# A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure
## 4 Physical Properties of Foam Concretes
### Air Void Structure and Porosity  
The strength and durability properties of cement-based materials are affected by the porosity, permeability, pore size and distribution of the material [27]. If we consider the pore structure of foam concrete in general, it has three types of porosity: gel pores, capillary pores and air pores [83]. While capillary and air pores affect the strength properties of foam concrete, gel pores have no effect on the strength [27]. The pore distribution and size of the foam concrete directly affect its mechanical and physical properties. Therefore, the properties of the porous structure are very important for foamed concrete [20]. The SEM images in Figure 2 can be shown as an example of the pore structure formed by the addition of foam [84]. When compared to synthetic-based foaming agents, protein-based foaming agents create smaller and homogeneous air spaces at high foam concrete densities [8]. In the study conducted using synthetic, plant and animal glue/blood-based surfactant, the pore walls of foam concrete containing SS were thicker and less connected than others. The smaller pore size of the SS-containing foam concrete and its low pore connection enabled the foam concrete to gain features such as high compressive strength, low water absorption and stronger frost resistance [36]. The air voids in the hardened concrete have two different effects on the concrete. On the one hand, the strength decreases with the increase in air content in concrete. Additionally, in hardened concrete, a well-designed air system can improve freezing-thawing resistance [40]. Providing an optimal air void system in foamed concrete is very important to produce a material with a high strength/weight ratio [48]. The narrowness of the air voids contributes to the increase in strength. With the increase in the amount of foam, mixtures with narrower air void size distribution show higher strength [27,84]. Increasing the foam dosage increases porosity [59]. [PERSON] and [PERSON]'s [20] study compared the performance of protein and synthetic foaming agents. As a result of the study, it was reported that the size of the air voids increased with the decrease in the density of the foam concrete for both types of foam. The reason for this can be shown as the amount of foam increases, there is an increase in the amount of air voids, and therefore the overlapping of the air voids can be combined. The combined air spaces create a wide distribution of pores, which results in lower strength [27]. In the use of glass and plastic wastes as filling material, glass wastes provide more uniform distribution of voids and a less interconnected void structure. This is attributed to the finer glass waste powder. It was reported that thinner fill materials create uniform air pockets [85]. The use of fly ash as a filler provides a more homogeneous distribution of air voids compared to fine sand. It provides a good and homogeneous coating on the bubble, preventing bubbles from overlapping and coalescing, thus helping to distribute the air voids evenly [27]. [PERSON] and [PERSON] reported that FA [27], when used in foam concrete, provides a good homogeneous coating on each bubble, and that these coatings help the uniform distribution of air voids and prevent blisters from coalescing. These properties lead to higher strengths. The increase in the amount of water provides the enlargement of the air voids [15,80,86,87]. Growing pores have less void surface area. In addition, the increase in the amount of water increases the number of capillary pores. In fact, for the same porosity, the increased fraction of capillary pores leads to a decreased number of air pockets. The pore size is reduced by the use of SA. Increased amounts of SA also result in increased ability of the slurry to contain air bubbles and as a consequence, the size of the pores is decreased [15]. The use of superplasticizer helps in improving the void structure [84].