Magnesium phosphate cement

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. 

The most important characteristic of the magnesium oxide powder used in these cements is its reactivity (Glasson, 1963). Magnesium oxide needs to be calcined to reduce this, otherwise the cement pastes are too reactive to allow for placement. Surface area and crystal size are important and relate to the calcination temperature (Eubank, 1951 Harper, 1967 Sorrell Armstrong, 1976 Matkovic et ai, 1977). The lower reactivity of calcined magnesium oxide relates to a lower surface area and a larger crystallite size. 

There are several types of cement and mortar (cement plus filler)  

Cement formation between MgO and various acid phosphates involves both acid-base and hydration reactions. The reaction products can be either crystalline or amorphous some crystalline species are shown in Table 6.5. The presence of ammonium or aluminium ions exerts a decisive influence on the course of the cement-forming reaction. 

These cements are soluble in water and so of no practical significance. 

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Hard-burned magnesias may be used in a variety of appHcations such as ceramics (qv), animal feed supplements, acid neutralization, wastewater treatment, leather (qv) tanning, magnesium phosphate cements, magnesium compound manufacturing, fertilizer, or as a raw material for fused magnesia. A patented process has introduced this material as a cation adsorbent for metals removal in wastewater treatment (132). 

Table 6.5. Crystalline phosphate species found in magnesium phosphate cements...

Figure 6.10 Action of heat on ammonium magnesium phosphate cements (Abdelrazig Sharp, 1988).

In particular we thank Dr J. H. Sharp for supplying original photographs for use in the section on magnesium phosphate cements and for critically reading the draft manuscript and making constructive suggestions. On clinical matters we have benefited from a 20-year collaboration with Dr J. W. McLean OBE. 

Magnesium oxysulfate cements, 75 417 Magnesium perchlorate, dessicant, 8 360 Magnesium peroxide, 75 415 78 395 Magnesium phosphate, 75 415 416 Magnesium phosphate cements, 5 500t 75 416... 

O. S. Krylov, I.N. Medvedeva, G.N. Kas yanova, Yu.P. Tarlakov, and S.A. Mertsalova, Characteristics of magnesium phosphates formed during hardening of magnesium phosphate cements, UDC 546.46 185, Translated from Isvestia Akademi Nauk SSSR, Neorganicheskie Materialy, 12 [3] (1976) 566—568. 

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. 

Magnesium Phosphate Cement Derived from Ammonium Dihydrogen Phosphate... 

The commercial magnesium phosphate cement SET-45, produced by Master Builders, sets in 15 min and hardens in 30-60 min. The compressive strength reached 24 MPa in the first hour and 47 MPa after 24 h. After 1 month the compressive strength reached 56 MPa (8122 psi). 

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