Thermogravimetric (TG) and differential thermal analysis (DTA) tests were carried out at room temperature to analyze the causes of mass loss and chromaticity changes of samples at high temperatures (Fig. 8). Variations in loess with temperature were observed to occur within three main stages: 23–200 °C, rapid decline of TG curve and unstable rise of DTA curve; 200–650 °C, steady decline of TG curve. DTA has changed from a steady rise to a steady decline. And 650–900 °C, the TG curve gradually stabilized and the DTA curve continued to decline steadily. These are described in more detail below.
(1) At temperatures of 23–200 °C, there is an obvious endothermic valley in the DTA curve (the mass loss is relatively obvious). The main change was the evaporation of free water, adsorbed water, and partially bound water from the samples (Li et al., 2015; Rau et al., 2014). The rate of water loss was rapid in this range, causing the highest mass-loss rate. However, changes in chromaticity were not obvious (Evrard et al., 2019; Cheng et al., 2019).
(2) At temperatures of 200–650 °C, from the DTA curve, it can be seen that the endothermic valley mainly appears in this stage too, which is in the state of mass loss. the main change was due to the precipitation of mineral structure water and crystal water. In this temperature range, the samples were completely dehydrated (Wang et al., 2000; Sun et al., 2016b). In addition, CaCO3 decomposition, emitting CO2 (Eq. 5). There was also a small amount of mass loss (Hajpál, 2002; Milheiro et al., 2005) and the mass-loss rate was low. At the same time, due to the higher temperature, the Fe2+ in the cement can be separated more easily. The Fe2+ gradually oxidized to Fe3+ and the color of the soil gradually tended towards red (Hu et al., 2013; Murad and Wagner, 1998; Manhães et al., 2002).
(3) At temperatures of 650–900 °C, it can be seen from the DTA curve that there is an exothermic peak in this temperature range, and the mass loss stops. The oxidation of iron became more thorough, with the intensity of the red color indicative of the degree of iron oxidation (Manhaes et al., 2002; Milheiro et al., 2005).
From the above analysis, we can see that before 650 °C, it is mainly the process of dehydration and the combustion of organic matter, as well as the decomposition of CaCO3, so that their original space is filled with air. The thermal conductivity of air is much lower than that of liquid and solid matter. Therefore, the thermal conductivity of the sample decreases rapidly at this stage (Paul et al., 2002; Fig. 9a).
At 650–900 °C, the main change of the sample is that the iron ions are gradually oxidized with the increase of temperature. The chromaticity changes obviously and the mass loss tends to be stable (Wang et al., 2021a). Therefore, we also carried out microscopic observation on the samples with binocular stereoscopic microscope and scanning electron microscope (Fig. 10). As can be seen from Figure 10a, there is no obvious crack in the sample at 500 °C. At 600 °C, obvious cracks appeared, while at 900 °C, the cracks increased and multiple cracks appeared. From Figure 10b, there are fewer pores in the sample at 500 °C. The porosity in the sample tends to increase at 600 °C. At 900 °C, the pore size in the sample increases, and the number of pores in the sample increases.
When heating up, the water in the sample evaporates, and the shrinkage of the surface is greater than that of the interior, which leads to cracks in the interior of the sample. In the cooling process, due to the fast cooling rate of the sample surface, the thermal stress caused by the difference between internal and external temperature also promotes the formation of cracks (Wang et al. 2003; Monteiro and Vieira 2004). The thermal expansion coefficient of different minerals is different, which leads to the deformation and evolution of adjacent minerals into fractures (Ahmad et al. 2008; Hummel, 2010; Wang et al., 2021b). Therefore, the increase of cracks and pores can reduce the thermal conductivity (Hideo and Hideyuki, 2004; Fig. 9b).
Figure 11 shows that in the range where there was a high mass-loss rate (23–300 °C), the thermal conductivity decreased rapidly, while in the ranges with lower mass-loss rates (300–500 °C and 600–900 °C) (Hiraiwa and Kasubuchi, 2000), the thermal conductivity of the loess samples decreased relatively slowly. This is mainly because the thermal conductivity of materials with lower moisture content is also smaller (O"Donnell et al., 2009; Duan, 2016). Overall, the mass-loss rate M of loess samples had strong linear relationships with thermal conductivity λ, thermal diffusion coefficient α, and specific heat capacity c. The models are as follows (Eq. 6; Eq. 7; Eq. 8):
λ = 0.875 - 6.352M (R2 = 0.935) (6)
α = 0.605 - 3.803M (R2 = 0.816) (7)
c = 1.543 - 4.481M (R2 = 0.913) (8)