Estimating the original cement content and water-cement ratio of Portland cement concrete and mortar using backscattered electron microscopy

In a previous study, a method was proposed to estimate the cement content, water content, free water-cement ratio (w/c) and degree of hydration of hardened Portland cement pastes that have unknown proportions. The method is based on measuring the volumetric fractions of capillary pores, hydration products and unreacted cement using backscattered electron microscopy and the volumetric increase of solids during cement hydration. It has the advantage that it is quantitative and does not require comparison with reference samples made with the same materials and cured to the same hydration degree as the unknown sample. However, the method was tested on neat cement pastes only, hence limiting its practical application. In this paper, the extension of the method to Portland cement mortars and concretes is presented, and results are provided, obtained from samples made with three cement types and a range of aggregate content (40-70% vol.), w/c ratio (0.30-0.70) and curing age (3-90 days). The study also involved sealed-cured, wet-cured and field-exposed samples. It is observed that bleeding and aggregate absorption can have a significant effect on the free w/c ratio. Overall, good agreement between the estimated and actual values is found. The error in estimating free w/c ratio using the proposed method is no more than 0.05 for all samples tested in this study.