Microstructural Aspects of the Fracture of Hardened Cement Paste

For microstructural studies, samples of hardened cement paste were fractured, impregnated with epoxy using a procedure that does not produce drying cracks, and examined using a scanning electron microscope. Cracks generally appear to pass around unhydrated cement grains and along calcium hydroxide cleavage planes. Cracks are often branched or forked at their tips, and a few microcracks are sometimes observed just ahead of the tips. Perhaps more importantly, the crack path is often offset, with the interjacent material between these offset crack segments forming a bridge across the crack. Even though the crack plane is most likely continuous in three dimensions, these bridging materials may provide local regions of ligamentary tractions across the crack, thereby providing a plausible explanation for the rising fracture resistance ( R -curve behavior) of hardened cement paste.