# A Detailed Review on Foam Concrete Composites: Ingredients, Properties, and Microstructure
## 7 Microstructure Investigations of Foam Concretes  
In this section, the effects of the contents of foam concrete on the microstructure will be shown. In the SEM image given in Figure 3, the w/c ratios and the effect of the added additives are compared. The foam concrete in the image on the left has a w/c ratio of 0.60 and is unadulterated. The one on the right is foam concrete produced with a w/c ratio of 0.35 using a superplasticizer and micro silica. The use of micro silica and superplasticizer helped to create a more robust structure [116]. \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**References** & **Cement and Additives** & **Foaming Material** & **Density (kg/m3)** & **Thermal Conductivity (W/mK)** \\ \hline
[46] & PC + GGBFS & H\({}_{2}\)O\({}_{2}\) & 150–300 (dry) & 0.05–0.070 \\
[40] & MPC & H\({}_{2}\)O\({}_{2}\) & 300–1000 (dry) & 0.136–0.347 \\
[86] & PC + FA & Protein & 975–1132 (bulk) & 0.225–0.264 \\
[26] & PC + FA & Protein & 970–1307 (dry) & 0.24 \\
[113] & PC + FA & Synthetic & 860–1245 (dry) & 0.021–0.035 \\
[114] & PC + FA + SF & Synthetic & 1100–1600 (dry) & 0.40–0.57 \\
[115] & PC & Protein & 650–1200 (dry) & 0.23–0.39 \\
[17] & GFC & - & 585–1370 & 0.15–0.48 \\
[51] & PC + FA & Protein & 594–605 (Unit weight) & 0.154–0.162 \\
[9] & PC + BT & - & 300–600 & 0.06–0.15 \\ \hline \hline \end{tabular}
\end{table} 
Table 12: Thermal conductivity results of different studies. The effect of the increase in the w/c ratio on the pore structure of foam concrete is given in Figure 4. The increase in w/c increased the pore diameter and made the pores more rounded. The reason for this can be shown as the decrease in viscosity with the increase in the amount of water and thus the difficulty of maintaining the bubbles. As a result, bubbles can easily coalesce, and larger pores can be formed [72]. Figure 4: The effect of change in w/c ratio on pore structure [72]. Figure 3: Microstructure of two foam concrete; (**a**) w/c = 0.60, containing no microsilica content; (**b**): w/c ratio = 0.35, with 10% microsilica content. Reprinted/adapted with permission from Ref. [116]. 2009, Elsevier Science &Technology Journals. The microstructure of foam concrete is given in Figure 5 when the WMP replaced silica sand and RHA replaced cement. When the sample without additive (F40R0M0) is considered, it is seen that the cement and silica sand particles form a compact structure with CSH. With the addition of marble waste (F40R0M25), there appears to be an increase in the amount of CSH. This is due to the presence of SiO\({}_{2}\) in the structure of the marble waste. The presence of RHA (F40R10M0) has extended the size of CSH. With the presence of both (F80R20M50), foam concrete had larger CSH gels [18]. Looking at the SEM images (Figure 6), there was no significant difference in the microstructure of the paste with the use of CBP, up to 7.5%. The microstructure of the paste with up to 30% CBP was looser than the control group. The reason for this can be shown to decrease the hydration products in the cake significantly. Since CBP has higher silica content and lower calcium content than cement, the amount of calcium decreases, and the amount of silica increases as the ratio increases [22]. Figure 5: SEM images of foam concrete samples (F: foam content, R: rice husk ash, M: waste marble powder). Reprinted/adapted with permission from Ref. [18]. 2021, Elsevier Science &Technology Journals. Figure 7 shows SEM images of foam concrete samples with and without FA. For both samples, MAP products can be observed as well crystallized crystals. When looking at (c) and (d), some FA particles are seen. Some large pores are observed in (a) and (b). FA can improve the pore structure by filling the pores. This property may contribute to the improvement in the strength and thermal insulation of FA [41]. In Figure 8, SEM images of foamed concretes with densities of 500 kg/m\({}^{3}\) and 1000 kg/m\({}^{3}\), respectively, are given.