Microstructural study of hardened cement paste by backscatter scanning electron microscopy and image analysis

The characterization of the microstructure of a material should include two aspects: the identification of the microstructural elements in the material, and the quantitative characterization of these microstructural elements. In this research, the microstructure of hardened cement paste (hcp) was studied using BEI on polished surfaces coupled with image analysis. The microstructural elements evaluated included (a) pores detectable at the magnifications used, (b) CH crystals, (c) residual unhydrated cement particles, (d) hydrated phenograins, consisting of both hydration shells and fully hydrated cement grains, (e) combined phenograins, i.e. (c) and (d) combined as they occur in individual features, and (f) the C-S-H in the groundmass. The quantitative analyses assayed include both area fraction measurement and geometrical measurement of individual features. The latter provides information on size, shape, size distribution and mutual arrangement of these individual features. Various hcps were investigated to quantitatively elucidate the effects of w:c ratio, age, and the incorporation of superplasticizer and silica fume on hcp microstructure. It was found that quantitative image analysis of hcp can yield interesting, and sometimes unexpected insights into the microstructure of hcp. The incorporation of superplasticizer and the incorporation of both superplasticizer and silica fume showed very strong effects on the hcp microstructure. On the other hand silica fume alone did not show strong effects. In another portion of this research, the fractal dimension characteristic of the pore system of hcp was investigated using an image analysis technique. It was found that the detectable pore system in cement pastes shows two fractal regimes: a structural fractal dimension exhibited at low resolution (D$\rm\sb{s} = 1.08\sim 1.15)$ and a textural fractal dimension exhibited at higher resolution $\rm (D\sb{t} = 1.22\sim 1.49).$ Finally, using a stereoscopic SEM method, it was found that the fracture surface of cement pastes shows two distinct fractal regimes: a structural fractal regime at low magnifications descriptive of the gross structural irregularities, and a textural fractal at high magnifications descriptive of the textural details of the fracture surface. The structural fractal and the textural fractal seem to have universal values of 2.02 and 2.12 respectively