# Potential of Advanced Consolidants for the Application on Sandstone
## 1 Introduction  
Alkoxysilanes have been the most widely used stone consolidants since the 19 th century [1]. Two compounds have been dominant: methyltrimethoxysilane (MTMOS) and tetraethoxysilane (TEOS). A number of products based on TEOS and its oligomers are available on the market. Their advantages are well known: their versatility, chemical stability, low viscosity, the ability to form silicon-oxygen-silicon (Si-O-Si) bonds [2], harmlessness of the compounds (e.g., water, ethanol) formed during gelation, avoidance of over-consolidation of the stone surface and, last but not least, cost effectiveness and ease of use. Silicon-oxygen bonds are contained in many minerals, thereby contributing to the compatibility between stone and TEOS-based alkoxysilanes. Owing to the advantages listed above, such consolidants easily penetrate deeply into the stone's porous system [3]. They can replace an original binder lost during weathering and, thereby, significantly enhance the cohesion of the stone grains and, thus, overall structure [1]. However, there are serious drawbacks concerning the application of TEOS-based consolidants. Specifically, the resulting silica gel shrinks and tends to develop cracks inside the stone due to the stress caused by increasing capillary force during aging and drying [1; 3]. This particular situation leads to the formation of brittle gel fragments that create a secondary capillary network in the stone, which can accelerate the rate of deterioration. Since the xerogel formed by the gelation of TEOS and oligomeric ethyl silicates are compact with only very narrow micropores (1-2 nm in width), the water vapor permeability is substantially hindered, which represents a major danger for the stability of the treated stone. Furthermore, the physico-chemical and structure properties of these consolidant xerogels differ from those of natural sandstones, which consist of quartz particles and an inorganic binder. This issue has been of major research interest in stone consolidation applications over the past twenty years. There are several approaches to overcoming the above-mentioned shortcomings. [PERSON] et al. showed that by adding drying control chemical additives (DCCA), such as formamide, the surface tension of the sol was changed and, thus, the evaporation rate was reduced, leading to the suppression of crack formation [4]. [PERSON] et al. also reduced cracking by the addition of suitable surfactants (alkylamines), which decreased capillary pressure and surface tension [5]. Another strategy involves the incorporation of elastic segments, such as hydroxyl-terminated polydimethylsiloxane (PDMS), into the xerogel structure; these segments effectively removed the stress caused by capillary pressure [5; 6; 7; 8; 9]. Another more general approach is based on the incorporation of various oxide nanoparticles, such as SiO\({}_{2}\), TiO\({}_{2}\), Al\({}_{2}\)O\({}_{3}\) and ZnO (so-called particle-modified consolidants, PMC) [3; 8; 10; 11; 12; 13; 14; 15]. Due to the increase in the pore size of PMC-based xerogel and the improvement of its mechanical properties, cracking was reduced, and the consolidation effect enhanced. However, while they have shown significant improvements, these 'new' formulations exhibit considerable limitations due to the complexity of the treated stone objects. During the development of any novel consolidant, the selection of a suitable stone substrate is fundamental. Theoretically, three main substrates can be used: (i) a naturally aged substrate, (ii) an artificially aged substrate, and (iii) stone as quarried [15]. The first option provides the advantage of weathering under real world conditions and potentially reliable technological transfer into conservation practice. However, there are issues, namely the insufficient availability of stone samples for extensive laboratory testing and the natural inhomogeneity. The modification of stone by artificial weathering (second option) is rather more problematic as the aging processes differ considerably from those in nature.