# Characterization of Natural Gypsum Materials and Their Composites for Building Applications
## 2 Materials and Methods
### Thermal Properties  
The thermal conductivity, the specific heat, and the thermal effusivity were measured using the Hot Disk Thermal Constants Analyzer Apparatus (Figure 2).  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Test** & **Investigated Data** & \begin{tabular}{c} **AG and SG** \\ **Sample Material** \\ \end{tabular} & \begin{tabular}{c} **Amount/Dimension** \\ \end{tabular} \\ \hline thermo-gravimetric analysis & thermal stability & powder & about 2 gr \\ chemical composition & crystallographic structure and & powder & about 2 gr \\ physical properties & \begin{tabular}{c} density and porosity \\ \end{tabular} & natural rock & 60 gr \\  & \begin{tabular}{c} powder \\ \end{tabular} & \begin{tabular}{c} 5 gr \\ \end{tabular} \\ thermal performance & thermal conductivity & natural rock & \(59\times 30\times 8\) mm\({}^{3}\) \\  & \begin{tabular}{c} powder \\ \end{tabular} & \begin{tabular}{c} 300 gr \\ \end{tabular} \\ mechanical properties & \begin{tabular}{c} transversal and longitudinal speeds, \\ Young and shear modules, coefficient \\ of Poisson, Vickers hardness \\ \end{tabular} & natural rock & \begin{tabular}{c} small pieces and \\ \(59\times 30\times 8\) mm\({}^{3}\) \\ \end{tabular} \\ Acoustic performance & \begin{tabular}{c} Absorption coefficient and \\ Transmission Loss \\ \end{tabular} & \
\begin{tabular}{c} gypsum powder mixed \\ with water and dried \\ \end{tabular} &
\begin{tabular}{c} cylindrical 29-mm and \\ 100-mm samples \\ \end{tabular} \\ \hline \hline \end{tabular}
\end{table}
Table 1: Characteristics of the examined samples and the related tests.  
The principle consists in imposing a uniform heat flux in a plane, separating two symmetrical samples (powder in a backer) from a large extension with respect to the probe and by placing the probe between them (in the middle of the samples). For the rocks characterization, the probe was placed between the sample (upper side) and a polystyrene pane (lower side). The hot disk method is based on the evaluation of the temperature trend on the sensor surface that is simultaneously a heat generator and a temperature probe. It consists of a double 10 \(\upmu\)m thick nickel spiral, sandwiched between two sheets of Kapton or Mica insulation, depending on the operating temperature. The use of nickel makes the level of accuracy very high, thanks to its well-known temperature coefficient of resistivity. A constant electrical power is enabled into the probe; the heat is generated and the temperature variation is measured by the spiral sensor; the temperature raises on the sensor surface, and the generated heat and the thermal properties (thermal diffusivity and specific heat) of the material surrounding the probe are calculated [18]. When temperature of the sample increases, the electrical resistance increases too, and the thermal conductivity can be obtained by multiplying the thermal diffusivity a (mm\({}^{2}\)/s) and the specific heat \(\gamma_{\mathrm{P}}\) (MJ/m\({}^{3}\)K) measured by the hot disk.  
In this study, the thermal conductivity tests were limited to room temperature (20 \({}^{\circ}\)C).