A combined SEM–Calorimetric approach for assessing hydration and porosity development in GGBS concrete

A combination of semi-adiabatic calorimetry and Scanning Electron Microscopy (SEM) is employed for characterising the hydration process and pore structure development of cementitious pastes. The efficiency of this method is investigated by obtaining hydration curve parameters for four different concrete mixes manufactured using varying combinations of limestone blended cement (CEM II/A-LL) and ground granulated blast-furnace slag (GGBS) over a period of six months after casting. Embedded thermocouples recorded the internal temperature development associated with heat of hydration released in the first hours after casting. Hydration monitoring was continued by analysing SEM images taken from broken concrete specimens at various time intervals. Reliable hydration quantification using this approach requires the aggregate particles to be identified and filtered out of the image; this is achieved using a semi-automatic image processing methodology developed for detection and segmentation of aggregates from the concrete paste. Grey-level thresholding and the inflection point method are employed to determine the area fraction of the void space and assess porosity. Hydration degrees are then determined by applying thresholding methods to distinguish the hydrated and anhydrous cement particles. Corresponding hydration curve parameters were obtained based on the experimental data, and the resulting curves were compared with those obtained based on commonly used cement composition models.