Study On Modification Of Phosphogypsum And Its Influence On Cement Propertie

Phosphogypsum is a solid waste produced in the wet-process phosphoric acid.A large amount of accumulated phosphogypsum not only occupies land,but also pollutes the surrounding environment.For this reason,The resource utilization of phosphogypsum needs to be increased to reduce the negative impact on the environment..At present,the use of phosphogypsum in cement-based Wet-mix mortar or other cement products is a new way to expand the utilization of phosphogypsum resources.However,soluble phosphorus,fluorine in phosphogypsum and a large amount of dissolved SO₄^2-ions will seriously affect the hydration process and cementing strength of cement,thus restricting the application of high-content phosphogypsum in cement base building materials and existing the problem of low-content.Therefore,this paper uses quicklime,cationic starch,cement and fly ash to modify phosphogypsum.On the basis of research on modification of phosphogypsum powder,modified phosphogypsum balls are prepared by ball forming technology,and then the influence of modified phosphogypsum prepared by these two modification methods on cement performance is analyzed and compared.Firstly,the surface modification of phosphogypsum with lime,cationic starch and cement is carried out.After curing for a period of time,the modified phosphogypsum powder is produced.The influence of the content of lime,cationic starch,cement and fly ash on the dissolution performance of phosphogypsum is studied.The morphology of the modified phosphogypsum powder is analyzed by SEM-EDS.The results show that the p H value of modified phosphogypsum increases with the increase of lime content,and the suitable content is 4%.The solubility of modified phosphogypsum powder decreases with the increase of gelatinized cationic starch and increases with the increase of curing time.The optimum dosage is 10%and curing time is 3 days.The SO₄^2-concentration in filtrate is 0.56g/L when soaked in water for 8h.At the same time,the amount of SO₄^2-dissolved in water is less than that in saturated Ca（OH）₂ solution.By SEM analysis,the surface of phosphogypsum powder modified by lime-cationic starch forms a dense film.With the increase of cement,fly ash content and curing time,the solubility of lime-cement-fly ash modified phosphogypsum powder decreases.When the content of both cement and fly ash is 10%and the suitable curing time is 7d,soak 8h SO₄^2-concentration is 0.30 g/L;By SEM analysis,the surface of modified phosphogypsum powder is wrapped by a flocculent layer,which is analyzed by EDS.It is the cement hydration product,such as calcium silicate and alunite.The effect of lime-cement-fly ash-gelatinized cationic starch modified phosphogypsum is better than that of single cationic starch and worse than that of cement-fly ash.Secondly,with the same amount of raw lime,gelatinized cationic starch and fly ash,appropriate amount of cement is changed to prepare phosphogypsum balls with certain strength.The effects of cement content and water consumption on the physical properties of phosphogypsum balls during pelletizing were studied.The results show that the solubility of phosphogypsum ball decreases with the increase of cement content,and the suitable cement content is 20%.The fineness modulus of phosphogypsum balls obtained with higher water consumption during pelletizing process increases the water absorption and solubility of phosphogypsum balls in 1h,and the accumulation density,apparent density and void ratio of phosphogypsum balls decrease.The water absorption,solubility and crushing value of phosphogypsum balls with different particle sizes increase with particle size and decrease with curing time.The bigger the spherical shape of phosphogypsum ball and the bigger the particle size of phosphogypsum ball,the more serious the internal hollow of phosphogypsum ball obtained by SEM analysis.Finally,the standard consistency water consumption,setting time and early strength of modified phosphogypsum cement paste,undisturbed phosphogypsum cement paste and mortar cement paste are compared,and the hardened cement is characterized by XRD,SEM,cement hydration heat and mercury pressure methods.The results show that the water consumption for standard consistency,setting time and early strength of modified phosphogypsum cement paste are lower than those of original phosphogypsum cement paste.The standard consistency,setting time and strength of phosphogypsum ball cement paste are all higher than those of sand cement paste.However,the effect is much better than that of modified phosphogypsum powder cement paste.According to XRD analysis,the main phases of cement paste hardened with modified phosphogypsum and undisturbed phosphogypsum are Ca SO₄·2H₂O,hydrated C-S-H,Ca（OH）₂ and alunite.According to SEM micro-morphology analysis,7d alunite of cement hardening sample doped with modified phosphogypsum is mainly short cylindrical,and 28d cement hardening sample is denser and has higher strength.The pore size of modified phosphogypsum cement hardened paste analyzed by MIP is smaller than that of unmodified phosphogypsum cement hardened paste.