method,and the mix ratio of type II anhydrite based self-leveling material was determined.The cementitious materials are:70%type II anhydrite,15%lime,10%fly ash,5%sulphoaluminate cement,and 1%Na₂SO₄;Admixtures:0.2%polycarboxylic acid superplasticizer,0.005%polyether defoamer,0.05%HPMC water-retaining agent;The water-binder ratio is 0.35.The obtained performance indexes of type II anhydrite based self-leveling material are:initial setting time4h40min,final setting time 6h30min,3d flexural strength 2.39MPa,3d compressive strength12.63MPa,28d flexural strength 7.09MPa,28d compressive strength 36.25MPa,28d tensile bond strength 1.71MPa,shrinkage-0.008%.The above properties meet the standard T/CBMF82-2020《Gypsum-based Self-leveling Mortar》.