SEM–BSE image analysis technique is one of promising approaches to determine water/cement ratios in hardened cementitious materials. In this study, a method using under–pixel information in the BSE images is proposed. It is hypothesised that a grey level of each pixel reflects the under–pixel porosity. It is also assumed that the porosity smaller than the resolution results in the shifts in position of the grey levels of specific phases. The amounts of shifting are combined with the Powers and Brownyard model so that the degree of hydration is directly estimated from the BSE images. The method was applied to hardened cement pastes with different water/cement ratios. There was a good agreement between estimated and real water/cement ratios. An estimate within an error of ±0.02 was attained except an extremely low water/cement ratio. The shift in the grey level histogram is useful information on microstructure in cement pastes.
Keywords: water–cement ratios, hardened cement pastes, under–pixel porosity, backscattered electron images, grey level histograms, mean atomic number, image analysis, calcium hydroxide, unhydrated cement, capillary pores, cement gel, Powers and Brownyard model, modelling, microstructure, concrete structures, concrete durability