Acid-Base Cements CBCs

Geopolymers are another type of intermediate products that lie between cements and ceramics [7]. A geopolymer is made by pyroprocessing naturally occurring kaolin (alumina-rich clay) into metakaolin. This metakaolin is then reacted with an alkali hydroxide or sodium silicate to yield a rock-Uke hard mass. Thus, a chemical reaction, which is not fully understood, is employed to produce a hard ceramic-Uke product. Though this product is produced like cement, its properties are more like a sintered ceramic. It is dense and hard like a rock. 

These examples of CBCs are only a few examples of such class of materials. A much wider range of such materials that share attributes of cements and ceramics are formed by acid-base reactions. Such cements are discussed below. 

Acid-base cements are formed at room temperature but exhibit properties like those of ceramics. They are formed by reaction of an acid with a base. Normally this reaction produces a noncoherent precipitate. If, however, the reaction rate is controlled properly between certain acids and bases, coherent bonds can develop between precipitating particles that will grow into crystalline structures and form a ceramic. The acidic and alkaline components neutralize each other rapidly, and the resulting paste sets rapidly into products with neutral pH. 

Much of the initial development in CBCs occurred because of search for suitable dental cements. Wilson and Nicholson [8] have an excellent review of acid-base cements in their book with the same title. They discuss the following three types polyalkenoate, oxysalt, and phosphate bonded cements. 

These are mainly polymeric cements formed by bonding of polyions (or macroions)which are anions with small cations called counterions. Good examples are polycarboxylate cements [9], glass-ionomer cement [10], and polyphosphonic cements [11,12]. Zinc polycarboxylate, glass polyalkenoate, and resin glass polyalkenoate are some examples 

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To form a CBC, control over the dissolution of the bases is crucial. The bases that form acid-base cements are sparsely soluble, i.e., they dissolve slowly in a small fraction. On the other hand, acids are inherently soluble species. Typically, a solution of the acid is formed first, in which the bases dissolve slowly. The dissolved species then react to form the gel. When the gel crystallizes, it forms a solid in the form of a ceramic or a cement. Crystallization of these gels is inherently slow. Therefore, bases that dissolve too fast will rapidly saturate the solution with reaction products. Rapid formation of the reaction products will result in precipitates and will not form well ordered or partially ordered coherent structures. If, on the other hand, the bases dissolve too slowly, formation of the reaction products will be too slow and, hence, formation of the gel and its saturation in the solution will take a long time. Such a solution needs to be kept undismrbed for long periods to allow uninterrupted crystal growth. For this reason, the dissolution rate of the base is the controlling factor for formation of a coherent structure and a solid product. Bases should neither be highly soluble nor almost insoluble. Sparsely soluble bases appear to be ideal for forming the acid-base cements. 

Overall, this review of minerals and acid-base cements provides insight into formation of CBCs in which control over dissolution of at least one of the participating components is crucial to formation of the ceramic. For this reason, a considerable part of this book is invested in describing the dissolution chemistry of oxides that form CBPCs. 

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