Study On Coated Phosphogypsum Reinforced Aluminous Rock Mineral Polymer Material

Wet phosphoric acid is the main raw material of high concentration phosphate compound fertilizer.About 4.5～5 tons of phosphogypsum（PG）can be produced in the production of 1 ton of phosphoric acid.At present,our country has a huge amount of PG in both the cumulative stock and annual output,but the overall effective utilization rate is low with lacking of high-value utilization methods which can consume large amount of PG.The problem on PG has become the biggest bottleneck for the sustainable development of the domestic phosphorus chemical industry,especially the phosphate fertilizer industry related to food security.Applications of PG in high strength and high value-added materials are restricted because of its poor water resistance and chemical properties.In this paper,three kinds of organic polymer emulsions,such as silicone acrylic emulsion,styrene acrylic emulsion and pure acrylic emulsion,were used to treat the PG by coating,and the organic coated PG（CPG）was prepared to improve the chemical properties of PG.The CPG was used as reinforcement aggregate to prepare high-performance aluminous rock mineral polymer composites.The coating effect,structure,properties and mechanism of the CPG and CPG reinforced aluminous rock（CPG/AR）mineral polymer composites were systematically researched by physical and chemical analysis,TG,FTIR,XRD,SEM-EDS,XPS,MLA and Micro-CT.The coating treatment process of organic emulsion on PG is as follows:the PG is dried and dispersed after twice washing with water;the PG was firstly coated and granulated by using the initial organic emulsion with an amount of 20～25 percent mass of the PG at medium to high rotating speed;then,a second spray coating were working with using a water-diluted organic emulsion of mass ratio of 1:1 at low rotating speed after the first coated PG particles were dried and graded;lastly,the final CPG particles can be obtained when the spray coated PG particles were dried.The mechanism of the organic emulsion coating on PG belongs to a combination of physical coating and chemical coating.The CPG prepared by coating silicone acrylic emulsion on PG has the best comprehensive properties and coating effect.After twice coating treatment,the CPG surface tends to be smooth and round.The organic polymer emulsion forms a strong three-dimensional organic emulsion film network inside the CPG.Compared with the raw PG,the resistance to acid,alkali and water are significantly improved;the sulfate ions dissociation degree of CPG in water sharply reduced to 5.7%of the initial dissociation degree,and the water resistance is increased by more than 50%;the bulk density and water absorption of CPG are significantly reduced,and also the thermal stability is improved.The physical and chemical properties of silicone acrylic emulsion coating of PG is beneficial to future utilization process of PG.The basic preparation parameters of CPG/AR mineral polymer composites prepared at normal temperature and pressure by using the CPG,aluminous rock and sodium silicate are1.5 of sodium silicate modulus,1.8:1 of the solid-liquid ratio,up to 40%of the CPG addition,and curing in airtight condition.CPG with certain gradation can produce obvious strengthening effect on aluminous rock mineral polymer composites.With the 40%of CPG addition,the flexural strength and compressive strength of the CPG/AR mineral polymer composite can reach 4 MPa and 16 MPa respectively,and the CPG/AR mineral polymer composite occurs non-brittle fracture under shear stress with showing some toughness;the interface between the surface of CPG and matrix is firmly bonded,and the strengthening mechanism of CPG on aluminous mineral polymer composites conforms to the crack deflection theory of particle reinforced inorganic composites.The apparent density of the composite is only 1.75 g·cm^-3,which is significantly lower than that of lightweight concrete and clay brick;the addition of CPG can also improve the thermal stability of aluminous rock mineral polymer composite,which is beneficial to the stable application of CPG/AR mineral polymer composite at a certain higher temperature.The research results of micro analysis of CPG/AR mineral polymer composites shows that the CPG is well dispersed in the matrix of aluminous rock mineral polymer material in intact structure;it is wrapped by the matrix presenting as a three-dimensional network structure,forming an obvious transition zone and a firm interface combination.The phase analysis results by XRD and MLA show that there is a large amount of new zeolite phase in CPG/AR mineral polymer composite,and the zeolite content at the interface is a little high to about 40%.The result of XPS indicates that the situation of sulfate ion diffusion from CPG towards the matrix is almost non-existent,and the chemical environment of calcium and oxygen in the raw material system changes obviously;the peak fitting results also shows that there may be silicate and aluminosilicate with zeolite structure and silica gel contained in the CPG/AR mineral polymer composite,which fully proves the occurring of chemical reaction process in the preparation of mineral polymer.Thermodynamic calculation of chemical reaction shows that the chemical reaction of forming the zeolite products between aluminous rock and sodium silicate in the raw material system of mineral polymer composites can be carried out spontaneously at normal temperature,and the main component working and participating in the chemical reaction and is kaolinite,which provides silicon and aluminum sources for the matrix components of composites;sodium hydroxide plays a substantial stimulating role in the decomposition reaction with kaolinite;mineral polymerization mainly forms heulandite and gismondine.The coating of silicone acrylic emulsion film on the surface of CPG can play a role of template agent and induce the diffusion of zeolite phase towards phosphogypsum coating,or make the amorphous silicon-aluminum polymer gel preferentially crystallized on its surface,and siloxane group on the surface can form a relatively stable and firm interface binding zone with zeolite phase through a certain chemical action,thus playing a key role in the stability and performance enhancement of the composite structure.With many obvious advantages of the CPG/AR mineral polymer composites,such as the mild preparation conditions,high strength and low density,it can be used to prepare high-performance and lightweight composite plates,which are using for ceiling of engineering decoration and large hall rooms,sound absorbing wallboard and wall lining in tunnels.The research results can provide valuable basic research data and ways for the preparation of mineral polymer（or composite）materials directly using aluminous rock and its tailings at normal temperature and pressure,and also the high value-added utilization of CPG as the reinforcement of mineral polymer composite materials.