LOW-TEMPERATURE SYNTHESIS OF HEXAGONAL ANORTHITE VIA HYDROTHERMAL PROCESSING

Hexagonal anorthite (CaAl2Si2O8) has been prepared by hydrothermal pro cessing of monocalcium aluminate and quartz at temperatures as low as 200-degrees-C. The successful development of this phase is dependent u pon several processing parameters, including the hydration of the calc ium aluminate precursor material to the hydrogarnet phase (Ca3Al2O6.6H 2O) prior to hydrothermal treatment and the use of quartz as opposed t o amorphous sources of SiO2. Quartz has partial solubility in the hydr ogarnet lattice for additions up to 40 wt%. Increased SiO2 substitutio n has been shown to reduce the conversion of hydrogarnet to Ca4Al6O13. 3H2O thereby increasing its thermal stability and improving its streng th characteristics at temperatures greater than 200-degrees-C. Quartz additions greater than 43 wt% lead to the formation of CaAl2Si2O8 as t he sole reaction product. The moderate temperatures involved in formin g this anhydrous material are an order of magnitude lower than those n ecessary to form this phase by melt crystallization, making it a true chemically bonded ceramic. The reaction can form a bonded matrix with strengths up to 40 000 psi (280 MPa). Strengths are limited due to den sity changes during anorthite formation, but the matrix is thermally s table up to 1000-degrees-C.