Evaluation Of The Characteristics Of Phosphogypsum Ecological Bricks And The Effect Of Improved Substrate Vegetatio

As phosphogypsum contains harmful substances such as phosphorus,fluorine and free acid,large amount of stockpiles caused a series of environmental problems,and its harmless treatment and resource utilization is the key to solve the problem.At present,phosphogypsum has been widely used in building materials to prepare no-burn bricks,gypsum bricks and gypsum plates,but the research of using phosphogypsum to prepare ecological bricks has not yet been reported.The development of phosphogypsum ecological bricks is of great significance for the ecological restoration of mines,mountain vegetation restoration,road slopes,river slopes,high and steep rock slope re-greening construction and market engineering.In this paper,the experimental study on the material selection of phosphogypsum eco-brick was carried out by adding different proportions of cement and bamboo fiber to phosphogypsum as the main raw material to investigate the changes of basic properties such as compressive strength,porosity and water absorption of phosphogypsum eco-brick under different ratios,and the effect of adding bamboo fiber on the basic properties of phosphogypsum eco-brick.At the same time,the vegetation effect was explored by preferentially selecting the vegetation substrate with phosphogypsum as the main body and adding different proportions of biomass charcoal and quicklime for improvement.The main conclusions of the experiments are as follows.（1）The phosphogypsum eco-brick was prepared with phosphogypsum as the main raw material,and the mass ratios of phosphogypsum to cement were designed as9:1,8.5:1.5,8:2,7.5:2.5 and 7:3.The test results showed that:with the increase of cement admixture,the compressive strength of phosphogypsum eco-brick gradually increased,the water absorption gradually decreased and the softening coefficient gradually increased,among which,the mass ratios of phosphogypsum to cement were 7.5:2.5 The compressive strength of phosphogypsum eco-brick at 28d age was 8.02 MPa,9.68 MPa,and reached MU6.7 strength level at 7:3.（2）The ratio of phosphogypsum to cement mass ratio of 7.5:2.5 and 7:3 was selected as the basic raw material ratio of ecobricks.By adding 2%,4%,6%and 8%（percentage of total dry mix mass）of bamboo fiber,it was concluded that adding bamboo fiber in appropriate amount could effectively improve the porosity of ecobricks,and the higher the content of bamboo fiber,the more obvious the increase of porosity.When the phosphogypsum:cement is 7:3 and bamboo fiber is added at6%,which is the best test ratio,the eco-brick reaches 15.6%porosity at 28d age,the compressive strength is 6.73 MPa,and the softening coefficient is 0.80.The hollow brick prepared with the same material has a compressive strength of 6.82 MPa,15.6%porosity,and 30%porosity.（3）The test results of XRF and XRD indicate that the main chemical composition of phosphogypsum is Ca O,SO₃and Si O₂,etc.The main mineral composition is hemihydrate gypsum（Ca SO₄-2H₂O）and a small amount of permeable calcium phosphate（Ca HPO₄-2H₂O）,silicon dioxide（Si O₂）,etc.The reaction of phosphogypsum with cement is mainly from Ca SO₄-2H₂O,Ca（OH）₂and Si O₂.The hydration products after the reaction between phosphogypsum and cement are mainly Ca SO₄-2H₂O,Ca（OH）₂and Aft.Ca SO₄-2H₂O and Si O₂are mainly derived from phosphogypsum,Ca（OH）₂is produced by the hydration of C₂S and C₃S in cement clinker,Ca（OH）₂and phosphogypsum undergo secondary hydration reaction to produce C-S-H gel and Aft,C S-H gel will form a waterproof film on the surface of gypsum,which increase the compressive strength of eco-brick.The longer the hydration time,the higher the cement content,the more Aft products are generated,the better the strength and water resistance of eco-brick.（4）The plant growth effect of phosphogypsum was measured by adding5%-20%biomass charcoal and 5%quicklime to improve the substrate.The results showed that the PC3 treatment group（85%phosphogypsum+10%biomass charcoal+5%quicklime）was a high-quality ratio of modified phosphogypsum substrate suitable for ryegrass growth.79.14%of ryegrass germination was achieved in the PC3treatment group compared with the control group CK（no biomass charcoal applied）,an increase of 49.12%,and 1.14±0.14 g aboveground fresh weight,an increase of60.56%.Photosynthetic pigment content increased,chlorophyll content and carotenoid content were 1.40±0.11 mg-g-1 and 0.34±0.07 mg-g-1,respectively,an increase of 34.62%and 78.95%,respectively.（5）The heavy metal leaching experiments on phosphogypsum and bamboo fiber eco-brick（PG-5 eco-brick with 6%bamboo fiber addition）samples aged 28 d using distilled water and 2%（mass fraction）citric acid as leaching agent,respectively,showed that the concentrations of Cu,Zn,Se,As,Pb,Cd heavy metals after leaching reached the standard of surface water environmental quality standard（GB3838-2002）The concentrations of Cu,Zn,Se,As,Pb and Cd reached the standard of Class III water bodies in GB3838-2002,and the concentration of Cr reached the standard of Class V.Among them,the ecobricks leaching results compared with the raw material of phosphogypsum,the bamboo fiber ecobricks under two leaching environments its heavy metal leaching significantly reduced,indicating that phosphogypsum through cement curing to form ecobricks to achieve the solidification of heavy metals and the resource utilization of phosphogypsum.