Study On Microstructure And Mechanical Properties Of Reactive Powder Concrete

Reactive Powder Concrete?RPC?is a new type of ultra-high performance concrete,which exhibits excellent mechanical properties,durability,etc.,and has great application prospect in modern civil engineering field.RPCs are characterized by very low water-cement ratio and complex cementitious systems,moreover,heat curing and autoclave curing are often adopted,so the microstructure of RPC is quite different from ordinary concrete.At present,the research work on RPC is focused on the preparation technology and macroscopic performance,and the research on its micro-mechanism is still lack,which restricts the further optimization of RPC performance and further application scope expand.In view of the above problems,on the premise of mastered preparation technology,the basic mechanical properties of RPC were researched,and the hydration hardening mechanism and microstructure of RPC were mainly studied through a series of advanced testing techniques.The main research and achievements accomplished in this paper are as follows:Based on the traditional mix proportion and the maximum density theoretical model,the typical mix proportion of RPC was determined by combining with the actual preparation situation.The influence of the variation of mix parameters and heat curing regime on the mechanical properties of RPC were studied.Results showed that,reducing the water-cement ratio,controlling the sand-cement ratio,increasing the steel fiber volume fraction,and applying the heat curing can improve the flexural and compressive strength of the RPC specimens.Improving the heat curing temperature properly,and ensuring a certain constant temperature are favorable for the development of RPC strength.Uniaxial tensile test results showed that,the tensile strength,peak strain and initial elastic modulus of RPC are obviously improved with the increase of the steel fiber volume fraction.In addition,the tensile failure mode of the specimens changes from brittle failure to plastic failure,and shows excellent post cracking ductility.The concept of critical fiber volume fraction was introduced to evaluate the improving effect of steel fiber on the brittleness of RPC specimens,and the mechanism of cracking resistance,enhancing and toughening effect of steel fibers were explained.The hydration and hardening mechanism,as well as the microstructure formation of RPC under different curing systems were studied by means of XRD,NMR,FE-SEM,BSE-EDS and BSE-IA.Results showed that,the main hydration products in RPC are C-S-H gel,CH and AFt at standard curing regime.Under 90?heat curing regime,the activity of silica fume is obviously improved,and a large number of C-S-H gels with high degree of polymerization are formed,not only enhance the compactness of hardened paste,but also improve the microstructure of aggregate-cement paste interface transition zone,so that the mechanical properties of RPC has significantly improved compared with that cured under standard curing regime.Under autoclave curing regime,the activity of quartz powder has been activated,and C-S-H gels transforms into one or more crystalline phases such as tobermorite,jennite and xonotlite.The chemical composition and morphology of these crystalline phases depends on the composition of cementitious systems and autoclave temperature.With the increase of curing temperature,the hydration degree of RPC cementitious system is improved,and the porosity of hardened paste is further reduced.The transition of C-S-H gels,tobermorite and xonotlite,as well as the formation of microstructure of RPC hardened paste could be well explained from the hydration model of C₃S.