Investigation And Application Of Micro Characterization Methods For Cementitious Materials

Cementitious materials are characterized with multi-scales and multi phases.Many macroscopic behaviors originate from the nano-scale properties of cementitious materials.It is fundamental requirement to understand the mechanism at the nano-scale level for further material tailoring to achieve better concrete.In this study,the characterized methods for the micro mechanical properties,microstructure and morphology of cementitious materials are investigated.The coupled techniques of the modulus mapping of scanning probe microscope（SPM）,the in-situ and discrete nanoindentation（NI）,the discrete microindentation（MI）,the back-scattered eletron microscope image analysis（BSE-IA）,energy-dispersive X-ray spectroscopy（EDS）,optical image analysis,X-ray computed tomography（CT）,mercury intrusion porosimetry（MIP）,thermal gravity analysis（TGA）,etc.are adopted to characterize the cementitious materials at nano-and micro-scales.The following aspects are discussed:Firstly,the application of SPM-modulus mapping in cementitious materials is proposed.The SPM-modulus mapping allows to measure the mechanical properties without causing plastic deformation to the materials and with much higher spatial resolution compared to NI technique.For better utilizing SPM-modulus mapping to identify and quantify the microstructure morphology of individual phases in cementitious materials,this study explored several critical aspects related to the experimental protocol and the data processing during SPM measuring for both ordinary Portland cement paste and slag-blended cement paste.Two data processing methods are proposed to extract the line data and the area data from the mapping images.The effects of the pixel spacing,the surface roughness of sample and the magnitude of loads on the measured mechanical properties are discussed.The results of this study will help to select the proper testing parameters for identifying different phases and quantifying their sizes under a nondestructive testing condition at a much higher resolution.Secondly,the vacuum impregnation method is adopted to prepare the epoxy impregnated concrete and paste samples.The aspects of microscopic properties of impregnated and non-impregnated samples are evaluated and compared in terms of the surface elevation,the indentation modulus,and the thickness of the interfacial transition zone between the aggregate and the paste matrix(ITZ_agg-paste)as well as that between the residual C₃S clinker and the hydration product(ITZ_C3S-HP).The results show that the epoxy impregnation is favored in the microscope image analysis,nanoindentation measurement and characterizing the ITZ_C3S-HP by SPM modulus mapping.It is not recommended for characterizing the thickness of ITZ_agg-pastegg-paste by SPM modulus mapping and for measuring the micromechanical properties at the paste scale by MI.Thirdly,the comprehensive investigation on the chemical-mechanical-morphological properties of CSH gel in both ordinary Portland cement paste and slag-blended cement paste is conducted by using the coupled techniques of SPM-modulus mapping,NI and EDS.The thickness of the inner C-S-H（IP）layer is precisely measured for the first time by modulus mapping,it varies with the types of unreacted cores as well as the addition of the supplementary cementitious materials.An interface transition zone（ITZ）is found between the unreacted C₃S grain and the surrounding inner C-S-H layer.Fourthly,the mechanical properties of the five types of C-S-H in OPC and slag-blended pastes are measured,and they are not significantly affected by their chemical compositions.A good correlation between the storage modulus and the indentation modulus of the individual phases is found.Moreover,the modulus at paste scale is measured by MI.And the differences between the measured paste modulus are found to be decided by the varied volume fractions of different phases at CSH scale.Fifthly,the effect of individual phases including the CSH,the residual clinker and the capillary pore on calculating the homogenized modulus of the hardened cement paste is quantified.It is found that the effective modulus of pore needs to be set as 12 GPa rather than0 GPa as the input for homogenization.As for the solid phases,the variation of the clinker modulus has limited influence on the homogenized modulus of the hardened cement paste.Whereas,the homogenized paste modulus increases significantly with the increasing fraction of high density CSH and the decreasing fraction of low density CSH.The results of this study can help to select appropriate input parameters for multiscale modeling the mechanical properties of cementitious materials.Finally,the coupled methods of TGA,BSE and NI are conducted to quantify the hydration degree of cement-slag system;the couple methods of MIP,BSE and CT are conducted to measure the distribution of pore in the paste sample influenced by the freezing and thawing.