Hydrothermal treatment of calcium aluminate—fly ash—sodium metaphosphate cements

Calcium aluminate cements, blended and unblended with class F fly ash, were modified with sodium metaphosphate solution and autoclaved in steam at 200 and 300°C. The 200°C-1 day autoclaved specimens with fly ash/calcium aluminate cement ratios (w/w) of 100/0, 90/10, 80/20 and 70/30, did not give crystalline reaction products. Increasing the temperature to 300°C led to the formation of crystalline hydrate compounds, such as Na-P-type zeolite, analcime, boehmite and hydroxyapatite. Although the hydrothermal cures were extended to 28 days, the unblended fly ash cements (100/0 ratio) has only the Na-P-type zeolite reaction product, together with unreactive mullite and quartz as major components of the fly ash. Incorporating calcium aluminate cement into the fly ash not only increasingly promoted the rate of reaction between the sodium metaphosphate solution and fly ash, thereby eliminating mullite and quartz, but also aided the formation of analcime, which was re~ponsible for strengthening and densifying the specimens. The combination of a well-formed analcime phase and moderately grown hydroxyapatite crystals gave a compressive strength of over 25 MPa and a porosity of less than 40%.