Microstructural study of the interfacial transition zone in concrete using backscatter-mode scanning electron microscopy with image analysis
Abstract
In order to understand the behavior and to predict the properties and service performance of concrete, one has to first understand its microstructural features. A number of concrete models are based on the idea of the existence of an interfacial transitional zone (ITZ) around aggregate grains, constituting a third phase with properties different from the aggregate and bulk cement paste phases. Although the ITZ concept has been accepted in concrete technology for many years, the details of differences between the microstructure of the ITZ and the bulk cement paste have not been adequately established. In this study, the microstructure of concrete was studied using backscatter mode scanning electron microscope coupled with image analysis on polished surfaces. The specific features quantitatively evaluated included (a) pores greater than 0.25 μm, i.e. those detectable at the magnification used; (b) calcium hydroxide; and (c) unhydrated cement particles. The quantitative image analysis provide area fraction measurement of each of these as a function of the distance away from the aggregate surface. In addition, paste area not specifically identified as pore area, calcium hydroxide, or unhydrated cement was recorded separately as area fraction of ‘hydrated cement’. The presumed basis for the development of the ITZ is a local deficiency in cement grains near the aggregate surface. This microstructural effect is valid and shows up strongly in all the concretes examined. However the supposed development of zones of appreciably higher porosity and uniformly higher calcium hydroxide contents in the ITZ around the aggregate grains are not confirmed. It is found that there is only a small statistical excess of porosity near the interface. The local variations in porosity laterally at any given distance around any given aggregate grain are much greater than this modest statistical excess porosity. Calcium hydroxide contents also shows similar lateral variations around grains and also major differences from grain to grain. Some grains have an appreciable portion of their perimeters covered by calcium hydroxide deposits extending through a significant part of the width of the ITZ; adjacent grains may be mostly or entirely free of calcium hydroxide deposits. Models postulating that ITZs constitute a “third phase” that has significant and uniform excess porosity compared with bulk cement paste do not appear to be well based. Thus, the percolation concept applied on the concrete seems to be misleading.
Degree
Ph.D.
Advisors
Diamond, Purdue University.
Subject Area
Civil engineering|Materials science
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