# Potential of Advanced Consolidants for the Application on Sandstone
## 3 Results and Discussion
### Characterization of the Consolidant Xerogels
#### 3.2.2 Mechanical Properties of the Xerogels in Microscale  
First, the elastic character of the xerogels was confirmed by the frequency sweep--constant force measurements--which showed that their storage modulus did not depend on the frequency (see Figure 1a).  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Xerogel**} & **Micropore** & **Mesopore** & **BET Surface** & **Pore Width/** & **Total Porosity** \\  & **Volume/** & **Volume/** & **Area 1/** & **Area 2/** & **nm** & **2/** \\  & **cm3 g-1** & **cm3 g-1** & **m2 g-1** & **nm2 g-1** & **\%** \\ \hline SiGel-d & 0.15 & 0.00 & * & 0.8 & 25 \\ SiGel-oa & 0.00 & 0.23 & 198 & 5.0 & 34 \\ SiGel-SiO2-oa & 0.00 & 0.24 & 187 & 6.6 & 35 \\ SiGel-SiO2-methyl-oa & 0.00 & 0.26 & 223 & 5.6 & 39 \\ SiGel-SiO2-octyl-oa & 0.00 & 0.24 & 209 & 5.4 & 35 \\ SiGel-TiO2-oa & 0.00 & 0.22 & 207 & 5.0 & 33 \\ SiGel-ZnO-oa/d & 0.14 & 0.17 & 55 & 1.9; 4–5 & 41 \\ KSE OH & 0.06 & 0.27 & 379 & 1.9; 4–5 & 38 \\ SP FX & 0.00 & 0.01 & 12 & 2–4 & 2 \\ \hline \hline \end{tabular}  
* BET equation is not applicable in narrow micropores. 2 Total porosity was calculated from the total pore volume.  
\end{table}
Table 3: Textural properties of the xerogels determined by sorption measurements.  
The mechanical properties in microscale of the xerogel were determined by a nanoindentation testing. Using the stress-strain curve, the contact stiffness and reduced elastic modulus, representing the Young's modulus of elasticity and indentation hardness, respectively, were obtained. The example of the nanoindentation curves is shown in Figure 1b.  
The catalyst used decisively affected the mechanical performance of the xerogels (Table 4). Both reference xerogels SiGel-d and KSE OH, for which an organometallic catalyst was used, exhibited much higher hardness than those catalyzed by n-octylamine. The indentation hardness correlated with the Young's modulus and the contact depth. The shape of the nanoindentation curves for the harder xerogels exhibited the character of glass-like deformation, while that for softer xerogels was polymer-like. The difference in the mechanical properties is probably due to the compact structure containing only narrow micropores of the xerogels catalyzed by organometallic compounds. The xerogels prepared using an n-octylamine catalyst showed lower values of both mechanical parameters, which is connected with their more open mesoporous structure.  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Xerogel** & **Young’s Modulus/GPa** & \
\begin{tabular}{c} **Hardness/** \\ **GPa** \\ \end{tabular} &
\begin{tabular}{c} **Contact Depth/nm** \\ \end{tabular} \\ \hline SiGel-d & \(7.74\pm 0.52\) & \(1.04\pm 0.09\) & \(182.21\pm 9.35\) \\ SiGel-oa & \(4.49\pm 0.14\) & \(0.41\pm 0.02\) & \(297.88\pm 6.50\) \\ SiGel-SiO\({}_{2}\)-methyl-oa & \(3.83\pm 0.06\) & \(0.45\pm 0.01\) & \(285.80\pm 4.43\) \\ SiGel-TiO\({}_{2}\)-oa & \(4.72\pm 0.03\) & \(0.33\pm 0.00\) & \(349.96\pm 2.10\) \\ KSE OH & \(11.44\pm 0.25\) & \(1.32\pm 0.05\) & \(175.75\pm 4.34\) \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mechanical properties of the xerogels determined by nanoindentation.  
Figure 1: Mechanical properties of the xerogel on a microscale level. (**a**) Variation of the storage modulus with the sweep frequency; (**b**) load-displacement curves; (**c**) Dependence of Young’s modulus (**c**) and hardness (**d**) on indent displacement.  
Generally, the hardness and elasticity were depth dependent (Figure 1c,d); those of the outermost surface layers being higher compared to the bulk, probably due to the faster hydrolysis and condensation of the ethyl silicate species within the surface layers of the xerogel block. However, from the consolidation viewpoint, the consolidation should be homogeneous within the stone profile. This feature was reasonably fulfilled using our consolidants, better than with the commercial ones. The storage modules determined by this method are in agreement with those in Figure 1a.