Silicate clays decomposition

In contrast to the decompositions of many solids (including the hydrates discussed in the previous chapter), the dehydrations of hydroxides show some common patterns of behaviour in two broad groups the dehydroxylations of (i) simple hydroxides (Mg(OH)2, Ca(OH)2, etc.) and (ii) extended silicates (clays, minerals. 

Significant replacement of monovalent metal cations on layer silicate clay surfaces by protons can occur if the electrolyte concentration is very low. The long-term result, beyond hydrolytic exchange, is acidic decomposition of the clay structure in part, and release of structural AP or Mg + to solution. These multivalent cations may then readsorb onto exchange sites, influencing the rheological properties of clays in very dilute salts. Some of the anomalous behavior of Na -smectites suspended in... 

Layer silicate clay minerals typically dissolve incongruently, with the octahedral sheet being more susceptible to hydrolysis and decomposition by acid attack than the tetrahedral (silica) sheet. Consequently, base cations from exchange sites and from the octahedral sheet initially dissolve out of these minerals at much faster rates than silica. This means that the residue of weathering is typically a siliceous material depleted in Ca, Mg, K, and Na. 

Correns, C. W. (1963). Experiments on the decomposition of silicates and discussion of chemical weathering. Clays Clay Miner. 10, 443-459. 

The FR properties of polymer-layered silicate nanocomposites have been studied for a wide range of polymers, especially for organomodified montmorillonites (OMMT) in thermoplastics. Depending on the nature of the polymer, the decomposition pathway and decomposition products may change.8 A major consequence of the introduction of modified clays is the formation or the enhancement of charred structure, caused by cross-linking processes possibly catalyzed by the nanoparticles. 

Rapid increase in temperature is desirable at temperatures below those at which substantial liquid formation occurs (C9,B27,S21,C11,W9,G26). Most of the belite, and almost all of the other product phases, subsequently either melt or react in the presence of the melt, and there is no merit in promoting crystal growth or removal of imperfections, which would impede these processes. Slow heating may also allow the decomposition products of the clay minerals to transform into less reactive phases. It can also lead to the formation of microstructures unfavourable to the later reactions Chromy (C9) found that it allowed the belite shells around the silica particles to thicken, producing composites slow to react with lime. In contrast, rapid heating increases movement of the liquid phase, when this forms, and thus improves the mixing of the calcareous and siliceous constituents (Cl 1). 

Diatomaceous earth is composed of the siliceous skeletons of microorganisms. It is pozzolanic, but its use in concrete is much restricted by its very high specific surface area, which greatly increases the water demand. Some clays react significantly with lime at ordinary temperatures, but while this property can be of value for soil stabilization, their physical properties preclude their use in concrete. Many clay minerals yield poorly crystalline or anrorphous decomposition products at 600-900 C (Section. 3.3.2), and if the conditions of heat treatment are properly chosen, these have enhanced pozzolanic properties. Heat-treated clays, including crushed bricks or tiles, can thus be used as pozzolanas in India, they are called surkhi. Other examples of natural rocks that have been used as pozzolanas, usually after heat treatment, include gaize (a siliceous rock containing clay minerals found in France) and moler (an impure diatomaceous earth from Denmark). The heat-treated materials are called artificial pozzolanas, and this term is sometimes used more widely, to include pfa. 

Ash particles are formed through the thermal decomposition or dehydration of inorganic minerals associated with the coal. Calcium carbonate and clay are the most abundant mineral impurities, with lesser amounts of sulfides, chlorides and oxides also present. The shape of the ash particle is dependent on many factors, two of which are the amount of time and temperature to which the coal is exposed in the combustion chamber (Fisher et al., 1978). The spherical shape, most commonly associated with fly ash particles, shows that complete melting of silicates occurs at high temperature. These spheres may be solid, hollow (cenospheres) or encapsulating spheres (plerospheres). 

The mechanism of dehydration of layer silicates [51] is believed to differ from the mechanism of dehydration of crystalline hydrates and this has been used to explain the observed differences in parameters E (in vacuum) and in decomposition temperatures. This view is reflected, in particular, in the fact that the special term dehydroxylation is used in the literature to describe clay dehydration. However, this belief is not justified. As can be... 

When clay minerals are treated with dilute acids ( activation process), protons may attack the silicate layers via the interlayer region and exposed edges. Octahedral ions such as AP and Mg are extracted into the interlayer which promotes a rapid decomposition. However, many mont-morillonites resist such a treatment, even when using concentrated acids . Formation of layer defects allows the number of anchoring points for new cations at the surface layer to increase. 

The clay minerals are basically described as hydrous silicates and can be divided into two types on the basis of their natural locations, i.e., (i) residual clay, which is produced during surface weathering of rock by various means and is generally found in the place of origin, (ii) transported clay (or) sedimentary clay, which is produced by the chemical decomposition of rock and could be separated out from the original deposit through erosion... 

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