ABSTRACT
Phosphate cements are the main representatives of acid-base cements. In these the setting/hardening process is brought about by a reaction between a more acid and a more basic compound. The product is then a salt or a hydrated salt produced in this reaction. The acid reactant may be an inorganic acid or an acid salt (for example monoammonium phosphate, or acid phosphates and polyphosphates of ammonium and alkali metals), or even an organic chelating agent such as polyacrylic acid [(CH2-CH(COOH)-)J or eugenol [CH2=CH.CH2.C6H3(OCH3)H]. The basic constituent is usually a weakly basic or amphoteric metal oxide with a moderately small ionic radius (MgO, CaO, ZnO), but may also include other substances that are more basic than their acid counterpart. Examples of such materials are tetracalcium phosphate [Ca4(PO4)2O], wollastonite (CaO.SiO2), or an acid-decomposable aluminosilicate-type glass. Most reactions of this type take place at ambient temperatures, but some require an elevated temperature. Some of the hardened products produced by such reactions have been called chemically bonded ceramics because of their unusual properties, which were unattainable in the past by reactions taking place at these temperatures.
