# A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure
## 6 Durability of Foam Concrete
### Elevated Temperature Resistance  
When exposed to high temperatures, foamed concrete experiences extreme shrinkage due to high evaporation rates. However, compared to normal concrete, foam concrete has an acceptable FR [57]. FR is related to the changes in the mechanical properties of foam concrete when exposed to high temperatures [3]. Generally, the compressive strength feature of foam concrete increases up to 400 \({}^{\circ}\)C. The reason is that high temperature stimulates the reactivity of the binders. However, the strength gradually decreases afterwards [17; 18; 24].  
As the temperature that foam concrete is exposed to increases, hardness loss occurs. It was reported that this loss of hardness starts after 90 \({}^{\circ}\)C regardless of the density [80]. It was reported that foam concretes with a density of 950 kg/m\({}^{3}\) can withstand fire up to 3.5 h and concrete with a density of 1200 kg/m\({}^{3}\) for up to 2 h [3]. Cavity structures help to reduce the effects of high temperature on foam concrete [94]. The pore structure of foam concrete is generally related to density, and it was reported that it is not affected by high temperatures. For this reason, the loss of strength at high temperatures is caused by the changing chemical components of foam concrete [80].  
Mineral additives and aggregates affect the properties of foam concrete after exposure to high temperatures. Pozzolanic additives can provide an increase in strength with an increase in temperature. The compressive strength increased after the foam concrete containing RHA and WMP was exposed to 200-400 \({}^{\circ}\)C. At temperatures above 400 \({}^{\circ}\)C, due  
\begin{table}
\begin{tabular}{c c c} \hline \hline
**Reference** & **Remarks** & **Equations** \\ \hline \multirow{2}{*}{[28]} & Sand as fine aggregate & E = 0.42 \(\mathrm{\upmu_{c}}\)\({}^{1.18}\) \\ \cline{2-3}  & Fly ash as fine aggregate & E = 0.99 \(\mathrm{\upmu_{c}}\)\({}^{0.67}\) \\ \hline \multirow{2}{*}{[109]} & E (kN/mm\({}^{2}\)), \(\mathrm{f_{cu}}\) (N/mm\({}^{2}\)) and \(\mathrm{\rho}\) (kg/m\({}^{3}\)) are the estimated elastic modulus, cubic compressive strength, and air-dry density, respectively. & E = \((\mathrm{\rho}/2400)^{2}\times(\mathrm{f_{cu}})^{1/3}9.1\) \\ \hline \multirow{2}{*}{[110]} & Density from 200 to 800 kg/m\({}^{3}\) & E = 5.31 W – 853 \\ \hline \multirow{2}{*}{[111]} & [PERSON]’s equation & E = 33 W\({}^{1.5}\)\(\mathrm{\sqrt{f_{c}}}\) \\ \hline \hline \end{tabular}
\end{table}
Table 11: Equations used for elastic modulus.  
to water loss in crystallization, changes in the Ca(OH)\({}_{2}\) content as well as changes in morphology and formation of micro cracks cause a decrease in compressive strength [18]. The thermal resistance of geopolymer foam concrete is evaluated on the changes in compressive strength and volume after exposure to high temperatures. [PERSON] et al. [17] worked entirely on foam concretes produced with a combination of FA and FA-slag. A 100% increase in compressive strength up to 800 \({}^{\circ}\)C was experienced in geopolymer foam concrete (GFC) with FA. However, in GFCs prepared with FA-slag combination, an increase in compressive strength up to 100 \({}^{\circ}\)C was observed, and then the compressive strength decreased. Because it is much more degraded with the loss of chemically bound water than gels rich in calcium formed by the FA-slag combination.  
Cracks occur in foam concrete as the exposed temperature increases. It was reported that cracks occur on the foam concrete surface after 400 \({}^{\circ}\)C and increase with the increase in temperature. At the same time, the cracks seen in foam concretes with high density are more numerous [14]. Moreover, the methods of cooling the samples (with air or water) affect the formation of cracks. It was observed that the slowly cooling (by air) samples had a greater tendency to crack. An increase in the amount of cracks increases the loss of strength [18].