# Acoustic Applications of a Foamed Geopolymeric-Architected Metamaterial
## 2 Materials and Methods
### Reactive Slurry Preparation, Foaming, and Curing Procedures  
Figure 1 describes the mixing, foaming, and hard-setting procedures and the formation of monomeric chemical precursors while preparing the foamed samples. These phases start with the alkaline-activated metakaolin reacting slurry preparation (\"0\" and \"1\" in the upper part and right part of the Figure), continue with the addition of the viscosity stabiliser and foaming additives (\"2\" and \"3\" in the Figure), and finally, to the end with the mould casting and hard setting in controlled temperature and humidity conditions (\"4\" in the Figure). \begin{table}
\begin{tabular}{c c c} \hline
**Compound** & **Metakaolin** & **Sodium Silicate** \\ \hline SiO\({}_{2}\) & 52.2 & 27.40 \\ Al\({}_{2}\)O\({}_{3}\) & 45.1 & - \\ Na\({}_{2}\)O & 0.22 & 8.15 \\ K\({}_{2}\)O & 0.15 & - \\ TiO\({}_{2}\) & 1.75 & - \\ Fe\({}_{2}\)O\({}_{3}\) & 0.42 & - \\ CaO & 0.04 & - \\ MgO & 0.04 & - \\ P\({}_{2}\)O\({}_{5}\) & 0.08 & - \\ H\({}_{2}\)O & - & 64.45 \\ \hline \end{tabular}
\end{table} 
Table 1: Chemical composition (by weight %) of the metakaolin and sodium silicate solution used to prepare the foamed geopolymer. Sodium hydroxide pellets were first dissolved in the silicate aqueous solution in a ratio of 0.13/1, sealed, left to cool down and equilibrated at room temperature for 24 h before use. Next, the manually mixed metakaolin powder and activating solution slurry (mixed in a solid-to-liquid ratio of 1.4/1.0) was sonicated for three minutes using a vertical mechanical ultrasound sonicator (UP100H, [PERSON], Germany). After sonication, the freshly prepared geopolymeric suspension was promptly added with 10% by weight of the oligomeric dimethyl-siloxane mixture and stirred for 10 min in a mixer (AM40-D ARGO LAB, Carpi, Italy) operating at 800 rpm. Foaming was obtained by adding 1% by weight of aluminium powder in the activated slurry and further stirring it for 5 min at 1000 rpm. The reactive and additivated geopolymeric slurry composition is reported in Table 2.  
The salates polycondensation process, occurring in the reactive foamed slurry and transforming it from a liquid of increasing viscosity and elasticity into a solid glass, consists of three main steps [9; 10; 11]: \begin{table}
\begin{tabular}{c c} \hline \hline
**Raw Materials (wt\%)** & **Foamed Geopolymer** \\ \hline Metakaolin & 33.7 \\ NaOH & 6.5 \\ Sodium silicate & 49.8 \\ Polysiloxane & 10.0 \\ Al 1 & 1.0 \\ \hline \hline \end{tabular} \({}^{1}\)aluminium powder.  
\end{table} 
Table 2: Geopolymer slurry composition that was used to prepare the foamed samples. Figure 1: Scheme of the foamed sample preparation by geopolymerisation. 1. Free tetrahedra of hydrated silicate [OSi(OH)\({}_{3}\)]\({}^{-}\) and aluminate [Al(OH)\({}_{4}\)]\({}^{-}\) ionic precursors are formed [8; 9] when metakaolin dissolves in the concentrated alkali solution (Equation (1) and upper part of Figure 1). \[\text{(Metakaolin) Al}_{2}\text{Si}_{2}\text{O}_{7}\text{ (s)}+4\text{OH}^{-}\text{ (l)}+5\text{H}_{2}\text{O}\to 2\text{Al(OH)}_{4}{}^{-}\text{ (l)}+2\text{[OSi(OH)}_{4}]{}^{-}. \tag{1}\]  
2. Inorganic linear and cyclic sialate oligomers are formed by polycondensation of the hydrated silicate and aluminate hydroxyl groups [63; 64]. Namely, the principal oligomers initially formed in the alkaline reacting liquid determined by NMR [64] are sialates with a ratio Si/Al = 1/1, silicate-siloxo with a ratio Si/Al = 2/1, and sialate-disiloxo with a ratio Si/Al = 3/1, and are reported in Figure 1 as phase \"0\". 3. The sialates' oligomer poly-condensation reaction continues increasing macromolecular weight and complexity by branching and crosslinking, reaching a gelled rubbery state before hardening into an amorphous glass (phases from 1 to 4 in Figure 1). The addition of the aluminium powder has been described as influencing the evolution of the forming geopolymer [55].