Hydration of sodium phosphate-modified high alumina cement

High strength can be achieved in high alumina cement (HAC) through the incorporation of phosphate-based additions at levels of 10 and 20 wt. %. In order to establish the mechanism that results in higher strength, the effects of a variety of condensed sodium phosphates (NaPO₃)_n, (NaPO₃)_n · Na₂O, Na₅P₃O₁₀, and (NaPO₃)₃ were studied. The influence of these additions on the kinetics of hydration was studied using isothermal calorimetry. The phosphatic additions enhanced reactivity, but x-ray diffraction analyses did not reveal evidence of new crystalline phosphate-containing hydration products. Microstructural evolution was examined in real time using environmental SEM, and hydration products exhibiting distinct morphologies were observed. The features exhibited ranged from amorphic to polygonal shapes, plates, and fibers. These frequently formed between crystalline calcium aluminate hydrate grains and by doing so appear to provide a means to enhance the strengths of these cements. In spite of the morphological variations, companion energy dispersive x-ray analysis showed that the compositions of these products did not vary widely. Their ranges of compositions are 52-60 wt. % Al₂O₃, 20-26 wt. % P₂O₅, and 20-24 wt. % CaO.