6 - Phosphate bonded cements

Summary

General

The phosphate bonded cements described in this chapter are the products of the simple acid–base reaction between an aqueous solution of orthophosphoric acid and a basic oxide or silicate. Such reactions take place at room temperature. Excluded from this chapter are the cementitious substances that are formed by the heat treatment of aqueous solutions of acid metal phosphates.

The most important of these are the refractory cements formed by the heat treatment of aluminium acid phosphate solutions. This subject has been well reviewed by Kingery (1950a), Morris et al. (1977), Cassidy (1977) and O'Hara, Duga & Sheets (1972). The chemistry of these binders is extremely complex as the action of heat on acid phosphates gives rise to polymeric phosphates, with P–O–P linkages, and these are very complex systems (Ray, 1979).

Here we are concerned with the cement-forming reaction between orthophosphoric acid solutions and basic oxides and silicates where the reaction is much simpler. Polymeric phosphates are not involved, there are no P–O–P bonds, and the structural unit is the simple [PO₄ tetrahedron.

Orthophosphoric acid solutions

Concentrated solutions of orthophosphoric acid, often containing metal salts, are used to form cements with metal oxides and aluminosilicate glasses. Orthophosphoric acid, often referred to simply as phosphoric acid, is a white crystalline solid (m.p. 42·35 °C) and there is a crystalline hemihydrate, 2H₃PO₄.H₂O, which melts at 29·35 °C.