Development Of Phosphogypsum Based Dry Mixed Mortar And Influence Mechanism Of Admixtures

A large amount of phosphogypsum is discharged every year in China,and the content of calcium sulfate in dihydrate is more than 90%.The utilization rate is very low due to the influence of impurities,which is only about 30%in China.It is urgent to broaden the utilization way.It is difficult to directly use phosphogypsum because of its short setting time and lower strength than natural gypsum.Therefore,it is necessary to add retarder and water retaining agent.The selection of the type and amount of additives is one of the core technologies in the preparation of gypsum building materials,which not only improves the properties of gypsum paste,but also seriously affects the strength of gypsum hardened body.In view of this,this paper uses phosphogypsum powder,vitrified beads,sodium citrate?SC?and methyl cellulose?MC?to prepare phosphogypsum based lightweight plastering gypsum,and analyzes the influence of additives and the amount of vitrified beads aggregate on the physical properties of light gypsum plaster.The results show that the addition of sodium citrate can increase the fluidity of plaster paste and delay the setting of gypsum paste.The water retention rate first increases and then decreases,and the flexural strength decreases first and then increases.With the increase of the content of methylcellulose,the fluidity of the paste decreases,the setting time prolongs,the water retention rate increases obviously,the flexural strength of the hardened body decreases at first and then increases,while the compressive strength changes on the contrary,and the tensile bond strength changes positively with the content.With the increase of lightweight aggregate content,the slurry flow decreases and the setting time is shortened.The water retention rate increases first and then decreases.The flexural strength and compressive strength increase first and then decrease,and the tensile bond strength increases positively.According to the influence law,the phosphogypsum:vitrified beads:SC:MC=97?93:3.0?7.0:0.7?1.2:0.2?0.5.The properties of the samples meet the requirements of GB/T 28627-2012"gypsum plaster".Secondly,the bonded gypsum with excellent performance was prepared by using phosphogypsum,degradation of polyamide calcium salt?PE?,Hydroxypropyl methyl cellulose ether?HPMC?and Polyvinyl alcohol?PVA?as raw materials,and the influence of additive content on the physical properties of bonded gypsum was analyzed.The results show that the setting time and initial setting interval of bonded gypsum increase with the increase of the content of PE retarder.The mechanical strength of hardened body is improved by low content of PE retarder,while the hardening strength performance is decreased obviously with the increase of PE retarder content.The addition of cellulose ether can significantly improve the tensile bond strength of gypsum products,but the flexural strength and compressive strength of the samples will decrease with the increase of cellulose ether content.With the increase of PVA content in gypsum,the water consumption and flexural strength of paste decrease gradually,the compressive strength increases first and then decreases,and the tensile bond strength increases first and then decreases.According to the influence law of admixtures,the ratio of phosphogypsum:PE:cellulose ether:PVA=100:0.05?0.20:0.05?0.20:0.10?0.50,which meets the performance requirements of JC/T1025-2007"bonded gypsum".The flexural strength,compressive strength and tensile bond strength of the optimal ratio are 5.41mpa,16.8mpa and 1.07mpa,respectively.Finally,on the basis of this study,five kinds of retarders and three kinds of water retaining agents were selected to study the effects of single and multiple admixtures on the properties of phosphogypsum.The influence mechanism of retarders and water retaining agents on phosphogypsum was analyzed by XRD of hydration products,calcium ion concentration of hydration liquid phase,microstructure of hardened body and X-ray photoelectron spectroscopy.The results show that:except Ca,90%Ca+10%MC,SC and PE,the other retarders and water retaining agents reduce the fluidity of gypsum paste and increase the water consumption for standard consistency of paste;the setting time of building gypsum paste is prolonged by retarder;the free water is reduced by water retaining agent,the water retention rate is increased,and the setting time of building gypsum paste is accelerated;the extension range of setting time of the composite admixture of retarder and water retaining agent is reduced;the retarding agent can prolong the setting time of gypsum paste The results show that the strength of the hardened body of building gypsum decreases seriously with the increase of the dosage of coagulant and water retaining agent.After 1 day hydration of some samples with retarder,the XRD diffraction peak of the hardened body still has half water phase.With the hydration expansion in the later stage,the strength of the hardened body decreases at 7 days.Under the same conditions,the retarding effect of PE in phosphogypsum is better,and the strength reduction range is far less than that of citric acid,and even low content can enhance the strength of gypsum.When the retarder and water retaining agent is added,the concentration of Ca²⁺in the hydrated liquid phase of phosphogypsum changes obviously,and the better the retarding effect is,the highest calcium ion concentration is in the early hydration liquid phase of the sample,and the ion concentration decreases more slowly with time;sodium citrate,sodium tripolyphosphate and sodium hexametaphosphate obviously change the crystal growth habit of the hydrated gypsum,and the crystal growth habit of the hydrated gypsum is changed from the long needle shape of the blank sample The addition of water retaining agent promotes the formation of needle like crystals with different orientations and changes the degree of overlapping between the crystals of hydration products.The binding energy of Ca²⁺in the samples of eight kinds of admixtures mixed with phosphogypsum deviated in varying degrees,which indicated that there was a certain degree of chemical reaction between the groups of additives and calcium ions.Due to the selective adsorption,the sizes of dihydrate gypsum crystals in some samples decreased in the long axis direction,but increased in the short axis direction.PE retards coagulation by inhibiting the nucleation of Ca²⁺and SO₄^2-in solution.