Solidification by Chemical Bonding in Nature

Chemical bonding as a means of solidihcation is very widely observed in nature. Formation of sedimentary rocks, such as carbonate rocks, is an excellent example. Carbonate rocks are formed by the reaction of calcium oxide with the carbon dioxide from the sea water [14]. Sea organisms also use this process and construct seashells. The organisms that flourish in calcium-saturated solutions of sea water change the alkalinity of the solutions slightly and precipitate calcium carbonate, which is used to form protective shelters such as shells and conches. 

Yet another example of natural chemical bonding is solidification of desert soils to build bird houses in Egypt. Nile silt was mixed with desert grass and river water to make tower shapes on concrete roofs (Fig. 1.2). Holes made in these towers provide shelter to the birds, which eventually are sold for food. The minerals from such rich soils consolidate when they are mixed with water and are dried in the intense sun of the desert. 

Solidification of minerals in carbonate rocks or in sea shells, or that of silica and alumina in lateritic or desert soils, is a very slow process. It takes years and centuries and even geological time to consolidate some minerals. Unfortunately, little is known on the exact chemical reactions and the resulting hardening process. As a result, the chemical hardening in nature cannot be translated into technological applications where accelerated hardening and solidification are desired. 

Temple at Angkor Wat in Cambodia built with thatched lateritic soil. (Photograph courtesy of Barbara and James Franch.) 

The exception is chemical reactions that form mineral rocks. These reactions have been explored by geochemists, and some interesting processes have been proposed to exploit them. A prominent example is the mineral accretion process, developed by W. H. Hilbertz, an architect, to build homes in the ocean [17]. Hilbertz proposes to use this process to 

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