Study On Modification And Mechanical Properties Of Geopolymer Recycled Concrete

Geopolymer recycled concrete was prepared by using fly ash and mineral powder instead of traditional Portland cement as cementitious materials and recycled aggregate(RCA)instead of natural aggregate(NCA)as coarse aggregate.The recycled aggregate was modified by soaking in sodium silicate solution,polyvinyl alcohol solution and silane emulsion.Standard mortar test and orthogonal test were designed to explore the effects of alkali activator modulus and content,water-binder ratio,fly ash content and sand ratio on the performance of geopolymer concrete,and the best mix ratio of concrete was determined.Using recycled aggregate instead of natural aggregate,geopolymer recycled concrete is prepared according to the best mix ratio,and the effects of different modification methods and substitution ratio of recycled aggregate on the working and mechanical properties of recycled concrete are explained.The uniaxial compression test of recycled concrete is designed and the stress-strain curve is drawn.The compressive strength,elastic modulus and Poisson’s ratio are calculated to explore the failure mode of concrete.The microstructure of concrete fragments was observed by scanning electron microscope(SEM),and the failure mechanism was further clarified in combination with macroscopic failure morphology.The main conclusions are as follows:(1)Sodium silicate solution has no obvious effect on water absorption and polyvinyl alcohol solution has no obvious effect on apparent density.The apparent density of silane modified recycled aggregate increased,while the crushing index and water absorption decreased significantly.(2)The best alkali activator modulus is 1.2 and the alkali content is15%.The orthogonal test determined that the best ratio is water-binder ratio0.45,the content of fly ash is 30%,and the sand rate is 35%.(3)Compared with NCA concrete,the slump and slump-flow of RCA concrete increase,while the compressive strength and splitting tensile strength decrease significantly.After modification,the strength of concrete is improved,and the strength of concrete modified by silane emulsion is the highest.(4)The failure of recycled concrete specimen is large pieces,and the fracture surface runs through the recycled aggregate.After modification,the number of cracks in concrete increases but the width decreases,and the shedding phenomenon of surface concrete is improved.The cracks of silane modified concrete are small and distributed relatively evenly,accompanied by transverse and oblique cracks.(5)The results of scanning electron microscope(SEM)show that the interface transition zone between silane modified recycled aggregate and geopolymer matrix is compact and continuous,and there is no obvious crack in the transition zone(ITZ),indicating that the interfacial adhesion between recycled aggregate and geopolymer matrix is significantly enhanced after silane emulsion modification.