Study On Waterproof Performance Of Building Blocks Made Of Phosphogypsum

Phosphogypsum is an industrial by-product produced when phosphate ore is used in phosphoric acid production.It contains phosphoric acid and its salts,fluoride,heavy metals,a small amount of radioactive substances,organic substances and etc.The annual output of phosphogypsum in China is nearly 70 million tons.A large amount of phosphogypsum produced as a by-product is stockpiled due to the complex impurity types and low utilization rate,thus causing many problems such as waste of land resources,soil and water pollution.Therefore,the state has been vigorously advocating and supporting the efficient utilization of phosphogypsum.Phosphogypsum has a lot of excellent properties.High calcium sulfate content makes it an excellent substitute for natural gypsum.The construction gypsum board and block made of phosphogypsum are widely used.However,due to the high solubility of calcium sulfate dihydrate which is the hydration products of phosphogypsum as well as the high porosity of microstructure,the waterproofing property of phosphogypsum building products is poor,which affects the use environment and stability.This project mainly focuses on the building block made by phosphogypsum-based composite cementing material which uses phosphogypsum,cement and fly ash as the main raw materials.By adjusting the ratio of phosphogypsum,cement,fly ash and glass bubbles,and adding the self-developed composite activator,the waterproof performance of the building block is greatly improved.At the same time,X-ray diffraction,scanning electron microscope,laser particle size analyzer and other testing methods are used to further study on the microstructure and waterproof mechanism of the waterproof block.The research results mainly include the following aspects:(1)Through experiments,the project studies the influence of the mixture ratio of phosphogypsum,cement and fly ash on the performance of phosphogypsum-based composite cementing material.Finally,the optimal mass ratio is determined that phosphogypsum:fly ash:cement:quicklime is 50:30:15:5,and the water-to-material ratio is 0.4.The 3d compressive strength of phosphogypsum-based composite cementing material is about 8MPa,7d 9MPa,28d 12Mpa.The water requirement for standard consistency is 39.6%,the initial setting time is 9 minutes,and the final setting time is 13 minutes.(2)By orthogonal experiments,the project studies the influence of adding different amount of mineral powder and cellulose in the self-developed stimulant as well as adding different amount of the self-developed stimulant and vitrified microsphere on the performance of phosphorus gypsum block,and finally identifies the best proportion of the amount.By single factor experiment,the project also studies the influence of self-developed stimulant on the performance of phosphorus gypsum block.Result of the experiments is when the amount of phosphogypsum is 500g,the amount of self-grinding activator is 10g including 50%mineral powder and 2%cellulose,the glass bubbles is 0.2 cubic centimeter.(3)The properties of the samples made by the best ratio of raw materials and additives are tested in all aspects:the softening coefficient is 0.85,the water absorption rate is about 5%,the compressive strength of 28d is about 15MPa,the thermal conductivity is 0.12w/(m·K).There is no bryozation.The freezing-thawing resistance is good.The above performance fully conforms to the performance requirements for wall materials used as waterproof building blocks(4)X-ray diffraction is used to analyze the phase of the sample,and it is found that the phosphogypsum block made by experiments contains CSH gel,AFt and CASH gel.Through SEM scanning electron microscope,it is found that in the crystal microstructure,there are not only a large number of gels wrapped on the grains,making the grains thicker and the pores smaller,but also flocculent gels coated at the joints of the grains,which effectively prevent the erosion of the internal structure by water and significantly improve the water resistance of the phosphogypsum block.Four points on the gel are selected for EDS analysis.According to the elemental analysis results of EDS,it was further confirmed that the gels existing in the structure where the grains overlap and around the grains include both CSH gel,ettringite,and CASH gel.The existence of the above flocculation structure makes the crystal microstructure with smaller void and lower porosityl more compact,the grain structuremore more stable,and the properties of softening coefficient and compressive strength obviously improved compared with ordinary gypsum block.