Study On The Influence Of Modified Phosphogypsum In Cement-based Wet-mixed Morta

In construction projects,a large amount of wet-mix mortar needs to be used,which leads to an increase in the demand for fine aggregates.However,the mining of natural aggregates will seriously damage the local ecological environment,and there is an urgent need to find a substitute for mortar aggregates.Phosphogypsum is a solid waste discharged during the production of wet process phosphoric acid,which can be used as a raw material for the preparation of cement-based mortar.However,when the amount of phosphogypsum admixture increases,there is a defect that the mechanical property of cement-based mortar decreases and the durability performance deteriorates.Seeking to increase the amount of phosphogypsum in wet-mix mortar will greatly enhance the resource utilization rate of phosphogypsum.Therefore,in this paper,phosphogypsum was firstly modified and pretreated to prepare a high-mix phosphogypsum wet-mix mortar,and the study on the durability performance of modified phosphogypsum in cement-based wet-mix mortar was carried out.Firstly,fresh phosphogypsum was modified by quicklime,aged for 7 d and sieved into four different particle sizes of 0-53μm,53-106μm,106-150μm and 150-212μm,and then the phosphogypsum of different particle sizes was mixed into cement-based wet mix mortar at a fixed admixture rate of 30%.The effects of different phosphogypsum particle sizes on the workability and mechanical properties of mortar were investigated,and the physical phase composition and microstructure of mortar were analyzed using TG-DSC,SEM and MIP.The results showed that.Phosphogypsum modified by quicklime can remove most of the soluble phosphorus and fluorine.As the particle size of modified phosphogypsum increases,the setting time of mortar shortens,the water retention rate and consistency loss first decreases and then increases,however,the tensile bond strength,compressive strength shows a trend of first increasing and then decreasing.When mixed with 53-106μm modified phosphogypsum,the mortar had 90%water retention,23 h setting time,19%loss of consistency,28 d compressive strength of 10.0 MPa and tensile bond strength of 0.25 MPa,and the performance indexes all met the requirements of M10 wet-mix mortar in JGJ/T 25181-2019"Ready-mixed mortar".The TG-DSC analysis showed that the C-S-H content in mortar gradually increased as the particle size of modified phosphogypsum increased.When modified 53-106μm phosphogypsum was incorporated,the C-S-H in the mortar increased significantly,and the amount of C-S-H in the mortar continued to increase slowly as the particle size of phosphogypsum increased further.The MIP analysis shows that with the increase of phosphogypsum particle size,the harmless pores and slightly harmful pores in the mortar gradually increase,however,the harmful pores and multi-harmful pores first decrease sharply and then increase slowly.When mixed with 53-106μm phosphogypsum,the mortar with the least harmful and multi-harmful pores and the least total pore volume was 0.1309 m L/g.Secondly,phosphogypsum was modified by wrapping method,lime modified phosphogypsum was formed into balls and the modified phosphogypsum balls were wrapped with cement and fly ash and maintained for 7 d to obtain modified phosphogypsum balls with different fineness modulus.The mortar was prepared by using modified phosphogypsum balls instead of fine aggregate,and the exothermic process of hydration,physical phase composition and microstructure of the mortar were analyzed by XRD,heat of hydration and MIP.The results showed that modified phosphogypsum balls with d≤0.3 mm would dissolve a large amount of SO₄^2-at 24 h,modified phosphogypsum balls with d≥1.18 mm would dissolve a large amount of SO₄^2-at 72 h,and modified phosphogypsum balls with 0.3-1.18 mm would dissolve a large amount of SO₄^2-at 168 h.the fineness modulus of phosphogypsum balls decreased,the number of fine particles with particle size d≤0.3 mm increased,the setting time of mortar was prolonged,and the 7 d compressive strength decreased;the tensile bond strength and 28 d compressive strength first increased and then decreased.When mixed with fineness modulus 2.5 modified phosphogypsum balls,the mortar has 91%water retention,16%loss of consistency at 2 h,18 h setting time,0.26 MPa tensile bond strength,6.8 MPa and 10.6 MPa compressive strength at 7 d and 28 d,respectively,meeting the requirements of M10 wet-mix plastering mortar in JGJ/T 25181-2019"Ready-mixed mortar".The analysis of heat of hydration,XRD and MIP revealed that the modulus of phosphogypsum ball fineness decreased,the heat of hydration exergy increased in the early stage of mortar and decreased in the later stage,and the intensity of C-S-H diffraction peak in mortar decreased significantly when the modulus of phosphogypsum ball fineness was less than 2.5.The fineness modulus of phosphogypsum balls decreased,the harmless pores and slightly harmful pores in the mortar decreased,and the harmful pores and multi-harmful pores first decreased and then increased.The total pore volume of mortar prepared using phosphogypsum balls with fineness modulus of 2.5 was the least,only 0.1409 m L/g.Finally,the dissolution rate of SO₄^2-in modified phosphogypsum balls of different particle sizes was measured first,and the durability performance of wet mortar mixed with modified phosphogypsum balls was examined.The mortar durability performance was analyzed by the SO₄^2-dissolution rate of modified phosphogypsum spheres,mortar shrinkage,freeze-thaw cycling mass and strength loss,dry and wet alternating strength change and softening coefficient.The results also showed that modified phosphogypsum balls with d≤0.3 mm would dissolve a large amount of SO₄^2-at 72h.The mortar with 28 d shrinkage variation was small,and the mortar with phosphogypsum balls mixed with fineness modulus 2.5 showed the best durability performance with minimum mass loss and mechanical property loss,1.1%mass loss for 25 freeze-thaw cycles,8.3%strength loss,0.85 strength coefficient for 15 dry and wet alternations,0.99 softening coefficient for 1 d,0.91 softening coefficient for 4 d,and 0.85 softening coefficient for 7 d.