Magnesium Phosphate Ceramics

Other silicophosphate cements that use cation-releasing silicates are based on wolla-stonite [33], and serpentinite [34,35]. Naturally occurring phosphate cements have also been known [36]. In these cements, silicates are sparsely soluble and release cations (Ca, and Mg ), which react with the phosphate anions to form hydrophosphates and eventually convert to phosphates. This process is similar to that involving zinc phosphate cements, in which hydrophosphates form first, then convert to phosphates during aging. 

As discussed earlier, ceramic is formed by the reaction of calcined magnesium oxide (MgO) with a solution of phosphoric acid or an acid phosphate in these products. The quick-setting reaction results in products similar to those found in zinc phosphate ceramics. The major products can be represented by the formula, Mg(X2P04)2 H2O or MgXP04 H20, where X is hydrogen (H), ammonium (NH4), sodium (Na), or potassium (K). The reaction products are listed in Table 2.2. 

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These observations with a range of magnesium phosphate ceramics suggest the following guiding principle for forming most stable ceramics ... 

Similar to calcium dihydrogen phosphate, one may use magnesium dihydrogen phosphate (Mg(H2P04)2-H20) as an acid phosphate [5]. Not many commercial applications of this product have been identified and hence its commercial availability has been limited. As a result, though this product can produce excellent ceramics, it can only be used in specialty products such as dental cements. Formation of magnesium phosphate ceramics with this acid phosphate is discussed in Chapter 9 and its use in dental cement in Chapter 18. 

The process of fabrication of zinc phosphate cements is very similar to that of magnesium phosphate ceramics. Direct reaction with phosphoric acid is fierce and needs to be slowed down. This is done by the following methods. 

Unlike magnesium phosphate ceramics, phosphate bonded alumina ceramics consist of particles, whose surfaces are coated with berlinite (AIPO4), a crystalline orthophosphate. The bonding phase AIPO4 [8] is formed by the chemical reaction between the phosphoric... 

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). 

One great advantage with phosphate bonded ceramics in biomaterial or dental applications is the phosphate ions in their structure. Bones contain calcium phosphate, and hence phosphate bonded ceramics are generally biocompatible with bones. While chemically bonded calcium phosphate ceramics have been difficult to produce, magnesium and zinc based phosphate bonded ceramics have been more easily synthesized and used as structural and dental cements. 

The literature review in Chapter 2 reveals that divalent metal oxides such as oxides of calcium, magnesium, and zinc (CaO, MgO, and ZnO) are the major candidates for forming phosphate ceramics. These oxides are sparsely soluble in acidic solution, and as we shall see in Chapter 4, they are the most suitable ones to form ceramics. In addition, following the methods discussed in subsequent chapters in this book, aluminum oxide (alumina, AI2O3) and iron oxide (Fe203), which are abundant in earth s crust have excellent potential to form low cost CBPCs. For this reason, we have provided relevant information on these oxides. Table 3.2 gives some details. 

Figure 9.4 shows the X-ray diffraction output of a magnesium-potassium phosphate ceramic in which boric acid was added as a retardant. The amount of boric acid was 1 wt% of MgO in the powder blend. The X-ray diffraction pattern indicated that the polymeric coating on the MgO particles was a low-solubility magnesium-boron-phosphate compound, called liinebergite ... 

The literature cites several other Mg-based phosphate ceramics or cements that use different acid phosphates or salts of magnesium. Connaway-Wagner et al. [9] reacted ammonium tripolyphosphate with calcined MgO and formed a hard cement with strength of 13,000 psi (90 MPa). The reaction product was an amorphous phase of magnesium... 

A.S. Wagh, R. Strain, S.Y. Jeong, D. Reed, T. Krause, and D. Singh, Stabilization of rocky flats Pu-contaminated ash within chemically bonded phosphate ceramics, J. Nucl. Mater., 265 (1999) 295-307. M.M. Sychev, I.N. Medvedeva, V.A. Biokov, and O.S. Krylov, Effect of reaction kinetics and morphology of neoformation on the properties of phosphate cements based on magnesium titanates, Chem. Abstr., 96, 222252e. 

Paint pigments do not change colors on appHcation. Other common colors are violet from cobalt(II) phosphate [18475-47-3] pink from cobalt and magnesium oxides, aureolin yellow from potassiuim cobalt(III) nitrite [13782-01-9], KCo(N02)4, and cerulean blue from cobalt staimate [6546-12-5]. Large quantities of cobalt are used at levels of a few ppm to decolori2e or whiten glass and ceramics. Iron oxide or titanium dioxide often impart a yellow tint to various domestic ware. The cobalt blue tends to neutrali2e the effect of the yellow. 

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. 

These ammonium phosphates are made by reacting ammonium nitrate with phosphoric acid. The resulting compounds are very soluble in water. During formation of ceramics, ammonia is released and phosphate reacts with metal cations such as magnesium and forms the CBPC. Because of the evolution of ammonia, it is used for outdoor applications such as road repair material and hardly any indoor applications have been found for these products. 

S. -Y. Jeong and A.S. Wagh, Formation of chemically bonded ceramics with magnesium dihydrogen phosphate binder. Invention Report ANL-IN-99-037, 1999. Filed for patent. 

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