# A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure
## 5 Mechanical Properties of Foam Concrete  
### Compressive Strength  
Compressive strength is affected by parameters such as density, mixture components used, aggregate, mineral additive, water content, foam, curing and porosity. The amount of water has a significant effect on the compressive strength of foamed concrete. It was reported that small changes in the water content of foam concrete do not affect the strength as in normal concrete [1]. An increase in the w/c ratio can provide an increase in strength. The reason for this can be demonstrated by the formation of pores that grow with the amount of water. With the increase in large pores and capillary pores, the density of the air voids decreases and the strength increases [15]. [PERSON] et al. reported that the compressive strength of foam concrete showed an inverted V-shaped change with the increase in the water/cement ratio. If the w/c ratio is below the optimal limit, thinner-walled and irregular foams occur, and the compressive strength is negatively affected. The use of the w/c ratio above the optimal limit results in a poorer bubble holding ability. It causes pores to join and uneven distribution. This irregularity in the pore structure is subjected to stress concentrations. In addition, the increase in the amount of water triggers the formation of capillary channels, thereby reducing its strength [72]. The use of superplasticizer contributes to the increase in compressive strength by reducing the w/c ratio [85]. The aggregate type used is effective on the strength. [PERSON] et al. [51] used RCA that increased the porosity, and this situation caused a decrease in the compressive strength. Similarly, [PERSON] et al. [19] used RCA in geopolymer foam concrete. RCA content reduced compressive strength. This is because the strength of RCA particles is lower than sand. In addition, adding more water to obtain workability in RCA samples also negatively affected the strength. Thanks to the pozzolanic properties of FA, its use in foam concrete as fine aggregate contributes to the compressive strength [11]. [PERSON] and [PERSON]'s [79] study investigated the effect of the additive type on the compressive strength of foam concrete. The compressive strength of the samples using FA as filler material showed higher compressive strength values than those prepared with sand. Due to the late pozzolanic reaction of FA, the increase in compressive strength continues with the advancing age. When FA was used as filler, it was reported that the compressive strength of foam concrete was 1.7 and 2.5 times higher on the 56 th and 180 th days compared to the 28 th day [29]. The use of mineral additives affects the compressive strength. The use of silica fume in foam concrete increases the compressive strength [48]. Compressive strength in soil-based foam concrete increased with the use of silica fume and quicklime. It was reported that it increased from 4 MPa to 7.8 MPa with the addition of 20% silica fume. This may be thanks to the finer pores caused by silica fume [90]. In the study of [PERSON] et al. [49], the 7-day compressive strength of samples containing FA showed 85-90% of the 28-day compressive strength. In samples without FA, this rate decreased to 75-80%. The compressive strength of the geopolymer foam concrete prepared with the addition of FA reached 7.5 MPa in 28 days. With the addition of 20% slag to FA, the compressive strength increased to 12.6 MPa. However, adding more slag slightly reduced the compressive strength [17]. Additives added to foam concrete affect strength. It is obvious that the superplasticizer causes higher strengths thanks to the lower void size and pore connectivity [38]. Using SA reduces pore size. For this reason, the increase in dosage causes the strength to decrease [15]. Comparing foamed and non-foamed samples, as expected, the compressive strength of foamed samples is lower. The amount of slurry decreases as the amount of foam used increases. This situation causes a decrease in dry density.