Study On Impact Of Sewage Sludge Ash On High Temperature Resistance Of Cement Paste

With the rapid development of sewage treatment industry in China,the sludge discharge amount is increasing year by year.As the by-product in sewage treatment,sewage sludge contains a large amount of toxic and harmful substance.The lack of safe and reasonable treatment and disposal of sewage sludge would result in serious environmental pollutions.The heating and incinerating treatment of sewage sludge can reach the reduction,harmless and stabilization of sewage sludge effectively.On the other hand,the residual sewage sludge ash after incineration is mainly constituted from Si O₂,Fe₂O₃ and Al₂O₃ and exhibits pozzolanic activity,which has a potential for the application in cement-based materials as supplementary cementitious materials.At present,the researches about the effects and the corresponding mechanisms of sewage sludge ash on the high temperature resistance of cement-based materials are still scarce.Considering the crucial significance of the high temperature resistance of building materials on the security of building structures and their users,the lack of researches in this field greatly restricts the applications of sewage sludge ash as building materials.On account of this,this research prepared cement pastes with various dosage of sewage sludge ash and investigated the mechanical properties after thermal treatment with different temperatures.The microscopic characterization and test techniques including XRD,FTIR,DTA,MIP and SEM were performed to s tudy the impacts of sewage sludge ash on the evolution of chemical components and microstructure of cement pastes under thermal treatment.Based on this,the influencing mechanism of sewage sludge ash on the high temperature resistance of cement pastes was clarified.The results suggest that cement pastes with the sewage sludge ash dosage of10%exhibit higher residual compressive strength and volume stability than control group,which means the high temperature resistance of cement pastes is improved considerably.Under high temperature conditions,ettringite,portlandite,calcite and C-S-H gel in cement paste are decomposed.This leads to the coarsening of pore structures and the increase of porosity and pore volume in cement paste matrix.With the increase of temperatures,the skeleton structures are destroyed.The microstructures of cement paste matrix are incompact and the defects and cracks are increased in the matrix,which result in a sharped degradation and invalidation of the mechanical properties of cement pastes.Under the thermal treatment at 600-800℃,the incorporation of sewage sludge ash promotes the pozzolanic reaction of sewage sludge ash and portlandite,which generates a certain amount of C-S-H gel to strengthen the matrix.Under the thermal treatment at 900-1000℃,the incorporation of sewage sludge ash promotes the formation of gehlenite,which shows a molten state under high temperature conditions to fill pores and microcracks in the matrix.These chemical composition evolution leads to the significant reduction of the increasing range of porosity,the coarsening degree of critical pore size and the width and depth of cracks in the cement paste matrix after thermal treatment.Hence,the degradation degree of the mechanical properties of cement paste is decreased and the high temperature resistance is enhanced.