## 3 Materials Used for Restoration and Conservation of Masonry Elements
### Restoration/Conservation of Masorry Materials against Abiotic Factors
#### 3.3.3 Consolidation and Protection Agents  
Consolidation represents the treatment applied in order to restore its mechanical properties, affected by weathering [86]. At the border between cleaning and consolidation, desalination is often necessary; the removal of soluble salts is usually made by applying wet poultices consisting of clays or cellulosic mixtures [97; 98], the alternative treatments proposed being represented by the application of ohmic technologies [99] and electrochemical methods [98], limewater (calcium hydroxide solution [100], crystallization modifiers (ferrocyanides, borax, etc. [101; 102], or diammonium hydrogenphosphate [103]. As it emerges from the literature review, most of the authors treat the desalination process as a secondary process (accompanying the main consolidation procedure), thus usually the traditional, well-established procedure being followed. This, in turn, leaves serious room for improving, by the development of new materials. At this time, on the market are available a wide range of both organic and inorganic consolidants [104; 105] with proven efficiency in consolidating different types of masonry materials. The stone consolidants can be divided in three main classes: (i) organic products (alkoxysilanes, recently reviewed by [PERSON] et al. [106]); (ii) organic-inorganic mixtures and (iii) purely inorganic products (usually apatite materials), relevant examples being provided in Table 5.  
A good example regarding the application of organic products for the consolidation of stone materials is represented by the study of [PERSON] and [PERSON] [107]. The authors proposed the use of a composite polymeric material (tetraethoxysilane with additives as hydroxyl-terminated polydimethylsiloxane and cetyl trimethyl ammonium bromide) for the consolidation of sandstone. After treatment, the sandstone showed crack-free surface homogeneity, acid resistance, as well as salt crystallization resistance, the polymeric layer also exhibiting very good hydrophobic properties (demonstrated by a water contact angle of 110\({}^{\circ}\)). Generally speaking, the organic products (as well as many of the organic/inorganic composites) used as consolidants also possess very good hydrophobicity, which also makes them very good protective compounds. A particularly interesting study was recently published by [PERSON] et al. [108]. In a three-layer treatment, the authors offered a complex conservation solution for stone monuments, incorporating a self-cleaning (based on TiO\({}_{2}\) nanoparticles), consolidation (based on calcium oxalate nanoparticles) and a protective hydrophobic layer. The approach was successfully applied on different types of masonry materials (limestones of different porosity, ceramic materials and mortars). The use of in situ formed hydroxyapatite represents a very good alternative to other methods. It is achieved by the reaction of a phosphate salt and the substrate (marble, limestone, sandstone, sulfated stone, gypsum stucco, concrete, etc.). The topic was recently reviewed by [PERSON] [109], so a very thorough presentation of the method would be redundant. We have chosen to discuss only a modification of the method, proposed by [PERSON] et al. [110]. The in situ formation of a hydroxyapatite consolidating layer was achieved using aqueous solution of diammonium phosphate and two types of calcium nanosuspensions, calcium hydroxide and calcite (calcium carbonate), on limestone and sandstone. The authors observed an improvement of the compactness of the stones, accompanied by a risk of incompatibility (proven by the substantial reduction of water sorption, especially for sandstone). The authors also proposed the effective order of reactants deposition on the stone (the phosphate followed by the calcium source) and drew attention to a possible drawback of the method, respectively an elevated risk of yellowing on the surface.