Chemically bonded phosphate ceramics - A novel class of geopolymers

Geopolymers are cements or ceramics that mimic natural minerals. They are formed by polymerization of inorganic molecules containing aluminum, silicon, oxygen, and other elements. Under this broad definition, chemically bonded phosphate ceramics may also be considered as a new class of geopolymers. Phosphate ceramics are synthesized at room temperature, and they set rapidly like conventional polymers. They contain naturally occurring mineral phases, notably apatite. They are formed by an acid-base reaction between a metal oxide and an acid phosphate. Virtually any divalent or trivalent oxide that is sparingly soluble may be used to form these ceramics. They are hard and dense, and exhibit superior mechanical properties compared to conventional cement. They have found a wide range of applications such as dental cements, construction materials, oil well cements, and hazardous and radioactive waste stabilization. This paper summarizes the kinetics of formation of these phosphate geopolymers, their phase composition and unique properties, microstructure, and potential niche applications. Emphasis will be on apatite-forming geopolymers and their key role in various applications.