Mineral phosphate cements

Phosphate bonded ceramics have several advantages over their cements. Unlike polyalkenoate cements, phosphate based ceramics are entirely inorganic and nontoxic. Unlike Portland cement, which is formed entirely in an alkaline solution, these are acid-base cements, and are neutral. They are stable in a wider range of pH, and since they are made from natural minerals, the raw materials needed for their manufacture are readily available. For the same reason, they are also less expensive compared to other acid-base cements. They are self-bonding, i.e., a second layer will bond intimately to a first set layer. These attributes motivated further research into phosphate bonded materials for... 

The most common nonphosphalic accessory minerals associated with sedimentary phosphate rocks are quaife— clays, and two carbonates dolomite (CaMg(C03)2) and calcite (CaCOs). Carbonate-cemented phosphate rocks are particularly noteworthy because of their abundance. McClellan and Gremillion [14[ estimated that there are 3 tonnes of carbonate-cemented phosphate rock for every tonne of ore that could be handled by conventional milling and flotation processes (quartz- and clay-containing phosphate rocks). 

Even if the HAP remains the most important calcium phosphate, from an industrial point of view, the development of low temperature processes, particularly those concerning mineral cements and coating on metal, have led to the utihzation and emergence of other calcium phosphates which are more reactive. Table 12.1 lists the different calcium phosphates used as biomaterials. 

When freshly mixed, the carboxyHc acid groups convert to carboxjiates, which seems to signify chemical adhesion mainly via the calcium of the hydroxyapatite phase of tooth stmcture (32,34—39). The adhesion to dentin is reduced because there is less mineral available in this substrate, but bonding can be enhanced by the use of minerali2ing solutions (35—38). Polycarboxylate cement also adheres to stainless steel and clean alloys based on multivalent metals, but not to dental porcelain, resin-based materials, or gold alloys (28,40). It has been shown that basic calcium phosphate powders, eg, tetracalcium phosphate [1306-01-0], Ca4(P0 20, can be substituted for 2inc oxide to form strong, hydrolytically stable cements from aqueous solution of polyacids (41,42). 

Ceramicrete cures to create final waste forms that are analogs of naturally occurring phosphate minerals. These minerals have been shown to be relatively insoluble over geologic time scales. The final waste form is stronger than typical room temperature, hydraulic cements and performs in the manner of high-temperature fused ceramics. The technology has been evaluated in bench-and operational-scale tests on contaminated wastewater, sedimenL ash, and mixed wastes. 

In most applications, a small amount of binder powders is mixed with a large volume of inexpensive hllers and then the entire mixture is stirred in water to form the reaction slurry. For example, if the phosphate binders are used for manufacturing construction products, invariably the hllers are sand, gravel, ash, soil, or some mineral waste. The phosphate binders provide adhesion between the particles of these hllers and bind them into a solid object. Thus, these mixtures mimic conventional concrete mixmres in which Portland cement binder is mixed with large volume of sand and gravel to produce cement concrete. When phosphate binders are used, the products may be termed as phosphate concrete . In waste stabilization, the waste itself becomes the hller and the hnal product is termed as a waste form . 

The interatomic bonds that produce the crystalhne structures of minerals are briefly discussed first. This is followed by general mles used in constmcting models of crystal structures of phosphate minerals, then the crystal structures of orthophosphate mineral forms. The discussion is brief because the emphasis of the book is on practical aspects of novel phosphate ceramics and cements. Readers interested in more details are referred to Corbridge et al. [1] and Kanazawa [2]. 

Since calcium oxide is more than sparsely soluble and its reaction with phosphoric acid or a soluble phosphate is highly exothermic, researchers have used less soluble salts of calcium to react with the phosphates and form a phosphate ceramic [4-12]. In the acidic medium of the phosphate solutions, the salts of calcium dissolve slowly and release Ca (aq) into the solution, which subsequently reacts with phosphate anions and forms calcium phosphates. The best calcium minerals for forming CBPCs are combination of oxides of calcium and insoluble oxides such as silica or alumina, e.g., calcium silicate (CaSi03) and calcium aluminate (CaAl204), or even a phosphate of calcium such as tetracalcium phosphate (Ca4(P04)2 0). These minerals are reacted with acid phosphate salts to form phosphate cements. 

Table 13.2 lists phosphate cements developed using C S and C A as the starter powders. Semler [4] used a naturally occurring mineral, wollastonite (C S), as the source of Ca(aq) " and reacted it with H3PO4 solution that was buffered with A1 and Zn to produce a calcium phosphate quick-setting cement that hardened within 3-8 min and provided... 

The major interest in calcium phosphate cements has always been in their potential for biomedical applications. This is because bone contains hydroxyapatite (Ca5(P04)30H), a calcium phosphate mineral. Any material that could be used to bond bone or produce an artificial graft should contain this mineral for compatibility. In fact, much of the research in producing calcium phosphate-based cements or sintered ceramics was motivated by their biomedical applications. We will discuss applications of calcium phosphate cements in detail in Chapter 18. This section describes their materials development. 

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. 

The biocompatible CBPC development has occurred only in the last few years, and the recent trend has been to evaluate them as biocompatible ceramics. After all, biological systems form bone and dentine at room temperature, and it is natural to expect that biocompatible ceramics should also be formed at ambient temperature, preferably in a biological environment when placed in a body as a paste. CBPCs allow such placement. We have discussed such calcium phosphate-based cements in Chapter 13. Calcium-based CBPCs, especially those constituting hydroxyapatite (HAP), are a natural choice. HAP is a primary mineral in bone [3], and hence calcium phosphate cements can mimic natural bone. Some of these ceramics with tailored composition and microstructure are already in use, yet there is ample room for improvement. This Chapter focuses on the most recent biocompatible CBPCs and their testing in a biological environment. To understand biocompatible material and its biological environment, it is first necessary to understand the structure of bone and how it is formed. 

At present, there are only a few comprehensive publications on phosphate chemistry, minerals, and materials. Notable ones are Inorganic Phosphate Materials by T. Kanazawa [Kodansha, Tokyo, and Elsevier, Amsterdam (1989)], Phosphate Minerals by J. Nriagu and P. Moore [Springer-Verlag, Berlin (1984)], and a chapter on CBPCs in Acid-Base Cements by A. Wilson and J. Nicholson [Cambridge Univ. Press, Cambridge (1993)]. Much of the background information on phosphate materials is derived from these and other phosphate chemistry books. 

Figure 5 Three-dimensional plot of second tier zinc emissions to the US atmosphere for the period 1960-1995. Data from Councell et al. (2003), Nriagu and Pac3ma (1988), minerals.usgs.gov/miner s/pubs/commodity/cement/stat/tbll. txt, minerals.usgs.gov/minerals/pubs/commodity/phosphate rock/stat/tbll.txt, Pacyna (1986), USEPA (1998), Statistical Abstracts of the United States (1998), author s calculations.

Fig. 3.1.8. Sandy phosphorite from near Kursk, R.S.F.S.R. (so-called kurskite). The calcium-phosphate mineral forms both an anisotropic and apparently isotropic cement for the glauconitic sandstone. Magnification 67x. Reproduced with permission (McConnell,...

The method involving the Mo-V-P acid has been used in determinations of phosphorus in biological tissues [127], plant material [128], fruits [129], fish products [130], foodstuffs [131], phosphate minerals [132], cast iron and steel [133,134], niobium, zirconium and its alloys, titanium and tungsten, aluminium, copper, and white metal [135], nickel alloys [134,135], metallurgy products [136], molybdenum concentrates [137], silicon tetrachloride [7], cement [138], and lubricants[139]. The flow injection technique has been applied for determining phosphate in minerals [140] and in plant materials [141]. 

Calcium is found in rocks as the carbonate, fluoride, oxide, phosphate, and sulfate. In purified forms, each of these minerals has practical applications. Limestone, calcite, and marble occur in the form of calcium carbonate (CaC03) and are used as building materials. In addition, marble is carved to make statues. Portland cement—derived mainly from calcium carbonate and calcium silicates—is a staple of the building industry. Calcium carbonate is taken to relieve heartburn antacids like... 

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