Low-Temperature Synthesis of Hexagonal Anorthite via Hydrothermal Processing

Hexagonal anorthite (CaAl₂Si₂O₈) has been prepared by hydrothermal processing of monocalcium aluminate and quartz at temperatures as low as 200°C. The successful development of this phase is dependent upon several processing parameters, including the hydration of the calcium aluminate precursor material to the hydrogarnet phase (Ca₃Al₂O₆·6H₂O) prior to hydrothermal treatment and the use of quartz as opposed to amorphous sources of SiO₂. Quartz has partial solubility in the hydrogarnet lattice for additions up to 40 wt%. Increased SiO₂ substitution has been shown to reduce the conversion of hydrogarnet to Ca₄Al₆O₁₃·3H₂O, thereby increasing its thermal stability and improving its strength characteristics at temperatures greater than 200°C. Quartz additions greater than 43 wt% lead to the formation of CaAl₂Si₂O₈ as the sole reaction product. The moderate temperatures involved in forming this anhydrous material are an order of magnitude lower than those necessary 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 40000 psi (280 MPa). Strengths are limited due to density changes during anorthite formation, but the matrix is thermally stable up to 1000°C.