Modification And Performance Evaluation Of Phosphogypsum-based Building Gypsum

The main component of phosphogypsum is calcium sulfate dihydrate（CaSO₄·2H₂O）,which is a solid industrial by-product produced by the wet production of phosphoric acid.For each ton of phosphoric acid produced,about 4 to 5 tons of phosphogypsum are discharged.The phosphogypsum produced by the preparation of phosphoric acid in wet method contains a large amount of impurities which m ay affect the performance of the phosphogypsum-related products,making it difficult to comprehensively utilize.At present,most phosphate fertilizer enterprises deal t with phosphogypsum mainly by stockpiling,and supplemented by comprehensive utilization.According to statistics,China’s phosphogypsum stockpiles have exceeded300 million tons,and such a large amount of phosphogypsum storage and disposal takes up a large amount of land bring a lot of pollution to the surrounding environment.Along with the gradual depletion of high-quality natural gypsum resources,the research ideas of industrial by-product phosphogypsum have received extensive attention as an alternative resource.Improving the comprehensive utilization rate of phosphogypsum is great sig nificance to the sustainable development of China’s economy and environment.In view of the performance defects of building gypsum prepared by phosphogypsum,in this paper,the process parameters of phosphogypsum for building gypsum are optimized.Then the prepared gypsum-based building gypsum are modified by adding different admixtures.The effect of different modifiers on their properties is as follows:（1）The optimum technological parameters for the preparation of gypsum-based building gypsum:calcination temperature is 150℃,calcination is 5 h,and aging time is 3 d.The standard consistency water content of the building gypsum prepared under this process condition is 82%;the initial setting time is 9′30′′,and the final setting time is 15′30′′;the 2h flexural and compressive strength are 2.21MPa and4.43MPa,respectively.The flexural and compressive strengths are 4.36 MPa and 9.64MPa,respectively,of which the strength of 2 h reaches 2.0 in the GB/T9776-2008GB.（2）When the calcination temperature is 200℃⁴00℃,the main calcined product of phosphogypsum is type III anhydrite;when the calcination temperature is450℃⁵50℃,the main calcined product of phosphogypsum is slow-dissolving anhydrite;When the calcination temperature is higher than 550℃,the calcined product of phosphogypsum loses hydration ability without adding an additive.（3）The reinforcing agent mainly modifies the phosphog ypsum-based building gypsum through chemical action.In this paper,sodium sulfate,urea,aluminum sulfate and aluminum hydroxide were selected for single-mixing and mixed-mixing modification tests.The results show that the best effect of single additive modification is aluminum sulfate,the optimum dosage is 1.5%;the double-doping modification effect is best when the reinforcing agent is mixed,the best combination is 1.5%aluminum sulfate+0.7%urea.The reinforcing agent three-doping and four-doping modification effects are not as good as single factors.（4）The water reducing agent mainly modifies the phosphogypsum-based building gypsum through the surface physical action.In this paper,four kinds of calcium lignosulfonate,naphthalene water reducing agent,melamine water reducing agent and polycarboxylic acid water reducing agent are selected.The substance is tested by single addition and complex addition of water reducing agent.The results show that the modification effect of calcium lignosulfonat e is poor,and it is not suitable for use as a water reducer for building gypsum;the optimum blending amount of the other three water-reducing agents is:polycarboxylate water reducer（0.5%）,The melamine water reducing agent（0.3%）and the naphthalene wa ter reducing agent（0.3%）have the best modification effect,and the water reduction rate reaches 25.61%,and the strength of the modified gypsum test piece is obviously improved;The improvement of the physical properties of the building gypsum by the double-mixing agent of the water reducing agent is not obvious compared with the single-mixing,and the higher cost is increased,so that the double-doping is not selected when the water-reducing agent is used.（5）The retarder modification test of phosphogyp sum-based building gypsum was carried out by using sodium polyphosphate,potassium citrate and animal protein retarder.The results showed that the animal protein retarder had the best retarding effect,and It has little effect on the strength of building plaster.Therefore,animal protein retarder is preferred in the selection of phosphogypsum-based building gypsum retarder,and the specific dosage is determined according to actual needs.（6）The ratio of the composite modifier of the optimized experimenta l scheme is:1.5%aluminum sulfate+0.7%urea+0.3%melamine water reducing agent,and the water reduction rate of the phosphogypsum-based building gypsum after the modification by the composite modifier is reached.26.83%,compared with the blank group,the 2 h flexural and compressive strength of the modified gypsum block increased by 45.61%and 70.02%,respectively;the absolute flexural and compressive strength increased by 43.08%and 77.36%,respectively.The performance of phosphogypsum-based building gypsum has been greatly improved.（7）This paper carries out technical and economic analysis by processing100,000 tons of phosphogypsum annually to produce 600,000 tons of modified phosphogypsum-based building gypsum powder and processing it into gypsu m hollow block for sale.The results show that this method can effectively deal with the large amount of phosphogypsum deposited by phosphate fertilizer enterprises,solve the problem of land occupation and environmental pollution,and have good economic and environmental benefits,and the market potential is good in the future.