Study On Preparation And Key Properties Of Composite Cementitious Material Containing Phosphogypsum

Phosphogypsum is a kind of solid waste,ric h in calcium sulfate dihydrate,which can react with potential pozzolanic active materials to form ettringite to improve the strength of composite cementitious materials.It has been used in the field of composite cementitious materials such as cement retarder and super sulfa te cement.On the other hand,under the requirements of sustainable development,the industry is under pressure to reduce greenhouse gas emi ssions and energy use.Therefore,an achievable action for the construction industry,which accounts for 8%of globa l carbon emissions,is to find reliable low-carbon alternative materials.This study focuses on the preparation of composite cementitious ma terials containing phosphogypsum as the main raw material and sulfate activator,and carried out the following research:1.The effects of slag and phosphogypsum contents on the mechanical properties and microstructures of composite cementitious materials were studied.X-ray diffraction analysis（XRD）,thermogravimetric analysis（TG-DTG）,and scanning electron microscopy（SEM）were used to analyze the effects of slag and phosphogypsum contents on the hydration products,microstructure,and strength formation mechanism of composite cementitious materials.The results show that with the increase of slag,the strength of the composite cementitious material increases gradually.When the slag content is 50%,the 28-day compressive strength reaches a maximum of about 14 MPa.Compared with the composite material without phosphogypsum,the composite cementitious material with 10–20%phosphogypsum showed higher strength properties,in which the 28-day compressive strength exceeds24 MPa.The main reason for this is that the sulfate in phosphogypsum can cause the composite cementitious material to generate a large amount of ettringi te and accelerate the dissolution of red mud and slag,increasing the release of aluminates,silicates,and Ca²⁺to form more C-（A）-S-H and ettringite.In addition,a large amount of C-（A）-S-H makes ettringite and unreacted particles combine into a uniform and compact structure,thus improving the strength.When the content of phosphogypsum exceeds 40%,the 28-day compressive strength of the comp osite cementitious material drops below 12 MPa due to the presence of fewer hydration products and the expansion of ettringite.2.The influence of phosphogypsum specific surface area on the physical and mechanical properties of PBC was studied.The XRD,TG-DTG,FTIR,SEM,and MIP were used to analyze the hydration products,microscopic morphology,and pore structure of PBC.The experimental results show that compared with the original phosphogypsum-containing PBC,the coagulation time of PBC with ball milli ng increasing the specific surface area of phosphogypsum is greatly extended,and gradually increases with the i ncrease of the specific surface area of phosphogypsum,and the final setting time exceeds 70 h,resulting in no intensity for 3 days.However,compared with the original phosphogypsum PBC,the ball mill increased phosphogypsum has better 7-day and 28-day strength than the PBC with increased surface area.With the increase of the specific surface area of phosphogypsum,its strength first increased a nd then decreased,and when the specific surface of phosphogypsum was 1277 m²/kg,the compressive strength of P BC exceeded all samples in 28 days to reach 47.6 MPa.In addition,the coagulation time,mechanical properties,hydration degree,hydration produc t and pore structure of PBC were closely related to the specific surface area of phosphogypsum.It was found that appropriate spherical milled phosphogypsum could promote hydration after PBC after7 days,increase hydration products,reduce the porosity,and average pore size of PBC slurry,thereby densifying the microstructure of PBC and improving strength development after 7 days.Increasing the specific surface area of phosphogypsum by ball milling can reveal the influence of phosphogypsum fineness on th e hydration degree,physical and mechanical properties,and microstructure of PBC.3.In this study,the effect of modified phosphogypsum with different lime contents in PBC on the mechanical properties,volume expansion,hydration products,microstructure morphology,and pore structure was investigated.The experimental results show that lime can weaken the retar ding effect of water-soluble impurities in PS and shorten the setting time of PBC,and the coagulation-promoting effect is better with increasing lime content.With increasin g lime content,the 7-day and 28-day strengths of PBC first increased and then dec reased,and the volume expansion gradually increased;among them,the 28-day compressive strength of B ₀（0.5%）was as high as 48 MPa.Lime-modified phosphogypsum（0.5%）is beneficial fo r promoting the degree of hydration of PBC,increasing the gel pore vol ume,reducing the average pore size of PBC,and compacting the microstructure,thereby improving the mechanical properties.Furthermore,through t he volume expansion results,it was found that the expansion of PBC depends not only on the amount of ettringi te but also on the alkaline environment.The ettringite produced at lower Ca O concentrations is uniformly dispersed and not prone to expansion,wh ile the ettringite produced at higher Ca O concentrations easily accumulates on the surface of other substances and is prone to expansion.4.The effects of different contents of calcined phosphogypsum（3%,5%,7%,9%）on the mechanical properties,hydratio n reaction and microstructure of LC3,as well as the effects of high temperature calcination on the residual s trength and mesostructure of LC3 were st udied.The experimental results sho w that the addition of calcined phosphogypsum to the LC3 system is bene ficial to the formation of AFt,thereby improving the strength of LC3,especially in the early strength develo pment.After 1 day of curing,compared with the sam ple without calcined phosphogypsum,the compressive strength of the sampl e with 9%calcined phosphogypsum increased by 310.5%,which was weaker for the later strength.The results of XRD,TG,FITR,SEM,and MIP showed that the addition of calcined phosp hogypsum was beneficial to promote the hydration reaction of LC3 and gener ate more hydration products to refine the microscopic pore structure.In addition,high-temperature calcination experiments（200°C,600°C,900°C）showed that with the increase of c alcination temperature,the mesoscopic crack increased and widened due to the deco mposition of hydration products,resulting in a gradual decrease in residual strength.In summary,it is entirely feasible to replace gypsum by calcined phosphogypsum in the LC3 system,and at the same time,it can further reduce the production cost of LC3 and broaden the recycling path of phospho gypsum.In this study,different types of phosphogypsum-containing composite cementitious materials were prepared by using phosphogypsum as the main raw material and sulfate activator,and composite red mud,cement,slag powder,clinker,calcined clay,and limestone.The effects of phosphogypsum content,particle fineness,quicklime modified phosphogypsum and calcined phosphogypsum on the properties of phosphogypsum-containing composite cementitious materials were systematically studied.Provide scientific t heoretical guidance for the promotion and application of phosphogypsum in the field o f building materials.