Applications of CBPCs

These observations imply that, forming a phosphate ceramic requires either diluted phosphoric acid or a partially neutralized phosphate solution as a source of anions, and a sparsely soluble (slightly soluble) oxide or a mineral to provide cations. All ceramics are formed in an aqueous solution. In general, the following scheme seems to work best. 

Phosphoric acid may be diluted with water. This step provides the water fraction needed to form the ceramic. Monovalent alkali metal oxides, with their high aqueous solubility, may be used for partial neutralization of the acid, while sparsely soluble divalent oxides are good candidates for providing the cations. In particular, oxides of Mg, Ca, and Zn are preferred because they are inexpensive compared to similar oxides, and unlike oxides of Pb, Cr, Cd, Hg, and Ni, they are not environmentally hazardous (see Chapter 16). 

Aluminum oxide is the only trivalent oxide that has been used to form a ceramic some heat treatment is needed. Kingery claims to have observed a setting reaction between trivalent iron oxide and phosphoric acid, but this reaction may have been caused by traces of magnetite in the trivalent oxide. Pure trivalent iron oxide such as hematite (Fe203) does not react with phosphoric acid. Overall, trivalent metal oxides have a solubility that is only marginal and falls below that of even sparsely soluble divalent oxides, while the solubility of oxides of most quadrivalent metals (zirconium is an exception) is too low to form a ceramic. 

Overall the cation donors remain the key parameter in determining formation of the ceramics in a diluted or partially neutralized phosphate solution. For this reason, Chapters 4-6 are devoted to a dissolution model for the formation of these ceramics. In Chapters 9-13, this model will then be used to discuss formation of ceramics from common oxides. 

Development of superior CBPC products for the wide-ranging applications shown in Fig. 2.1 requires a fundamental understanding of their kinetics of formation and their properties. This topic is extensively addressed in Chapters 4-6. The dissolution model described in these chapters also helps in understanding the role of individual components in formation of ceramics and the end performance of the ceramics. In addition, the dissolution model explains how hazardous and radioactive components are stabilized in a phosphate matrix. The stabilization mechanisms are discussed in Chapters 16 and 17. 

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Wide-ranging applications of CBPCs are possible, because, as mentioned in the preceding section, CBPC matrix composites can be made with very high loading of either waste... 

Chapter 16 Applications of CBPCs to Hazardous Waste Stabilization... 

To be more specific. Chapter 2 provides an overview of Chemically Bonded Phosphate Ceramics. It is intended to streamline the earlier literature and present it in a suitable context. Since the many potential applications of CBPCs are likely to alfect the raw materials (such as phosphates) market, an overview of the raw materials, their general properties, and their manufacturing processes is given in the third chapter. Chapters 4-7 are devoted to the theoretical basis for formation of phosphate ceramics by chemical reactions, and much of the discussion in these chapters is based on thermodynamics. 

The last five chapters of the book are devoted to major applications of CBPCs. Chapter 14 covers CBPC matrix composites that are finding commercial applications in the United States. Discussed in Chapter 15 are drilling cements developed mainly by the U.S. Department of Energy laboratories with industrial collaborations. Applications of CBPCs in the stabilization of hazardous and radioactive waste streams are discussed in Chapters 16 and 17. Finally, recent advances in CBPC bioceramics are covered in Chapter 18. Appendixes A, B, and C compile relevant thermodynamic and mineralogy data that were useful in writing the book. They serve as a ready reference to researchers who venture into further development of CBPCs. 

A quick review of literature on phosphate cements and ceramics indicates that very little attention has been paid to CBPCs, and a large opportunity exists in developing practical applications of these materials. Westman [1] conducted the first review of work done during 1918-1973 on phosphate ceramic and cement materials. This review appeared in Ceramic Abstracts in 1977. He concluded that only 7% of the total number of articles on cements, lime and mortars were on phosphate-bonded materials and also found only one... 

The survey, presented above, however, does not present the full picture of the recent research in the CBPC area. The Abstracts have not covered many modem CBPC applications such as those in radioactive and hazardous waste management. The purpose behind writing this monograph is to cover such areas in which CBPCs have made major inroads. In the process, we have built a discussion on the foundation of basic science and technology behind formation of these materials. We, therefore, hope that this monograph will be a comprehensive source for a wide readership interested in the science of CBPC materials and their applications. 

Once the hydrofluoric acid (HF) is removed, the end product is a solid mass of calcium hydrophosphate and gypsum. This solid is termed as normal super phosphate (NSP). Though this fertilizer is an inexpensive product, because available P2O5 in NSP is very small (5-8%), it is not a good raw material for economic production of CBPCs. However, TSP can be used in some applications. Details of TSP use in ceramic formation are discussed in Chapter 13. 

Aggregates are typically high volume, low cost materials available at every site where concrete is used. They form the bulk part of CBPCs in most applications. Being inexpensive, they reduce the overall cost of the product and hence are key to production of viable CBPC products. 

In the acid-base reactions that form ceramics, this alkaline region is of little interest to us, because the reaction products that constitute the ceramic are neutral, and hence, the reaction is not driven to the alkaline side. We will not elaborate on the reactions in alkaline regions, except in waste management applications in Chapters 16 and 17, where we discuss the stability of CBPC products in highly alkaline waters. 

CBPCs may have an important role even in the production of artificial implants. Typically, one may exploit rapid-prototyping to produce exact shapes of the implants. From a practical standpoint, formation of a ceramic out of a paste would appear to be most suitable for rapid-prototyping processes [11]. Thus, coupling CBPC with rapidprototyping should lead to artificial body parts that not only match the namral bones in their composition, but in structure as well. The science of CBPCs paves the way for their use not only as dental cements and bioceramics for the 21st century, but as discussed in earlier chapters, many other applications as well. 

Following these two surveys, we conducted a literature review based upon Ceramic Abstracts for the years 1988-2002. The results are summarized in Table 2.1. The results presented in Table 2.1 indicate that there has been a significant increase in the literature on CBPCs in recent years. The major thrust of the research has been in biomaterials and dental cements. Though small in number, there have been several articles in structural materials applications, which also include oil well cements. Interest in conventional refractory materials has continued, and as expected, all the applications have been supported by research in materials structure and properties of the CBPCs. 

Depending on the purity and concentration, phosphoric acid is sold in dilferent grades. Generally, commercial grade phosphoric acid is 70 and 85 wt% concentrated. The pH of this acid is zero and hence it is a strong acid. For all practical applications in forming CBPC products however, either this acid is diluted, or reacted with aUcah metals to form acid-phosphates with a pH > 1. Figure 3.1 illustrates formation of these acid-phosphates. 

Monopotassium phosphate (MKP) is formed by reaction of chloride or carbonate of potassium with phosphoric acid and the phosphate is derived as a crystalline material in a pure form. Its main commercial applications are, food ingredient in cold drinks and in detergents, and now CBPCs have provided a new avenue for its commercial use. 

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. 

The most common apatite is Ca5(P04)30H and is called hydroxyapatite. Other forms include chloroapatite (Ca5(P04)3Cl), fluoroapatite (Ca5(P04)3F) and carbonate apatite or dahllite (Ca5(P04)3C03). These minerals are in pure forms, but it is also possible to generate them by partial replacement of one anion by another or one cation by another. For example, Ca may be replaced by Pb by ionic substitution, yielding pyromorphites [Pb5(P04)3(0H,Cl,F)]. As we shall see in Chapter 16, this mineral is very important in stabilizing the hazardous metal Pb. Also as discussed in Chapter 2 and shall be seen in later chapters, Mg-based CBPCs have many applications, and hence minerals such as Mg5(P04)3(0H,Cl,F) are also very common. 

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