Micromechanical Properties Of Cementtitious Materials Characterized By Nanoindentation And Multiphasic Analysis

Cementitious materials characterized of multi-phase and multi-scale are the most widely used building materials in the world.The performance of cementitious materials in structures is determined by their microstructures and micro-mechanical properties.Establishing multi-scale relationship between micro and macro properties of cement-based materials is a necessary way to realize performance-based material design in the future.Therefore,based on the experimental mechanics methods,the purpose of this study is to analyze the mechanical properties of micro-phases of cementbased materials and their influencing factors.Nanoindentation test was carried out on cement pastes with typical water cement ratio(w/c=0.2,0.4,0.6)and typical cement grade(P.O.32.5,P.O.42.5,P.O.52.5)to get the elastic modulus and hardness of them using deconvolution analysis.Also,to overcome the shortcomings of deconvolution analysis,cluster analysis method in machine learning was introduced to analyze the mixed experimental data of nanoindentation test.By means of material testing methods such as scanning electron microscopy(SEM)and X-ray diffraction(XRD),combined with data analysis methods such as gray level analysis,cluster analysis,XRD-Rietveld method and linear regression analysis,the microstructure and multiphase characteristics of cement-based materials were analyzed and the effects of test parameters,material parameters and high temperature environment on them were further investigated.The main results are as follows:1)The micro-morphology of cement-based materials was observed by SEM and found the micro-structure of cement-based materials became denser with decreasing water-cement ratio,increasing cement grade and increasing hydration degree.The phase identification and quantitative characterization of cement-based materials are carried out simultaneously by image processing.Also,the main mineral components of cementitious materials were analyzed by XRD,including C-S-H gel,CH crystal and unhydrated cement clinker and XRD-Rietveld method was used for quantitative characterization.2)The micro-mechanical properties of cement-based materials were tested by nanoindentation test and the elastic modulus and hardness of micro-phases of cement-based materials were obtained by deconvolution analysis and cluster analysis.The effects of cement-based material parameters,including cement strength grade,water-cement ratio and curing age,on elastic modulus,hardness and phase content were further analyzed.Finally,the empirical formulas of elastic modulus and hardness for cement strength grade,water cement ratio and indentation depth are obtained by linear regression.3)The changes of strength and mechanical properties of cement-based materials under four typical high temperatures(25 ℃,200 ℃,400 ℃,600 ℃)are analyzed from micro-scale to macro-scale and found the critical temperature range for micro-phase damage is around 300~400 ℃ that reveals the mechanism of high temperature affecting micro-phase.