Nature of the Chemical Bonding in CBCs

Properties of CBCs lie between ceramics and cements. These materials are formed at room temperature like cements, or may be synthesized at slightly elevated temperatures, but their structure is highly crystalline or glass-crystalline composite. The particles in CBCs are bonded by a paste formed by chemical reaction, as in cements, but the particles themselves are mostly crystalline. Their strengths are higher than those of cements but fall short of sintered ceramics. Their corrosion resistance is close to ceramics, but at the same time, they may be vulnerable to erosion like cements. The ease of formation of these ceramics, their rapid setting behavior and low cost make them very attractive for the various applications discussed in this book. 

Whether in minerals or man-made materials, the chemical bonding in CBCs is at room or warm temperatures, and this aspect distinguishes them from conventional sintered ceramics. Most of the CBCs are formed in the presence of water, though Wilson and Nicholson [8] have discussed several nonaqueous cements. In many of the aqueous CBCs, water is bonded chemically within their stmcture, but in some cases water may be expelled during the reaction. In aU cases, their formation is based on dissolution of individual components into an aqueous phase to form cations and appropriate anions. These ions react with each other to form neutral precipitates. If the rate of this reaction is controlled, then the reaction products will form network of connected particles and produce either well-ordered crystals or disordered structures. These CBCs comprise a composite of the crystallized and partly disordered structures. 

Some silicate minerals are also formed in a similar manner. The process is very slow, slower than even carbonate formation, because of the very low solubility of silicate minerals. In clay minerals, or in lateritic soils, silicates dissolve very slowly to form an intermediate product, silicic acid (H4Si04), which subsequently will react with other sparsely soluble compounds and form silicate bonding phases. Thus, a dissolution-precipitation process seems to be crucial to forming some silicate minerals. 

In hardened lateritic soils, the binding phase is so small that it cannot be isolated and identified. However, needle-shaped crystalhne growth was found in silicate-bonded red mud [22]. Such direct evidence is not available for natural soils, but the fact that soils harden when they are rich in alumina after wetting and drying cycles suggests that the dissolution-precipitation phenomenon controls hardening of these lateritic sods. 

There is evidence in the literature [14] of clay mineral forming in a laboratory environment. This is another example of CBC formation. Complex clay minerals such as montmorillonite [(Al,Mg)g(Si40io)3(OH)io-12H20], chlorite [(Mg,Fe)3(Si,Al)40io (OH)2 (Mg,Fe)3(OH)6], and serpentine [Mg3Si205(0H)4] have been synthesized in 

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