# Potential of Advanced Consolidants for the Application on Sandstone
## Materials and Methods
### Preparation of the Nanoparticle-Modified Consolidants  
The self-developed alkoxide-based consolidants were prepared by adding the nanoparticles (3 wt.%) to Dynasylan(r)40 [11; 13; 14]. Afterwards, catalyst n-octylamine (0.18 wt.%) (Alfa Aesar) was added. The mixture obtained was diluted with isopropanol at the ratio of 1:1. Concerning the consolidant containing ZnO nanoparticles, a mixture of n-octylamine (0.18 wt.%) and dibutyltin dilaurate (1 wt.%) was used because n-octylamine was not sufficient to achieve a formation of the gel. An overview of the consolidants is given in Table 1, wherein the designation of Samples, the type of nanoparticles and catalysts used are given, for example, SiGel-SiO\({}_{2}\)-methyl-oa means that the consolidant contains methylated SiO\({}_{2}\) nanoparticles and n-octylamine catalyst. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Source** & **Sample** & **Consolidating Agent** & **Solvent** & **Catalyst** & **Additive** \\ \hline IPC 1 & SiGel-d & 50\% oligomeric TEOS & 50\% IPA 2 & dibutyltin & - \\  & & & & & \\ IPC & SiGel SiO\({}_{2}\)-d & 50\% oligomeric TEOS & 50\% IPA & dibutyltin & SiO\({}_{2}\) \\  & & & & & \\ IPC & SiGel SiO\({}_{2}\)-octyl-d & 50\% oligomeric TEOS & 50\% IPA & dibutyltin & SiO\({}_{2}\)-octyl \\  & & & & & \\ IPC & SiGel-oa & 50\% oligomeric TEOS & 50\% IPA & n-octylamine & - \\ IPC & SiGel SiO\({}_{2}\)-oclyl-oa & 50\% oligomeric TEOS & 50\% IPA & n-octylamine & SiO\({}_{2}\)-octyl \\  & & & & \\ IPC & SiGel SiO\({}_{2}\)-methyl-oa & 50\% oligomeric TEOS & 50\% IPA & n-octylamine & SiO\({}_{2}\)-methyl \\  & & & & \\ IPC & SiGel TiO\({}_{2}\)-oa & 50\% oligomeric TEOS & 50\% IPA & n-octylamine & TiO\({}_{2}\) \\  & & & & & \\ IPC & SiGel ZnO-oa/d & 50\% oligomeric TEOS & 50\% IPA & dibutyltin & ZnO \\  & & & & \\ IPC & HAP & 23\% DAP & 77\% H\({}_{2}\)O & - & Ca(OH)\({}_{2}\) \\  & Remmers & KSE OH & 75\% oligomeric TEOS & 25\% MEK & dibutyltin & - \\  & & & & & \\ NanoPhos SA & SP FX & 20\% TEOS & 80\% IPA & n-octylamine & CaC\({}_{2}\)O\({}_{4}\) \\ \hline \hline \end{tabular}  
* IPC = J. Heyrovsky Institute of Physical Chemistry; 2 IPA = isopropyl alcohol; 3 DAP = diammonium hydrogen phosphate.  
\end{table} 
Table 1: Overview of the composition of self-prepared and commercial consolidants. The second type of self-developed consolidants included hydroxyapatite-based ones. A 2.0 M aqueous solution of diammonium hydrogen phosphate (DAP) was prepared by dissolving the salts in deionized water. This concentration (close to saturation at room temperature) was selected because the higher the DAP concentration, the more calcium phosphate phases are formed by the reaction with the calcium source. The commercial consolidant KSE OH (Rommers) was used as reference. Compared to our developed consolidants, KSE OH contains only 25% solvents, while ours contains 50%. According to the data sheet, this product should be suitable for the consolidation of weathered, friable natural stones, particularly sandstones, cast stone, renders and mortar. Another commercial consolidant SurfaPore FX WB was obtained from Nanophos SA (Lavrio, Greece). This innovative nano-material is suitable for enhancing the mechanical properties of worn building surfaces [16; 17]. To test, the consolidant xerogels aged for two months were used. The consolidants were applied to stone by capillary suction; the stone specimens were partially immersed (3 mm depth) in the liquid products for 3 h. The application time was determined by measuring the evolution of the capillary fringe (mm) on the lateral surface of the specimens. We concluded that 3 h was the time necessary for the wet fringe of the majority of the consolidants to reach the top of the specimens (3 cm). The samples were weighed before and immediately after the application of the consolidant to calculate the uptake. Additionally, they were re-weighed after complete drying (months after the consolidant application) to calculate the dry yield.