# A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure
## 6 Durability of Foam Concrete
### Thermal Conductivity  
The porosity and density of concrete are the two main parameters affecting the thermal conductivity value [51]. The change in the foam ratio affects the dry density, the change in the dry density affects the thermal conductivity [39]. As the dry density increases, the thermal conductivity increases.  
[PERSON] et al. [17], in their study investigating the mechanical, thermal insulation and acoustic properties of geopolymer foam concrete, determined that when the dry density increased from 585 to 1370 kg/m\({}^{3}\), thermal conductivity increased from 0.15 to 0.48 W/mK. The amount of porosity increases as the dry density decreases. Increase in porosity decreases thermal conductivity. Similarly, the w/c increase decreases thermal conductivity by increasing porosity [86]. In other words, thermal conductivity increases with dry density. GFC was reported to have better thermal insulation properties than Portland cement foam concrete (same density and/or strength).  
Thermal conductivity varies depending on the type of cement used and the foaming gas. The lower the thermal conductivity of the cement and the foaming gas used, the lower the thermal conductivity of the foam concrete [37,39,40]. [PERSON] et al. [37] studied the effect of foaming gas and cement type on the thermal conductivity of foamed concrete. For the research, foam concrete was prepared using four different foaming gases (air, hydrogen, oxygen, carbon dioxide) and three different cement types (MPC, SAC, OPC). The thermal conductivity of MPC-based foam concrete was higher than that of other cements. The thermal conductivity of foamed concrete using hydrogen foaming gas was the highest, and the one using carbon dioxide foaming gas was the lowest. The reason for this is that carbon dioxide gas has a much lower thermal conductivity (0.014 W/mK) than the ones for atmospheric (0.025 W/mK) and ammonia gasses (0.025 W/mK). Therefore, the use of carbon dioxide foaming gas is an effective method to improve thermal insulation [41]. Partial (30%) replacement of FA with cement helped to reduce the heat of hydration. The use of lightweight aggregates, with low particle density, among air voids artificially inserted into the mortar matrix, was advantageous in reducing thermal conductivity [1]. In the study performed by [PERSON] et al. [51], the thermal conductivity of foam concrete decreased with the RCA. This is thanks to the increased porosity with the use of RCA. The increase in porosity decreased thermal conductivity. Likewise, thermal conductivity decreased when RCA geopolymer was used in foam concrete. The uniform and increased amount of air void with the use of RCA may have provided this [19]. The SF improves the opening distribution, making the pores more uniform and closed circular, which increases the insulation performance [90]. The use of coir fiber reduced the thermal conductivity of foam concrete. Coir fiber has low thermal conductivity thanks to its high heat resistance. This can be shown as another example proving that materials with low thermal conductivity reduce the thermal conductivity of foam concrete. In addition, the formation of uniform air voids in concrete thanks to fiber addition is another factor that reduces thermal conductivity [60]. The thermal conductivity results of different studies are given in Table 12.