Investigations Into the Microstructure of Fresh Portland Cement Mortar

摘要

Specimens for backscatter SEM examination were prepared from freshly- mixed mortars, and from the same mortars hydrated for 6 hours, 12 hours, and 28 days. Small samples were fast-frozen in liquid nitrogen, sublimed at low temperature, and epoxy impregnated and polished. Reference specimens were also prepared from the same mixes at 6 hours and later, using conventional 60oC drying. Comparison of the freeze-dried and conventionally- dried specimens at 6 hours and beyond showed identical microstructural features. In the fresh state;the larger cement grains were found to be distributed inhomogeneously, 'pockets' of paste being either relatively densely packed with large cement grains, or largely bereft of such grains. These pockets gave rise to densely packed patches and porous patches in the mature hardened mortars. Layers of water on the order of 5 μm thick seemed to have been present around some of the sand grain surfaces in the fresh and very early age mortars. By 6 hours isolated crystals of Ca(OH)2 were deposited within these layers;subsequently they were filled in with more Ca(OH)2 and deposits of C-S-H. Several other features seen in fresh and very early-age mortars are briefly described.^