Magnesia-phosphate cements

There are several commercial rapid-setting cements used for the repair of airport runways and other eoneiete infrastructure where rapid setting and strength gain are mandated. These inelude magnesia-phosphate cements. 

Abdelrazig and co-workers studied the hydration (at 22°C) of MgO (75 g) and monoammonium phosphate (56 g) a composition ratio not dissimilar to commercial phosphate cements.P Mortars (Systems 1 and 2) and pastes (System 4) were investigated. The mortars (containing quartz sand) were prepared at water/solid ratios of 0.62 and 0.125 the pastes had water/solid ratios of 0.125. Thermograms (DTA) of mortar and paste are shown in Fig. 28a. The hydration time is one week. 

A small endotherm at 51°C is due to the dehydration of NaNH4HP04 4H20. The dehydration of the hexahydrate into the monohydrate (MgNH4P04 H20) is shown by the well-developed endotherm at 114°C. The small endotherms at 198°C and 232°C are attributed to the unreacted monoammonium phosphate and the dehydration of the monohydrate respectively. The sharp endotherm at 575°C is due to the a - jS polymorphic transformation of quartz. The exotherms at 614°C and 812°C are due to the crystallization of Mg2P207 and the formation of Mg3(P04)2 respectively. 

The compressive strength of mortar continues to develop for several weeks. If colloidal particles of hydrate form around nuclei of hexahydrate, it may account for continuous strength development. 

In mortars originally formed with both ADP and STPP, there is a small endotherm around 70°C. At early times, e.g., 5 minutes, the superposition of exothermic and endothermic effects (at about 107°C) results in a double endotherm indicating the presence of schertalite. A small exothermic peak above 8 00°C may be due to the formation of Mg(P04)2 coincident with an endothermal effect due to the melting of Na4P20y formed from STPP. 

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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. 

Abdelrazig, B. E. I. Sharp, J. H. (1988). Phase changes on heating ammonium magnesium phosphate hydrates. Thermochimica Acta, 129, 197-215. Abdelrazig, B. E. I., Sharp, J. H. El-Jazairi, B. (1988). The chemical composition of mortars made from magnesia-phosphate cement. Cement Concrete Research, 18, 415-25. 

Finch, T. Sharp, J. H. (1989). Chemical reactions between magnesia and aluminium orthophosphate to form magnesia-phosphate cements. Journal of Materials Science, 24, 4379-86. 

B.E.l. Abdehazig, J.H. Sharp, and B. El-Jazairi, Microstructure and mechanical properties of mortars made from magnesia-phosphate cement, Cem. Conor. Res., 19 (1989) 228-247. 

Abdelrazig, B.E.I. (1988) The microstructure and mechanical properties of mortars made from magnesia-phosphate cement. Cement and Concrete Research 18,415-425. Abdelrazig, B.E.I. (1989) The microstracture and mechanical properties of mortars made from magnesia polyphosphate cement. Cement and Concrete Research 19, 147-158. Abdelrazig, B.E.I, et al. (1984) Chemical reactions in magnesium phosphate cements. 

El-Jazairi, B. (1987) The properties of magnesia phosphate cement based materials for rapid repair of concrete. Proceedings 3rd International Conference on Structural... 

Hall, D.H., Steverrs, R., and El-Jazairi, B. (1998) Effect of water content on the structure and mechanical properties of magnesia phosphate cement mortar. Journal of the American Ceramic Society 81, 1550-1556. 

Sharp, J.H., and Winbow, H.D. (1989) Magnesia-phosphate cements, in Cement Research Progress (ed. W.E.Brown), American Ceramic Society, Westerville, OH, USA, pp. 233-264. 

The formation of magnesium ammonium phosphate monohydrate, MgNH4P04 H20 at low water contents was reported by Popovics, et al. t The use of excess water in magnesia-phosphate cement systems has been found to lead to increased porosity and strength reduction. 

Compressive strength, flexural strength, and the elastic modulus of magnesia-phosphate cement products are affected by composition and water content. The compressive strength, at different times, for seven phosphate-cement concretes is plotted in Fig. 

Figure 28. (a) DTA curves of magnesia-phosphate cement systems (b) DTA curves of... 

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