# A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure
## 4 Physical Properties of Foam Concretes
### Drying Strinkage  
Foamed concretes have the disadvantage of high drying shrinkage and are affected by foam volume, aggregate type, mineral additive, fiber content and water content. The cracking phenomenon is particularly related to the uneven volume change during the curing process due to the temperature difference caused by the heat of hydration under the thermally semi-adiabatic condition of the matrix [46].  
The shrinkage in foam concrete is a function of the foam volume. Therefore, it is related to the paste amount and paste properties. [PERSON] et al. [75] reported that although removing water from relatively larger artificial air pores does not improve shrinkage, artificial air voids may indirectly have some degree of effect on volume stability by enabling some shrinkage occurrence. This situation occurred more frequently at a higher foam volume [75].  
\begin{table}
\begin{tabular}{c c} \hline
**References** & **Unit Weight (kg/m\({}^{3}\))** \\ \hline
[78] & 868.8–2225 \\
[51] & 594–605 \\
[5] & 1679–2033 \\
[55] & 587–1040 \\
[10] & 650–1000 \\
[1] & 240–1350 \\
[79] & 800–1320 \\
[80] & 650–1000 \\
[9] & 300–850 \\
[81] & 100–300 \\
[57] & 280–1840 \\
[45] & 400–1200 \\
[11] & 970–1350 \\ \hline \end{tabular}
\end{table}
Table 6: Unit weight of foam concretes in the literature.  
Drying shrinkage increases with increasing density of foam concrete [59]. Higher amount of foam with lower densities reduces the amount of cement. Thus, the hydration products are reduced, and less shrinkage occurs [20]. The reason why the foam used has an effect on drying shrinkage may be the pore structure that occurs. It was reported that the pore size and pore attachment increased with increasing the amount of foam, thus decreasing the shrinkage [75]. A lower pore connection may help reduce drying shrinkage [36].  
An amount of shrinkage varying between 0.1% and 0.35% of the total amount of hardened foam concrete occurs. The main method used to prevent shrinkage is the use of fiber. The fiber-containing foam concrete resists shrinkage, resulting in less drying shrinkage. The increase in fiber content helps to increase the resistance to drying shrinkage and reduces the amount of shrinkage that occurs [61]. The type of fiber used may show different performance regarding shrinkage. [PERSON] et al. [60] reported the effects of PVA fiber and coir fiber on the drying shrinkage of foam concrete. The use of PVA fiber increases drying shrinkage. The reason for this was shown to be that early on, the PVA fiber retains water and shrinks, releasing the water as the concrete hardens. Reduced drying shrinkage was experienced with the use of coconut fiber. The fact that coconut fiber has water-retaining properties explains this situation. The use of sugar cane fiber also limits the changes in foam size, reducing soot shrinkage [61].  
FA has negative effects on drying shrinkage. In the study where FA was used as fine aggregate, there was an increase in drying shrinkage with the use of FA. This is due to the greater presence of free water in FA pastes and therefore more evaporating water [11; 75; 82]. The use of clay brick powder from construction and demolition waste as additional cement material was reported to improve the drying shrinkage behavior of foamed concrete [22]. Drying shrinkage of foamed concrete was reduced when silica sand was replaced with WMP. The reason for this may be the grain shape of the WMP and the pore size distribution of the foam concrete. As mentioned earlier, the pore structure affects drying shrinkage. The improvement in the pore structure reduces the drying shrinkage thanks to the decreased evaporation from the capillary pores [5].