Microstructures of reactive magnesia cement blends

The microstructures of reactive magnesia cement blends, consisting of mixtures of a reactive magnesium oxide (MgO), Portland cement (PC) and pulverised fuel ash (pfa), as observed with scanning electron microscopy are reported. To allow identification of the hydration products of MgO, mixtures of pfa and MgO only were also studied and mixtures of PC and pfa were included as a reference point for comparison. Mixtures with 90 wt% pfa, were found to have a microstructure, which consists of weakly bonded spheres stacked together; when the pfa content is reduced to 50 wt%, the microstructure changes to spheres packed into a solid matrix. For mixtures with a high MgO content, up to 50 wt%, however, the microstructures remain more open and particulate in nature due to the high water demand of reactive MgO. It is shown that these changes in microstructure are consistent with a simple estimate based on the volume fraction of solids formed during hydration. X-ray diffraction studies confirmed that in these blends MgO reacts mainly with water to form magnesium hydroxide, with hydrotalcite as a minor reaction product, and that for the reactive MgO used here hydration completes early on so that there should be no risk for damage due to later-age hydration. Moreover, comparison of X-ray diffraction patterns obtained from mixtures containing either PC and pfa or MgO and pfa with those of blends containing PC, MgO and pfa, shows little evidence of interaction between PC and MgO in terms of the hydration products that are formed.