| Research on the formation and evolution of microstructure of blended cement paste isthe fundamental work to improve the properties of blended cement. Currently,non-evaporable water, selective dissolution, thermal analysis, heat evolution, quantitativeX-ray diffraction, and mercury intrusion methods are used to characterize the hydrationprocess and microstructure of blended cement paste. These methods can characterize thechange of only one component of blended cement paste, and cannot describe the evolution ofall components of blended cement paste. Backscattered electron image (BEI) analysis cancharacterize the evolution of all components of cement paste directly and quantitatively, but itis hindered not only by the complicated procedures and time-consuming during the specimenpreparation but also by the artifacts during the data analysis. In addition, BEI method can notbe applied perfectly in determining the composition and structure of blended cement due tosmall difference in gray of supplementary cementitious materials and hydration products. Inthis paper, based on the backscattered electron images of Portland cement and SCMs pastes,BEI method was improved by gray curve separation and establishing macro, etc., In addition,the improved BES-IA was applied to investigate the formation and evolution ofmicrostructure of blended cement paste. At the same time, the BEI-GSR and NMR were usedto investigate the composition and structure of hydration products of blended cement paste.Details are presented as following:Firstly, sample preparation process and backscattered electron image statistical analysismethod were improved. The sample preparation procedure was improved by injecting epoxyresin with low viscosity to fix samples and the refinement of polishing by using ultra-finepolishing powder. Consequently, the quality of backscattered electron images was improvedsignificantly. Phases in the paste were divided accurately by morphological filter, advancedfilter, and gray curve separation steps. The repetitive operations are completed by theestablishment of macro, not only effectively improving the speed of statistical analysis of theimage but also reducing the operation mistake.The BEI characters of Portland cement and SCMs were investigated. The grayheterogeneities of slag particles were concentrated in0~0.1, indicating that the backscatteredelectron images of slag are homogenous. The gray heterogeneities of Portland cement and flyash particles were concentrated in0.1~0.2and0.2~0.6, respectively, which indicates that thebackscattered electron images of fly ash is non-homogenous. The composition and structure of hydration products of Portland cement and SCMs wereinvestigated by using GSR, TEM, and NMR. Fiber-like C-S-H gels with Ca/Si and Al/Siranging from1.7to3.0, and0to0.3, respectively, were found in Portland cement paste.Silicon tetrahedrons in C-S-H gels mainly existed in the form of Q1and Q2, and aluminum inAl[6]. In contrast, flake-like C-S-H gels with low Ca/Si were generated during the hydrationof GBFS, their Ca/Si and Al/Si mainly ranged from1.0to1.5and0.3to0.5, respectively.Silicon tetrahedrons in Q2site and the amount of Al and Mg in hydration products of GBFSwere significantly increased. The content of Al[4] is significantly increased, the amount ofAl[4] nearly equaled to that of Al[6], indicating that the Silicon of the hydration products ispartly replaced by Al[4]. In addition, for a given cementitious material, The Ca/Si ratio ofouter-hydration products was slightly higher than that of inner-hydration products.Hydration degrees of cement clinker and SCMs in Portland cement, reference cement(prepared by inter-grinding the mixture of clinker and SCMs), and gap-graded blendedcement pastes were investigated by BEI method. The28days hydration degree of Portlandcement paste is66%, which is similar with that of reference blended cement, indicating thatthe cementitous properties of clinker in both Portland cement and reference blended cementwas not fulfilled. In addition, both the hydration degrees of GBFS and fly ash in referenceblended cement were only28.4%and15.9%, respectively. In comparison with referencecement, the hydration degrees of clinker and GBFS in gap-graded blended cement paste wereincreased significantly. For instance, the28days hydration degrees of clinker and GBFS ingap-graded blended cement paste were as high as92.0%and84.0%, respectively, indicatingthat the cementtious properties of clinker and GBFS in gap-graded blended cement pasteshave been fulfilled.The formation and evolution of blended cement paste were followed by BEI method.After28days, the volume contents of hydration products, un-hydrated phases, and porosity ofPortland cement paste were62.8%,17.2%and21.1%, respectively. Compared with Portlandcement, the amount of hydration products in reference blended cement paste was reduceddramatically, while the amount of un-hydrated phases and porosity were increased. Thevolume contents of hydration products, un-hydrated phases, and porosity were41.6%,32.9%,and25.6%after28days curing. For gap-graded blended cement paste, the amount ofhydration products was increased significantly, while the un-hydrated phases content andporosity were decreased. For example, the volume contents of hydration products,un-hydrated phases, and porosity were57.3%,23.8%and19%, respectively, after28dayscuring. In comparison with Portland cement paste, gap-graded blended cement pastes had comparable amount of hydration products, un-hydrated phases, and porosity. The difference isthat the un-hydrated phases in Portland cement paste were mainly clinker, while those ingap-graded cement paste were mainly SCMs with low cementitous activities (e.g., fly ash). Itcan be concluded that the microstructure of gap-graded blended cement paste was improvedsignificantly, which is similar with or denser than Portland cement paste, therefore theproperties of gap-graded blended cements were improved dramatically. |
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