Preparation And Characterization Of Phosphogypsum-Straw Lightweight Wall Material

Phosphogypsum is one of the industrial waste residues produced in the process of phosphoric acid production（with main composition of CaSO₄?2H₂O）and its emissions are large.Only 15%amount of the phosphorus gypsum has been recycled,as well as using as cement retardant,construction materials,soil reagent,and the mine filler at present,and the remaining 85%is disposed as solid waste,which not only causes the waste of resources,but also causes the problem of environmental pollution.It has become a major problem for the environmental protection industry,and accelerating its resource utilization is the main way to solve this problem.In order to promote the resource utilization of phosphogypsum,especially to accelerate the utilization of phosphogypsum in wall materials,this paper took50%amount of phosphogypsum（PG）and 50%amount of mixture of the blast furnace slag（BFS）,cement and fly ash as the main raw materials to prepare a green cementitious material with good performance.Based on the cementitious material,the PG-straw light mortar was prepared by adding straw powder.The expanded perlite（EP）and expanded polybenzene particles（EPP）were added to reduce the bulk density of the lightweight mortar specimens,which could further act as lightweight wall materials（LWM）.By employing the universal testing machine,X-ray fluorescence（XRF）,scanning electron microscope（SEM）,thermal conducting coefficient measuring device and other testing devices,the properties of the resulting specimens of the cementitious material and the lightweight mortar,as well as the LWM one based on original phosphogypsum were characterized.In the preparation of specimens of cementitious material,which used the PG,BFS,and cement as raw materials,and abbreviated as PBC cementitious material,it was found that the compressive strength and softening coefficient of the as prepared specimens was increased at first and then reduced with the increase of cement content.When the cement content was 20%,the resulting specimens of 28 d compressive strength was 26.39 MPa.When the cement content was 40%,its 28 d compressive strength was only 11.70 MPa.Therefore,the 28 d compressive strength of the specimens can be significantly improved by adding appropriate content of cement.After the optimization of PBC specimens’s compositions,the fly ash was adding to replace the optimized mixture of BFS and cement,namely to prepare the PBCF cementitious materials.The early（3 d）compressive strength of the PBCF specimens was increased firstly and then reduced with the increase of fly ash adding amount.The specimens of PBCF of 15%fly ash adding amount had the the highest flexural and compressive strength of 28 d among all the as prepared PBCF specimens,which were 5.50 MPa and 28.68 MPa respectively.When the fly ash adding amount was more than 15%,the compressive strength of PBCF specimen was decreased.The softening coefficient of the PBCF specimens was increased firstly and then decreased with the increase of fly ash adding amount.The PBCF specimen of fly ash adding amount being15%had the softening coefficient of 0.91.The microstructure of the cross-section of PBCF specimen showed that a large number of AFt,C-S-H gel and excessive CaSO₄?2H₂O appeared in the hardened specimens,and they cemented each other to form a whole,forming the strengthened skeleton of the specimens.Straw powder（SP）was used as fine aggregate bonded by PBCF cementitious material to prepare PG-straw lightweight mortar specimens.The PBCF specimens consisted of 50%phosphogypsum,35%mixture of slag and cement（at mass ratio of slag:cement=3:2）,and15%fly ash.The bulk density of the hardened mortar specimens was decreased with the increase of SP adding amount.The flexural strength and compressive strength of 7 d and28 d for the hardened mortar specimens were decreased with the increase of adding amount of SP.As SP adding amount being 30%,the hardened mortar specimen had the lower compressive strength at 3 d,and its compressive strength at 7 d was 10.67 MPa.The loss rate of its compressive strength was 37.13%.In addition,the dry shrinkage and the thermal conducting coefficient of hardened mortar specimens were also decreased with the increase of SP adding amount,and their softening coefficients were≥0.86.The water absorption rate was 11.61%,and the water resistance was good.The EP was added into the above mentioned mortar specimens to prepare LWM specimens,and the mortar had 30%SP adding amount.The increase of EP content could effectively reduce the bulk density,compressive strength at 28 d,and thermal conductivity of the LWM specimens.When the EP content was 24%,the bulk density of the LWM specimens was 1.520 g/cm³,as well as its compressive strength at 28 d was 16.14 MPa,and its thermal conducting coefficient was 0.186 W?m^-1?K^-1.On the basis of this LWM specimen of 24%EP adding amount,the addition of EPP could significantly further reduce the bulk density of the modified LWM specimens,but the compressive strength of the modified LWM specimens at 7 d and 28 d with the increase of the content of EPP.When the content of polybenzene particles was 50%,the bulk density of the modified LWM specimen was 0.848 g/cm³,its compressive strength at 7 d was 4.78 MPa,and its compressive strength at 28 d was 6.78 MPa.In addition,the water absorption rate and thermal conducting coefficient of modified LWM specimens both were decreased with the increase of the EPP content.When the EPP content of modified LWM was 50%,its thermal conducting coefficient was 0.155 W?m^-1?K^-1,which can be used as the lightweight insulation wall material.