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 strengt h, the effects of a variety of condensed sodium phosphates (NaPO3)n, ( NaPO3)n . Na2O, Na5P3010, and (NaPO3)3 were studied. The influence of these additions on the kinetics of hydration was studied using isother mal calorimetry. The phosphatic additions enhanced reactivity, but x-r ay diffraction analyses did not reveal evidence of new crystalline pho sphate-containing hydration products. Microstructural evolution was ex amined in real time using environmental SEM, and hydration products ex hibiting distinct morphologies were observed. The features exhibited r anged from amorphic to polygonal shapes, plates, and fibers. These fre quently formed between crystalline calcium aluminate hydrate grains an d by doing so appear to provide a means to enhance the strengths of th ese cements. In spite of the morphological variations, companion energ y dispersive x-ray analysis showed that the compositions of these prod ucts did not vary widely. Their ranges of compositions are 52-60 wt. % Al2O3, 20-26 wt. % P2O5, and 20-24 wt. % CaO.