Effect Of Autoclaved Curing Time And Mineral Admixture On Microstructure And Mechanical Properties Of UHPC

Ultra high performance concrete（UHPC）is an advanced cement-based material with excellent mechanical properties and durability.It has a broad application prospect in the field of large-scale civil engineering.The curing system and mineral admixture have significant influence on the formation of UHPC microstructures,thus affecting its macroscopic mechanical properties and durability.Based on the NSFC Straits Foundation"Preparation and Engineering Application Foundation of ultra high performance concrete"（No.U1305245）and project of National 973 Plan first topic“Microstructure Formation and Regulation of Long-life Concrete Materials under Harsh Environments”（No.2015CB655101）.Using XRD,SEM-EDS,MIP,NMR and other modern testing techniques,the effects of autoclaved time,mineral admixtures,and magnesium sulfate erosion on the mechanical properties of UHPC and the microstructure of cement paste was studied.The main research results are as follows:The effect of autoclaved time on the mechanical properties and the microstructure of cement paste of UHPC prepared by cement and silica fume were studied.Results show that,with the autoclaved curing time varying from 0h to 16h,the compressive/flexural strength of UHPC increases first and then decreases,the porosity decreases first and then increases,and the porosity value attain a minimum and the compressive/flexural strength attain a maximum at 12h.The best time for autoclave curing is 8h¹2h.The hydration products of standard cured UHPC cement paste include C-S-H,Ca（OH）₂,AFt,AFm and TAH.Hydration products of the autoclaved curing UHPC cement paste include the C-S-H,Tobermorite,α-C₂SH,Ca（OH）₂,AFm and TAH.As the autoclaved curing time goes on,the hydration degree of both cement and silicon fume increases.Simultaneously,the amount of Al[4],mean chain length,Al[4]/Si in C-S-H increase,whereas,the Ca/Si and the relative AFm decrease.Amount of TAH increases with autoclaved curing time,receives a maximum at 4h,then progressly decreases from 4h to 16h.In addition,the influence of autoclaved curing on UHPC is minuscule between 12h to 16h.The effects of mineral admixtures on the mechanical properties of UHPC and the microstructure of cement paste were also studied.Under the condition of autoclaved curing at 210℃for 8h,with the increase of silica fume content,the compressive/flexural strength of UHPC increases gradually,and the compressive/flexural strength of UHPC decreases with the increase of the content of fly ash cenosphere.With the increase of silica fume content,the amount of hydration productα-C₂SH decreases gradually,the hydration degree of cement increases,but the hydration degree of silica fume decreases.The Si-O-Si covalent bond of silica fume breaks the bond in the alkaline environment generated by cement hydration,then forming[SiO₄].It can serve as a bridge to connect two dimers,while Ca/Si of C-S-H decreases and MCL increases.At the same time,[SiO₄]produced by hydrolysis of silica fume can react directly with Al（OH）₄^-provided by C₃A dissolution and Ca²⁺to form C-（A）-S-H,which leads to the increase of Al[4]content and the increase of Al[4]/Si.With the increase of fly ash cenosphere content,the hydration degree of cement is increased,the Ca/Si of C-S-H decreases and the Al[4]/Si increases gradually,simultaneously the AFt and AFm progressly decrease.The effect of MgSO₄ erosion on mechanical properties and microstructure of UHPC surface（1²mm）cement paste was studied.10%MgSO₄ erosion resulted in a slight decrease in mechanical properties but a smaller decrease in UHPC.MgSO₄erosion had an inhibitory effect on cement hydration,but it promoted the hydration of the silica fume.Meanwhile,it had a weak decalcification and dealuminization effect on the C-（A）-S-H,resulting in the decrease of Ca/Si and Al[4]/Si,the increase of MCL.SO₄^2-conjugates Al³⁺of C-（A）-S-H and TAH generated AFt and AFm with the erosion of MgSO₄,resulting in an increase in AFt and AFm and a decrease in TAH.With the increase of silica fume content,the effect of MgSO₄ erosion on the microstructure of UHPC surface cement paste gradually weakened.The effect of MgSO₄ erosion on the microstructure of UHPC cement paste is weak,and UHPC has good resistance to MgSO₄ erosion.Silica fume can improve the corrosion resistance of UHPC to MgSO₄.The results of this study can provide a reference for the optimization and regulation of UHPC microstructure and its preparation.