Exothermal peaks clay minerals

DTA can be applied to identify and estimate various t)q)es of clays and calcareous minerals (Fig. l). These minerals can be differentiated by determining the temperature at which the endothermic and exothermic peaks occur. For example, kaolinites exhibit an endothermic peak at... 

Soils and clays, in general, when calcined give off adsorbed, interlayer, and hydrated types of water. These effects produce endothermal peaks or loss of weight in DTA and TG, respectively. The endothermal peaks are followed by exothermal peaks that are caused by re-crystalliza-tion. Although many types of clay minerals such as montmorillonite, illite, and some shales show these effects, they are not suitable as pozzolans in concrete. Metakaolin, formed by heating kaolinite, seems to be the most suitable additive material for cement. Heating of kaolinite involves removal of adsorbed water at about 100°C and dehydroxylation at above 600°C, followed by the formation of metakaolinite, an almost amorphous product. The sequence of reactions is as follows ... 

Many accessory minerals present in a elay influence its properties in the plastic and hardened states. Some of the organic and inorganic materials that may be present in elays inelude elarin, vitrain, marcasite, pyrite, quartz, gypsum, limestone, and dolomite. Their presence is indicated by endothermal peaks caused by deearbonation or exothermic peaks caused by oxidation. DTA curves of some of the accessory materials that may be present in a clay are shown in Fig. lO.P ... 

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