Cement formation with ammonium polyphosphate

Sugama Kukacka (1983b) described cements based on magnesium oxide and a 56% aqueous solution of ammonium polyphosphate (APP). The po wder was a fine magnesium oxide that had been calcined above 1300 °C and had a surface area of 1 to 5 m g . The reaction was strongly exothermic the cements set within 3 minutes and developed an early strength of 13-8 MPa after 1 hour and over 20 MPa after 5 hours. 

9 Cement formation with aluminium acid phosphate 

Shinada Hiraoka (1974) examined cements formed by the reaction between magnesium oxide and concentrated aqueous solutions of aluminium dihydrogen phosphate. Later, Finch Sharp (1989) made a detailed examination of the cement-forming reaction and reported that the reaction yielded cements of moderate strength. 

They considered that cement formation was the result of an acid-base reaction leading to the formation of hydrates by a through-solution mechanism, by nucleation and precipitation from pore fluids. Two phases were found in the matrix, one amorphous and the other crystalline. The crystalline phase was newberyite. Finch Sharp concluded that the amorphous phase was a hydrated form of aluminium orthophosphate, AIPO4, which almost certainly contained magnesiiun. They ruled out a pure AlP04.nH20, for they considered that the reaction could not be represented by the equation 

As we have seen in Section 6.2, there is some evidence for supposing that zinc phosphate cements contain an amorphous aluminium phosphate or zinc aluminophosphate phase. Also, as we shall see in Section 6.5, amorphous aluminium phosphate is the binding matrix of dental silicate cement. 

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