# Waste Cork in Metakaolin-Geopolymer Matrix: Physico-Mechanical Characterization
## 3 Results
### Density and Porosity  
Depending on the ageing time of the samples, a difference in the density and porosity of a typical MK-based geopolymer can be noted (Table 3) [1]. Additionally, the cork is a material that is very sensitive to humidity and is porous. It is estimated that about 800 million cells are present in a single wine cork stopper due to the fact that the inner bark of the cork oak tree is composed of layers of living, growing tissues, formed of independent tetradecahedron cells (20 to 40 million per cubic centimetre structure) [22]. This cellular structure remains even after the cork has been processed to obtain stoppers.  
In Figure 4, it can be observed how the apparent density decreases as the percentage of cork increases after 28 days, as a light and porous material is inserted into the geopolymer matrix that lightens the compound. Regarding the real density, it remains almost constant with a slight decrease for the 9.1 sample.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Total H\({}_{2}\)O in the** & **H\({}_{2}\)O Loss after 28** & **H\({}_{2}\)O Retention after** \\  & **Formulation** & **Days** & **28 Days** \\  & **\%** & **\%** & **\%** \\ \hline GP0 & 28.92 & 9.50 & 19.42 \\ \hline GP-2.4 CW & 28.49 & 10.36 & 18.13 \\ \hline GP-4.8 CW & 29.71 & 10.94 & 18.77 \\ \hline GP-9.1 CW & 30.62 & 20.86 & 9.76 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Percentages of total water, water loss and retention in the different formulations after 28 days of ageing.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Sample**} & **Real** & **Real** & **Porosity** & **Porosity** \\  & **Density 7 Days** & **Density 28 Days** & **7 Days** & **28 Days** \\  & **g/cm\({}^{3}\)** & **g/cm\({}^{3}\)** & **\%** & **\%** \\ \hline GP-2.4 CW & 2.362 & 2.230 & 35.61 & 43.87 \\ \hline GP-4.8 CW & 2.320 & 2.182 & 38.91 & 48.20 \\ \hline GP-9.1 CW & 2.278 & 2.113 & 48.62 & 48.62 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Real density and porosity of geopolymers with CW after 7 and 28 days.  
Within the same formulation, the apparent density decreases with curing time (Figure 5). This is probably due to the fact that cork retains water, which, over time, manages to evaporate, making the material lighter, and therefore less dense and also more porous, as shown by the porosity trend of Figure 6.  
The trend of total porosity reflects what was observed in the first 28 days in Figure 3, which reports the water loss values during drying. The values reported in Figure 6 demonstrate the trend over the first 90 days of ageing, indicating the very high porosity value (approx. 56%) of the sample with the highest cork waste content. At this point, it should be  
Figure 4: Real density and apparent density of all geopolymers after 28 days.  
Figure 5: Apparent density of all geopolymers after 7, 28 and 90 days.  
Figure 6: Porosity of all geopolymers after 7, 28 and 90 days.  
remembered that geopolymers based on metakaolin present an intrinsic nanoporosity of about 35-40%, with nanopores in the mesoporous range (2-50 nm) [24]. The evaporation of condensation water through these very fine pores is difficult, but it enhances the presence of cork powder and some voids around it, due to the difficult workability of the material as a fresh geopolymer paste.