After exposure to high temperatures, clay undergoes a series of physical and chemical changes. The TA and TS samples which are heated in air become a brick red color at temperatures over 500°C because the iron-containing minerals in the clay are oxidized to Fe3+ (Murad and Wagner, 1998). At high temperatures, the Fe3+ cations can partially substitute for the Al3+ cations in the octahedral sites of the kaolinite structure (Manhães et al., 2002; Milheiro et al., 2005). A higher temperature means a more thorough reaction and a deeper red color. The NA sample which is heated in anoxic conditions is blue-gray in color because the organic matter in the clay is carbonized, and a series of chemical changes occur as a result, which is shown in Eqs. (1) and (2) (Toledo et al., 2004), thus resulting in a reducing environment in the crucible.
With a reduced firing temperature, the iron oxides are reduced to magnetite (Fe3O4) and wustite (FeO), both of which are dark in color (Houseman and Koenig, 1971). As the temperature rises, the blue-gray color begins to fade. This is because at a more rapid rate of heating, the Fe2+ cations react with a small amount of oxygen that has not yet been consumed, thus resulting in the formation of a small amount of Fe3+ cations which then partially substitute for the Al3+ cations in the octahedral sites of the kaolinite structure, so the Fe3+ ions are not reduced.
As for the core of the clay samples, the TA and TS samples which are heated in air at temperatures less than 800°C are grayish black. This is because the surface of the clay is first sintered, so that internally, it is a relatively closed anoxic system, and the water vapor inside cannot escape, thereby forming a reducing environment (Akinshipe and Kornelius, 2017), just like the NA sample. However, cracking penetrates the clay samples at temperatures that exceed 800°C, so that the inside of the clay samples is destroyed and exposed to the outside environment. Therefore, the core of the clay samples is no longer grayish black.
In order to understand the physical and chemical changes associated with heat, TG and DTG analyses were carried out by details of experiment here. The results are plotted in Fig. 9 which shows the TG curve of clay in a helium (He) atmosphere. At temperatures between 25-120℃, the TG curve slopes downwards rapidly which is mainly attributed to the rapid evaporation of the interparticle, adsorbed and interlayer water on the clay surface and interlayer space (Johari et al., 2010). At 300-600℃, the downwards trend of the TG curve is mainly due to the carbonization of the organic matter and the loss of the absorbed water in the clay samples. The absorbed water begins to evaporate when the temperature is above 300°C, and the organic matter begins to carbonize at temperatures between 400-500°C (Geng and Sun, 2018). At 600-730℃, an endothermic peak appears in the DSC curve, which is when the dehydration of the kaolinite occurs, as shown by Eqs. (3) and (4) (Sun et al., 2016b). At the same time, the melting of the flux components on the clay surface, such as potassium oxide (K2O), sodium oxide (Na2O) and calcium oxide (CaO), is initiated (Johari et al., 2010). Finally, at temperatures over 800℃, the clay mass does not change.
Temperature is a key factor that affects the shrinkage of the clay samples in this study. In general, increased temperatures result in an increase in shrinkage (Weng et al., 2003; Monteiro and Vieira, 2004). This is due to the loss of chemically and mechanically bound water (Karaman et al., 2006a). The capillary forces transport the liquid to the dried surface and shrinkage occurs as water is evaporated and the particles move closer together (Reed, 1995). At temperatures that exceed 700°C, the melting of the flux components such as K2O, Na2O, CaO on the surface of the clay is initiated; a liquid phase appears and the liquid fills the space between the solid particles (Johari et al., 2010). Due to the surface tension of the liquid phase, the unmelted particles are fused, which also causes volume shrinkage. There are two reasons for the formation of cracks. One is when the water evaporates, the rate of shrinkage on the sample surface is greater than the rate of internal shrinkage, which causes cracks in the sample. The second reason is that the thermal expansion of different minerals differs, so that thermal stress is produced between adjacent minerals, and deformation occurs between adjacent phases, which in turn evolves into cracks (Ahmad et al., 2008). Similarly, since the surface of the clay samples cools down at a faster rate and the inside of the clay samples cools down at a lower rate during the cooling process, the temperature differences between them produce thermal stress, which also promotes the formation of cracks. A higher thermal stress is produced when a sample is quenched in water, and therefore, the resulting cracks are much larger. In considering that some cracks will offset the shrinkage of soil samples, the rate of linear shrinkage is not entirely positively correlated with temperature in this study.
Shrinkage and sintering affect the hardness and compressive strength of clay (Manhães et al., 2002; Milheiro et al., 2005). The clay shrinks and the particles become more compact, so the hardness increases. At 700°C, the hardness of the samples increases due to melting and re-consolidation of the clay surface. As the temperature continues to rise, the clay surface is essentially unchanged, so the hardness does not change substantially. The shrinkage and sintering of the clay samples increase their compressive strength, while cracking reduces their compressive strength. At 700°C, cracking has not yet developed inside the clay samples, so the clay samples have the highest compressive strength when subjected to this temperature. With further increases in temperature, cracking develops inside the clay samples, and the compressive strength of the clay is reduced.
In addition, the temperature range of 700-800°C is an important range for clay because many of the physical properties (color, shrinkage, mass, hardness, and strength) greatly vary when the samples are exposed to this temperature range.