Ceramic fibre-reinforced calcium aluminate/flyash/polyphosphate cements at a hydrothermal temperature of 280°C

The usefulness was assessed of two ceramic fibres, α-Al₂O₃ and α-Al₂O₃/mullite mixture, for improving the toughness of sodium polyphosphate-modified fly ash/calcium aluminate blended (SFCB) cement, which is used for completing geothermal wells. At the hydrothermal temperature of 280°C, the reaction between the α-Al₂O₃ phase-based fibre's surfaces and silicate in the cement slurry was responsible for the development of a moderate bonding at the interfaces between the fibres and the SFCB cement matrix. This development improved fracture toughness to 2·7 times more than that of non-reinforced cements. In contrast, adding α-Al₂O₃/mullite mixed phase-based fibres did little to improve the fracture toughness because of the transformation of the mullite into Na-P type zeolite. This mullite → zeolite phase transition was caused by the hydrothermal reaction of mullite with the Na in the cement. This transition generated a strong linkage between the fibres and the matrix; however, it also converted the strong, flexible and tough fibres into fragile, brittle fibres. Correspondingly, cracks generated in the cement composites propagated through the fibres, but not at the fibre/matrix interfaces.