CBPC matrix

The modem technological needs of stmctural materials are not fulfilled entirely by these two types of materials. There is also a need for materials that exhibit properties in between cement and sintered ceramics. That need can be met by CBPC matrix composites—materials that are produced like cements at ambient or at slightly elevated temperatures, but exhibit properties of ceramics. These composites are attractive for many stmctural applications, including architectural products, oil-field drilling cements, road repair materials that set in very cold environments, stabilization of radioactive and hazardous waste streams, and biomaterials. 

CBPC matrix composites are formed by incorporating a small amount of CBPC binder in a much greater amount of a second-phase material. These components are then mixed with water to form a slurry that will react and form the composite. Varying the properties of the additive alters the composite so that one obtains a range of products with tailored... 

CBPC matrix composites can incorporate a high volume of industrial waste streams such as fly ash, mineral waste such as iron taUings and Bayer process residue from the aluminum industry (red mud), machining swarfs from the automobile industry, and forest product waste such as saw dust and wood chips. Table 14.1 lists some of these waste streams and potential products or applications. 

Though the data given in Table 14.2 are for Ceramicrete matrix composites, similar properties in other CBPC matrix composites are possible with different extenders. Therefore, overall CBPC matrix composites are versatile materials and have the potential for varied applications as structural and nuclear materials, as well as civil engineering applications in general. In this chapter, we discuss some common CBPC matrix composites and their applications in the construction industry. Additional applications are presented in subsequent chapters. 

The SEM micrograph of the ash product in Fig. 14.5 reveals these new amorphous phases as a coating of continuous glassy phase on cenospheres. Commercially, such spheres are separated from ash and are used in cement composite to reduce the product density. Likewise, in CBPC matrix composites they help to reduce the overall product density. 

These studies by Wagh and coworkers with glass hbers are only an indication of how hber-reinforced composites may be developed using CBPCs. Because of neutrality of the CBPC matrix and its formation at room temperature, a range of hbers may be added in the matrix that include natural hber (such as wood, cellulose, and cotton) and artihcial hbers (such as nylon). The greatest potential is in wood composites. Unlike the case of glass hbers, a bond should form between the natural hber surface and the CBPC matrix. This bond should produce superior hber-reinforced composites (cements). These areas are shll open for research, and hardly any work has been reported in the literature. 

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... 

This versatility allows one to develop CBPC matrix composites with specific properties required for niche applications (see Table 14.2), such as heavy Ceramicrete with iron oxide or light-weight Ceramicrete with cenospheres, 7-ray shield with iron oxides or any other heavy metal oxide, neutron shield with light elements such as boron, insulators with cenospheres and ash, and comparatively better conductor with metals. The remaining chapters in this book address some of the niche applications where considerable scientific... 

Most waste streams contain more than one contaminant. Some may have contaminants such as Hg, whose sulfide has a higher pAQp than its phosphate, and Ba, whose sulfide is soluble, but phosphate is insoluble. Even in these cases, sulfide treatment followed by phosphate ceramic formation is very effective. The sulfide treatment will produce insoluble HgS that will be microencapsulated in the phosphate matrix, but Ba will be converted partially into soluble BaS, which subsequently will dissolve and will be converted to insoluble phosphate in the CBPC matrix. Thus, the dual treatment is very effective even when several contaminants with varying solubility of sulfides and phosphates are found in the same waste. 

In another similar case, Wagh et al. [59] simulated Pu-contaminated ash from Rocky Hats (one of the DOE sites) by incorporating various hazardous contaminants, including Cr as Cr203, and stabilizing them in the CBPC matrix at a loading of 54 wt%. The TCLP results of their study are presented in Table 16.9. 

Salts of actinides are very common in waste streams. In particular, nitrates, chlorides, and sulfates are found in tank waste streams that were formed by neutralization of highly acidic solutions at several DOE sites, such as Hanford and Savannah River. The aqueous solubility of these salts is very high, and hence, it is a challenge to stabilize them. As we shall see in case studies, the CBPC matrix has good promise in handling these waste streams. 

Finally, some DOE sites also have stored hexavalent uranium fluoride (UFs). The approach to stabilize this compound is to calcine it to form stable uranium oxide. There is no study reported in the literature on treatment of fluorides using a CBPC matrix, but considering that fluroapatites are stable minerals, they should be applicable to stabilization of actinide compounds. 

Cs, Sr, and Ba are mostly found in salt waste streams as chlorides, nitrates, and sulfates and hence are soluble in water. Even Cs oxide is very soluble. Therefore, they readily react during phosphate stabilization and are chemically immobilized. We shall see in the case studies later in this chapter that such stabilization is very effective in a CBPC matrix. 

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. 

---