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 st eam at 200 and 300 degrees C. The 200 degrees C-1 day autoclaved speci mens with fly ash/calcium aluminate cement ratios (w/w) of 100/0, 90/1 0, 80/20 and 70/30, did not give crystalline reaction products. Increa sing the temperature to 300 degrees C led to the formation of crystall ine hydrate compounds, such as Na-P-type zeolite, analcime, boehmite a nd hydroxyapatite. Although the hydrothermal cures were extended to 28 days, the unblended fly ash cements (100/0 ratio) has only the Na-P-t ype zeolite reaction product, together with unreactive mullite and qua rtz as major components of the fly ash. Incorporating calcium aluminat e cement into the fly ash not only increasingly promoted the rate of r eaction between the sodium metaphosphate solution and fly ash, thereby eliminating mullite and quartz, but also aided the formation of analc ime, which was responsible for strengthening and densifying the specim ens. The combination of a well-formed analcime phase and moderately gr own hydroxyapatite crystals gave a compressive strength of over 25 MPa and a porosity of less than 40%.