Microstructural analysis of hardened alite paste

Abstract

New information about the pore structure of hydrating alite (impure tricalcium silicate) of 0.59 water∶alite ratio has been obtained by examining polished cross-sectional surfaces with a scanning electron microscope. Micrographs were used to analyse quantitatively the volume of porosity which was impregnated with epoxy and these results were compared with independent measurements of porosity using volumetric technqiues. The results suggest that pores less than 50 nm diameter are not impregnated by epoxy and that they may be incorporated as part of the pore structure of the calcium silicate hydrate; however, only the finest of these pores may remain in pastes formed at low water ∶ solid ratios. The calcium silicate hydrate formed in the late stages of hydration may not incorporate as much fine porosity as that formed during the early stages. The structure of the large “capillary pores” (i.e., pores greater than 50 nm diameter) changes with time from an open network to an array of disconnected pores. This study reveals that mature specimens are very heterogeneous and contain regions, hundreds of micrometres across, having few, if any, capillary pores. These regions are not found in young specimens.