Magnesium oxide phosphate cements

The most important of the phosphate bonded cements are the zinc phosphate, dental silicate and magnesium ammonium phosphate cements. The first two are used in dentistry and the last as a building material. Copper(II) oxide forms a good cement, but it is of minor practical value. In addition, certain phosphate cements have been suggested for use as controlled release agents. The various phosphate cements are described in more detail in the remainder of this chapter. 

Two methods are available for the preparation of the powder (Smith, 1969). In one, zinc oxide is ignited at 900 to 1000 °C for 12 to 24 hours until activity is reduced to the desired level. This oxide powder is yellow, presumably because zinc is in excess of that required for stoichiometry. Alternatively, a blend of zinc oxide and magnesium oxide in the ratio of 9 1 is heated for 8 to 12 hours to form a sintered mass. This mass is ground and reheated for another 8 to 12 hours. The powder is white. Altogether the powder is similar to that used in zinc phosphate cements. 

Wilson, Abel Lewis (1974), in a detailed chemical study of erosion in aqueous solution, found that in the first 24 hours of the cement s life the ions eluted were Zn, Mg, HPO " and H2PO4. Far more Mg ions were eluted than Zn ions, despite zinc being the major metal constituent of the zinc phosphate cement. These workers deduced that magnesium is far less firmly bound to phosphate than is zinc and that, consequently, its presence in the oxide is a source of weakness. These results were later confirmed by Anzai et al. (1977). 

Magnesium (or magnesia) phosphate cements are based on the reaction between ignited magnesium oxide and acid phosphates, which are generally modified by the addition of ammonium and aluminium salts. The phosphates may be either in solution or blended in solid form with the magnesium oxide. In the latter form the cement is formed by mixing the powder blend with water. 

Ando, 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. 

As discussed in Chapter 2, use of A1H3(P04)2-H20 was recognized during development of early dental cements. Finch and Sharp [6] studied the detailed chemistry of reaction of magnesium oxide and this acid phosphate that forms excellent ceramics. 

Calcium oxide is the main ingredient in conventional portland cements. Since limestone is the most abundant mineral in nature, it has been easy to produce portland cement at a low cost. The high solubility of calcium oxide makes it difficult to produce phosphate-based cements. However, calcium oxide can be converted to compounds such as silicates, aluminates, or even hydrophosphates, which then can be used in an acid-base reaction with phosphate, forming CBPCs. The cost of phosphates and conversion to the correct mineral forms add to the manufacturing cost, and hence calcium phosphate cements are more expensive than conventional cements. For this reason, their use has been largely limited to dental and other biomedical applications. Calcium phosphate cements have found application as structural materials, but only when wollastonite is used as an admixture in magnesium phosphate cements. Because calcium phosphates are also bone minerals, they are indispensable in biomaterial applications and hence form a class of useful CBPCs that cannot be substituted by any other. 

The reaction of phosphoric acid (usually in the form of ammonium phosphate) with magnesium oxide in the presence of water gives magnesium phosphate cements. These are used for rapid repairs, for example of pot holes in busy roads. 

Stierli, R. F., Gaides, J. M., and Tarver, C. C., Magnesia Cement Mixtnre Based on Magnesium Oxide and an Ammoninm Phosphate Capable of Reacting with the Mixture, German Patent 2,551,140,26 (May 1976)... 

These cements are based on the reaction product of phosphoric acid with other materials, such as sodium silicate, metal oxides and hydroxides, and the salts of the basic elements. Zinc phosphate is the most important phosphate cement and is widely used as permanent dental cement. It is also modified with silicones to produce dental-filling materials. Compressive strengths of up to 200 MPa are typical of these materials, which are formulated to have good resistance to water. Copper phosphates are used for similar applications, but they have a shorter useful life and are used primarily for their antiseptic qualities. Magnesium, aluminum, chromium, and zirconium phosphates are also used. ... 

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