## 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
A particular approach was developed by our group, regarding the application of synthesized hydroxyapatite and apatite materials, especially for the protection of limestone, but also for antimicrobial purposes [111; 112; 113; 114]. However, due to the significant color changes recorded, the optimization of these recipes still represents a subject for future research. The incorporation of basalt fibers (chopped fibers, continuous filaments or milled fibers) into mortars (prepared using natural hydraulic lime, dry premix and inert aggregates, respectively dry premix and inert aggregates with crushed bricks and tiles--_cociopesto_) was proposed by [PERSON] et al. [115]. The authors observed that the fibers addition increased the compressive properties of the mortars, while the chopped fibers imparted post-peak stress of the hydraulic lime mortar. For all the proposed materials, was also observed a decrease in the capillary water adsorption coefficient. Similar approaches were published by [PERSON] et al. [116] (using several pozzolanic additives), [PERSON] et al. [117] (using palygorskite and metkaolin), [PERSON] et al. [118] (using commercial phase changing materials), [PERSON] et al. [119] (using commercial cellulose nano-fibrils), [PERSON] et al. [120] (using natural polymers) or [PERSON] et al. [121] (using crushed lava granulates). However, regarding the reconstruction/consolidation mortars, the best approach, in our opinion, is the one presented by [PERSON] et al. [122], respectively designing the mortars according to the nature of the natural stone (in the cases presented by the authors marl limestone, nummulitic limestone, marble, biogenic calcite sandstones), and as, often as possible, reclaiming local materials. A particular case is represented by the earth (unfired) masonry materials (such as earth-blocks--cob, adobe, mud-bricks or mud mortars). In those cases, considering the limited viability of the materials, the best approach is their replacement/grouting with materials as similar as possible, or, at least, compatible [123; 124]. As already stated, many of the polymeric composites previously presented possess hydrophobic properties, thus having a protective role for the masonry materials. The usually applied conservation materials are primarily represented by siloxanes [125] and elastomers [126], with proven efficiency over a large variety of masonry materials. More interesting, the use of hybrid nanomaterial/polymeric coatings was proposed by some authors. [PERSON] et al. [127] proposed a mixture containing commercially available micrometric hydroxyapatite and multiple polymeric components (trimethyllop propane trimethacrylate, trimethoxyspropyl silane methacrylate monomer, vinyl terminated polydimethylsiloxane and 3-mercaptoethanolpropylTriethoxysilane). The mixture was applied by brushing on two calcarenitic stones, with different porosity and photo-cured by UV-lamp. The coatings led to a surface hydrophobic character of the treated material (demonstrated by the contact angle--129\({}^{\circ}\)-136\({}^{\circ}\), depending on hydroxyapatite amount, compared with the untreated sample--96\({}^{\circ}\)). Considering the hydrophobicity results and the observed color changes, the authors proposed an upper limit of 5% hydroxyapatite filler for future applications. [PERSON] et al. [128] incorporated nano-titania in a commercial silane resin (Alpha(r) SI30). The hybrid mixture was airbrushed on the surface of different types of stones (marble, dolomite), obtaining superior values of hydrophobicity, compared with the pure commercial product. In the case of marble, was even achieved a superhydrophobicity character (contact angle \(>\) 150\({}^{\circ}\)). The same approach was used by [PERSON] et al. [129].