# Characterization of Natural Gypsum Materials and Their Composites for Building Applications
## 3 Results and Discussion  
### Thermal Analysis and Chemical Composition  
Thermal analysis techniques were firstly used to obtain information about thermal stability of natural Moroccan gypsum materials. A two-step dehydration process was suggested by a significant change in the slope of the weight loss curve (Figure 5). For the SG sample, the main thermal effect was observed at 140 \({}^{\circ}\)C, with a total weight loss of 20.5 wt %. The AG material presented two thermal effects: the first mass loss of 19 wt % was observed before 160 \({}^{\circ}\)C, assigned to water evaporation, whereas the second one was observed at 712 \({}^{\circ}\)C, with a small weight loss of 3 wt % related to the combustion of little carbonates or organic matter, such as confirmed by chemical analyses. For both the natural gypsum samples, the main thermal effect corresponded to the conversion of dehydrated calcium sulphate CaSO\({}_{4}\) 2H\({}_{2}\)O to the plaster CaSO\({}_{4}\) 1/2H\({}_{2}\)O, but the presence of oxide or carbonate impurities can affect this conversion. The crystallographic structure of AG and SG samples was examined by XRD patterns, at different temperatures of calcination, in order to study the structural change during the dehydration phenomenon (Figure 6). The high intensity of the XRD peaks indicates that the AG and SG powders have a high crystallinity. Results show that the crystalline structure of these solids changed with the temperature of calcination. A pure CaSO\({}_{4}\) 2H\({}_{2}\)O phase was detected for the two raw gypsum products, while the heat treatment between 150-360 \({}^{\circ}\)C favored the formation of a hemihydrates CaSO\({}_{4}\) 1/2H\({}_{2}\)O monoclinic system. Beyond 360 \({}^{\circ}\)C, an orthorhombic phase of anhydride CaSO\({}_{4}\) was identified, such as that reported in [27; 28]. The vaporization of the H\({}_{2}\)O molecule leads to the change of the crystalline structure and the formation of the anhydrous CaSO\({}_{4}\). Using FT-IR spectroscopy, the associated absorption bands of sulfate of gypsum were detected around 1120-1150 cm\({}^{-1}\) and 669-604 cm\({}^{-1}\), in addition to those of the stretching vibrations of the H\({}_{2}\)O molecules, between 355-3410 cm\({}^{-1}\) (Figure 7). During dehydration, the intensities of the OH gypsum bands decreased strongly, until there was a total disappearance of the OH-stretching vibrations. In increasing the dehydration temperature, the absorption bands of the OH (H\({}_{2}\)O) shifted towards the high frequencies, with a maximum shift of 50 cm\({}^{-1}\) when the temperature changed from 20 \({}^{\circ}\)C to 150 \({}^{\circ}\)C, reducing a strong structural transformation related to the dehydration process. Figure 5: Thermal Gravimetric TG and Differential Thermal Analysis DTA curves for gypsum materials. SEM images of both gypsum samples at different temperatures of calcination show the needle shape structure related to the calcium sulphate crystals (Figure 8). For the uncalcined and calcined powder at 150 \({}^{\circ}\)C and 360 \({}^{\circ}\)C, there were less dense porous agglomerates. On the other hand, at a calcining temperature of 1100 \({}^{\circ}\)C, the two samples AG and SG showed denser agglomerates than the ones at lower temperatures. The shape of the agglomerates depends on the origin of the gypsum: the Safi sample showed round agglomerates, while the Agadir one presented needle agglomerates. The chemical composition of the SG and AG powders was measured via X-ray fluorescence spectroscopy (XRF) (Table 3). Results revealed that the weight percentages of the constituent elements varied between the two gypsum samples, confirming that the CaO and SO\({}_{3}\) oxides are the major  
Figure 8: SEM images of SG and AG samples treated at various temperature of calcination. Figure 6: XRD diagrams of AG (**a**) and SG (**b**) minerals at different temperatures. Figure 7: Infrared curves for Agadir (**a**) and Safi (**b**) gypsum at different temperatures.