## 3 Results and Discussion
### Characterization and Properties of CRMFoams
#### 3.1.4 Mechanical Properties  
The mechanical performance of the fabricated carbon-red mud foams (CRMFoam-2.5 and CRMFoam-5) was assessed by performing compression strength tests according  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Foam**} & \multirow{2}{*}{**Pyrolizing**} & **Apparent** & **Bulk** & **Water Ab-** & **Apparent** & **Thermal** & **Compressive** \\  & & **Porosity a** & **Density b** & **sorption c** & **Specific** & **Conductivity** & **Strength** \\  & & **(\%)** & **(g/cm\({}^{3}\))** & **(\%)** & **Gravity d** & **(W.m\({}^{-1}\) K\({}^{-1}\))** & **(MPa)** \\ \hline CRMFoam-5 & 1000 \({}^{\circ}\)C, N\({}_{2}\) & 75.1 & 0.44 & 172.9 & 1.76 & 0.0400 \(\pm\) 0.0041 & 0.22 \\ CRMFoam-2.5 & 1100 \({}^{\circ}\)C, N\({}_{2}\) & 65.2 & 0.54 & 121.7 & 1.55 & 0.0675 \(\pm\) 0.0048 & 0.66 \\ \hline \hline \end{tabular}  
* The apparent porosity (P) is the volume of the open pores, into which a liquid can penetrate, as a percentage of the total volume of the specimen. b The bulk density (B) of a specimen in grams per cubic centimeter is the quotient of its dry weight divided by the exterior volume, including pores. c The water absorption (A) expresses as a percentage the relationship of the weight of water absorbed to the weight of the dry specimen. d The apparent specific gravity (T) is the ratio of the mass of a volume of the impermeable portion of a specimen at a stated temperature to the mass of an equal volume of distilled water at a stated temperature.  
\end{table}
Table 2: Properties of CRMFoams.  
to ASTM C 1424-99 [56]. Stress-strain curves were plotted (Figure 5) to calculate the compressive strength of foams with different red mud contents (CRMFoam-2.5 contains 40 wt% and CRMFoam-5 contains 20 wt% of RM), and the results are shown in Table 2. It should be noted that both samples were tested at 55% compressive strain, and the compressive strength as well as the modulus of elasticity of all samples were calculated from the linear region (red dotted lines) of the profiles. For better understanding of the RM content-density-compressive strength relationship triangle, the values of foams' bulk density along with their RM content and compressive strength are being quoted in Table 2.  
The CRMFoam-2.5 sample exhibited the best compressive strength with 0.66 MPa, whereas CRMFoam-5 sample showed compressive strength of 0.22 MPa. Regarding the behavior of the elastic modulus, it can be observed that the sample with less apparent porosity (CRMFoam-2.5, 65.2%), exhibited higher value of modulus (12.9 MPa), whereas the CRMFoam-5 (75.1% porosity) a value of 1.56 MPa, confirming the literature reports regarding the relationship between porosity and elastic modulus for ceramic foam materials [57]. Generally, the compressive performance of foam materials varies on several factors such as density, microstructure, and reinforcing agents. The RM presents higher density than the other constituents of the hybrid foams, and therefore by increasing the RM content, the bulk density of the foams increased, leading to enhanced compressive strength of them, and more stiff structure.