Research On The Influence Of Recycled Powder On UHPC Performance And Microstructure

Ultra-high performance concrete(UHPC)is a new type of cement-based composite material with higher mechanical properties,better durability and toughness than ordinary concrete.It usually adopts cementitious materials,fine aggregates with different particle size gradations to achieve the tightest packing under the condition of extremely low water-to-binder ratio thereby preparing a cement-based composite material with excellent performance.However,the traditional UHPC cement has a large amount of cement,uses expensive raw materials,and has complex preparation and construction techniques,resulting in not many general engineering construction filed.Therefore,finding cheaper mineral admixtures to replace part of the cement and silica fume in the UHPC mix is one of the effective methods to solve the high cost and high energy consumption of UHPC.Recycled powder has a pozzolanic effect and can be used as a supplementary cementitious material to replace part of cement or silica fume,and it has a promoting effect on the hydration rate of cement.In this paper,aiming at some characteristics of recycled powder,on the basis of the tightest packing theory,the influence of two recycled powder namely concrete powder(CP)and brick powder(BP)on the mechanical properties and autogenous shrinkage of UHPC after replacing part of the silica fume is studied,and through X-ray diffraction,scanning electron microscope,nitrogen adsorption,nanoindentation and other microscopic experiments for mechanism analysis.The research results are as follows:(1)As the content of CP increases,the compressive strength and flexural strength first increase and then decrease,which is always lower than the control UHPC group doped with silica fume;as the content of BP increases,the compressive strength and flexural strength gradually decrease.Compared with CP,the UHPC strength of BP replacing silica fume is better.When the content is 15%,its 28 d compressive strength is the highest at 130 MPa,which is greater than the control UHPC group.(2)The effect of recycled powder content and water-binder ratio on the autogenous shrinkage of UHPC was studied.The results showed that the development of UHPC autogenous shrinkage value showed a trend of fast first and then slower,that is,the development rate of autogenous shrinkage was faster before 25 h,and gradually slowed down after 25 h.The content of recycled powder has a significant inhibitory effect on the autogenous shrinkage of UHPC.As the content of recycled powder increases,the autogenous shrinkage of UHPC is significantly reduced.When the content of CP and BP in UHPC reached 45%,the autogenous shrinkage was reduced by 51% and 55%,respectively.(3)Various microscopic test results such as scanning electron microscope and nanoindentation show that: After adding recycled powder,the internal structure of UHPC is denser,and the percentage of hydration product UHD C-S-H is relatively increased,which effectively improves the performance of the interface between the cement matrix and the fine aggregate,and improves the nanomechanical properties and interface transition of the cement matrix.The compactness and structural compactness of the area.At the same time,the incorporation of recycled powder improves the internal pore structure of UHPC and reduces the total porosity.