**4. Conclusions**

As a result of the present investigation, a new approach to the application of glycol alkoxysilane–chitosan hybrids, including THEOS–chitosan and MeTHEOS–chitosan, in the area of stone conservation of historic stone buildings is presented and suggested, with THEOS–chitosan used as a consolidant and MeTHEOS–chitosan as a water repellent. Several aspects have been covered. First, a more detailed characterization of the hybrids has been described and discussed, as we believe that it was important to address the lack of such information. Once the hybrids had been obtained by a very simple synthetic procedure, they were applied to three different historic building stones and their performance was evaluated in detail. The application of the formulations to the stones is water-based, which implies the elimination of organic solvents, as an important contribution, but also suggestive of their use in the conservation of either siliceous or calcareous natural stones. By this means, synergetic benefits arise from the interaction of alkoxysilanes and chitosan in the performance of the formulations. The effectiveness of the consolidation and hydrophobic treatments was evaluated by different spectroscopic methods, such as FTIR–ATR and SEM–EDX, and physical analysis, such as hardness measurements, in the case of the consolidation, water absorption, characterization of the dynamic and static contact angle, and energy surface determination. The evaluation of the effectiveness of treatments is considered positive in terms of the consolidation and water repellency.

Several perspectives will now be presented for further study of the interaction of the formulations with the calcareous material, which in the present case, is caliche. A primary suggestion is that the interaction between the consolidant and caliche occurred via the free amino group of the chitosan. It is very important to point out that data on colorimetric changes after treatments have not been obtained, bearing in mind that the assessment of such analyses is quite important when stones of historical buildings are treated. No apparent colorimetric changes were observed after treatment in any sample, although this was recorded by simple observations. However, corresponding analyses must be

performed and considered in perspectives; for example, the use of the Munsell method, which is commonly applied to observe chromatic variations as a result of sample treatment. Another interesting aspect that is currently under study, with preliminary results availables, is the use of the intrinsic fluorescence emission of chitosan. We have observed that such a property is maintained in the hybrid and might be a useful tool for ascertaining the effectiveness of the dispersion or penetration of the formulations on or inside the stone. Additionally, the antimicrobial property of chitosan is under study, which could lead to a formulation that will also prevent or solve cases of biodecay.

**Supplementary Materials:** The following are available online: Figure S1: FTIR–ATR for films of (a) MeTHEOS–chitosan and (b) THEOS–chitosan, Figure S2: 29Si CPMAS NMR of THEOS–chitosan film, Figure S3: 13C CPMAS NMR of MeTHEOS–chitosan film, Figure S4: 29Si MAS NMR of MeTHEOS–chitosan film, Figure S5: 29Si MAS NMR of MeTHEOS–chitosan film, Figure S6: FTIR– ATR spectrum of a Compañía sample without treatment, consolidated, and hydrofugated, Figure S7: FTIR–ATR spectrum of a Sostenes sample without treatment, consolidated, and hydrofugated, Table S1: SEM–EDX elemental composition for the MeTHEOS–chitosan hybrid, Table S2: SEM–EDX elemental composition for the THEOS–chitosan hybrid, Table S3: SEM–EDX elemental composition for the caliche sample without treatment, Table S4: SEM–EDX elemental composition for the consolidated caliche sample, Table S5: SEM–EDX elemental composition for the hydrophobic treated caliche sample, Table S6: SEM–EDX elemental composition for the Compañía sample without treatment, Table S7: SEM–EDX elemental composition for the consolidated Compañía sample, Table S8: SEM– EDX elemental composition for the hydrophobic treated Compañía sample, Table S9: SEM–EDX elemental composition for the Sostenes sample without treatment, Table S10: SEM–EDX elemental composition for the consolidated Sostenes sample, and Table S11: SEM–EDX elemental composition for the hydrophobic treated Sostenes sample.

**Author Contributions:** Conceptualization, J.C., M.M.-Z. and R.Z.-N.; methodology, M.M.-Z., I.B.-F., E.R.-O., G.Á.-G., A.G.-C. and A.V.; validation, J.C., M.M.-Z., R.Z.-N. and I.B.-F.; formal analysis, I.B.-F., M.M.-Z., G.Á.-G., E.R.-O., A.G.-C., R.Z.-N., M.M.-R. and A.V.; investigation, I.B.-F., M.M.-Z., E.R.-O., G.Á.-G. and A.G.-C.; resources, J.C., R.Z.-N. and G.Á.-G.; data curation, M.M.-Z., J.C., E.R.-O., A.G.-C., G.Á.-G., A.V. and M.M.-R.; writing—original draft preparation, J.C., M.M.-Z. and I.B.-F.; writing—review and editing, J.C. and M.M.-Z.; visualization, J.C., M.M.-Z. and I.B.-F.; supervision, J.C., M.M.-Z., G.Á.-G., A.G.-C. and E.R.-O.; project administration, J.C., G.Á.-G. and A.G.-C.; funding acquisition, J.C., A.G.-C., E.R.-O., A.V. and M.M.-R. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by CONACYT-México award 284510 supported by the Fondo Sectorial de Investigación para la Educación and the University of Guanajuato (Guanajuato-México).

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** The authors wish to thank the support of the following laboratories located at the University of Guanajuato: Laboratorio Nacional de Caracterización de Propiedades Fisicoquímicas y Estructura Molecular University of Guanajuato-CONACYT-México; laboratorio de Análisis Instrumental, "Q. Fernando de Jesús Amezquita <sup>L</sup>ópez", laboratory of Analytical Chemistry Services, Chemistry Department; laboratory in charge of Mario Avila (contact angle measurements). Finally, I.B.-F. wishes to thank CONACYT-México for the fellowship received (CVU 919318).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript; or in the decision to publish the results.

**Sample Availability:** Samples of the compounds are not available from the authors.
