Research On Strontium Calcium Sulfoaluminate And Its Hydration And Cementitious Mechanism

Calcium sulfoaluminate(CSA)cement is characterized by rapid hardening,high early strength,tailored expansion and compacted harden pastes,which is suitable for repairing and strengthening concrete structures.C4A3? is the main phase in CSA cement.C4-x(?)xA3? can be synthesized through substitutions of Sr2+ for Ca2+ in C4A3?,which can produce strontium calcium sulfoaluminate cement with higher performance.Object of this study is C4-x(?)xA3? which is the main phase of strontium calcium sulfoaluminate cement.Through XRD,Rietveld method,TG and SEM-EDS,crystal structure,formation process,hydration characters,characteristic hydration products and cementitious characters of C4-x(?)xA3? have been studied systematacially,the purpose of this study is to solve theoretical problems in application of strontium calcium sulfoaluminate cement.In aspect of research on crystal structure for C4-x(?)xA3?,XRD,FTIR,SEM-EDS were conducted to test C4-x(?)xA3? with different Sr2+ substitutions,and method of Rietveld refinement for C4-x(?)xA3? was confirmed,the results show that with increasing strontic substitutions amounts,C4-x(?)xA3? series phases transform from orthorhombic system to cubic system,while the strontic substitutions cannot change types of functional groups.Sr2+ ions are directed to replace Ca2+ ions in C4A3?,and actual substitution amounts are approach to but slightly larger than Theoretical substitution amounts,ACn contents in clinkers can be neglected.With strontic substitutions amounts increasing,lattice parameters of C4-x(?)xA3? show linear increasing.For research on formation process of C4-x(?)xA3?,XRD was conducted to test clinkers of C4-x(?)xA3? sintered at different temperatures and prolong time,and contents of C4-x(?)xA3? in the clinkers were determined by Rietveld quantitative phases analysis,meanwhile formation process of C4-x(?)xA3? was analyzed by kinetic theory.Results show that Formation process of C4.x(?)xA3$ can be divided into three stages:decomposition of raw materials,formation of intermediate phases and generation of C4-x(?)xA3?.Increasing strontic substitutions prolong time for C4-x(?)xA3? formation.Solid-phase reactions for C3SA3? and C2S2A3? formation accord with three-dimensional diffusion model,and Jander model yields the highest correlation coefcients.Values of activation energy for C4-x(?)xA3? are larger than that of C4A3?,and with strontic substitutions increasing,values of activation energy increase.In aspect of hydration characters and characteristic hydration products of C4-x(?)xA3?,calorimetry was used to test heat release of C4-x(?)xA3? and C4-x(?)xA3?-C?·H2,hydration products at different ages were tested by XRD and TG,contents of phases in hydration prodcuts were identified by Rietveld quantitative phases analysis,meanwhile crystal structure and thermostability of Sr-bearing ettringite were analyzed.Results show that with strontic substitutions increasing,hydration rate of C4-x(?)xA3? decreases gradually.Compared to C4A3?,strontic substitutions change types of hydration products,and the mian crystalline hydration product of C4-x(?)xA3$ is AFt,meanwhile a small amount of CAH10 and SrSO4 can be also traced and amounts of CAH10 and SrSO4 increase with strontic substitutions increasing.Compared to C4-x(?)xA3? hydrating without sulfate,C4-x(?)xA3?-C?·H2 system reacts faster.Compared to C4A3$,more C?·H2 can be comsumed by C4-x(?)xA3$,and increased strontic substituions can reduce time for almostly consuming C$·H2.AFt and SrSO4 are the main crystalline hydration products for C4-x(?)xA3$-C?·H2 system;when C$·H2 is almostly consumed,AFm can be traced.W/C mainly affects contents of hydration products at different hydration ages,larger W/C leads to yielding smaller amounts of crystalline phases at early stage,but larger amounts of crystalline phases when hydration tends to be completed.Decomposition processes of AFt yielded by C4-x(?)xA3? and C4A3? in condition of rising and constant temperature are similar,strontic substitutions cannot change thermostability of AFt.For research on cementitious characters of C4-x(?)xA3? compressive strength of harden pastes for C4-x(?)xA3? with different Sr2+ substitutions amounts was tested,XRD,Rietveld quantitiative phases analysis and TG were used to identify compositions of the pastes,SEM and MIP were used to analyze microstructure of the harden pastes,results show that substitutions of Sr2+ would decrease early strength of harden pastes hydrated by C4-x(?)xA3?.However,with strontic substitutions amounts increasing,strength increases firstly and then decreases,and strength of C2S2A3$ is highest at 28d.Hydrating with C?·H2 would increase early strength but decrease intermediate and long-term strength.AFt phases tend to accumulate compactly in harden paste hydrated by C2S2A3?.In microstructure of harden paste hydrated by C2S2A3? and C?·H2,sizes of AFt phases tend to be larger but the compactness decreases,which would lead to more capillary pores.