Application of Automated Image Analysis to the Study of Cement Paste Microstructure
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Department of Civil Engineering, University of Kansas, Lawrence, KS 66045 ABSTRACT
Digital acquisition and analysis of backscattered electron images provide powerful tools for the study of cement-based materials. The techniques can provide useful information on hydration phases, size distributions of unhydrated particles and voids, effects of changes in the watercementitious material ratio and the use of mineral admixtures, and the distribution of microcracks. The results of automated analyses of cement pastes with different water-cement ratios and pastes containing silica fume are presented. The analyses demonstrate that microstructural data vary significantly from image to image, requiring multiple images to limit the effects of scatter. The analyses also indicate that, although the pastes exhibit different degrees of hydration, the size distributions of the unhydrated cement particles are nearly identical. In contrast, the size distribution of larger voids differs significantly as a function of water-cementitious material ratio and with the use of silica fume as a partial replacement for cement. The calcium hydroxide content obtained based on image analysis exceeds but generally parallels that obtained with thermogravimetric analysis. The majority of microcracks in both nonloaded and loaded specimens occur through or adjacent to the lowest density hydration phase. INTRODUCTION
Backscattered electron imaging of polished surfaces is an important tool in studying the microstructure of cementitious materials1 -5. It offers an advantage over secondary electron imaging of fracture surfaces, since it allows for the evaluation of a representative cross-section, rather than a "specimen selected" surface that represents the weakest portion of the microstructure. In spite of this advantage, backscattered electron (BSE) imaging is still subject to the same weaknesses as other imaging techniques using the scanning electron microscope (SEM), including poor selection of SEM settings, leading to an inadvertent loss of information, and the human tendency to emphasize "interesting" features for study. Automated image analysis offers the potential of fully utilizing the capabilities of backscattered electron imaging, providing the power to process the large amounts of data needed to characterize this highly heterogeneous material. Taking full advantage of this technique requires the development of strategies for image acquisition and analysis to insure that a minimum of information is lost and that the data obtained from the analysis is representative of the material being studied. The procedures discussed and this paper can provide information on hydration phases, size distributions of unhydrated particles and voids, effects of changes in the water-cementitious material ratio and the use of admixtures, and the size and orientation distribution of microcracks. This paper describes the approach taken in a large-scale study of cement paste microstructure and presents results for three cement pastes. TEST SPECIMENS
The cement past
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