Maria J. Mosquera

New Nanomaterials for Consolidating Stone

A novel sol-gel synthesis, in which a surfactant acts to make the pore size of the gel network more coarse and uniform, is shown to provide an effective alternative for the consolidation of stone. The new mesoporous silica avoids the main inconvenience of current commercial consolidants, which is their tendency to crack inside the pores of the stone. Since the cracking of xerogels is a well-known drawback of the sol-gel process, the synthesis presented here can be extended to other applications. Finally, preliminary studies of the effectiveness of the novel surfactant-templated sol in consolidating a typical biocalcareous stone are also discussed.

Pore structure in mortars applied on restoration Effect on properties relevant to decay of granite buildings

Since mortars play a key role in buildings decay, their suitable choice is critical to the success of restoration projects. The focus of this paper is to characterise the pore structures of a set of mortars and correlate them with mechanical properties and vapour permeability, which are relevant to the decay of granite buildings. Water vapour transport was tested by means of a simple set-up developed in our laboratory. A good correlation was found between total porosity and the two parameters tested: strength and vapour diffusivity. Pore size distribution also showed a strong influence on diffusivity. A mix based on cement with a high sand proportion was considered as the most suitable for granite building restoration because it showed good mechanical properties and low free calcium content. A negative aspect was that this mix exhibited significantly lower vapour permeability than mortar containing lime; this could be explained by the smaller radius of its pores.

Surfactant-Synthesized Ormosils with Application to Stone Restoration

A challenging objective in monumental stone restoration is to synthesize crack-free silica materials for application as consolidants. Hydrophobicity is also a valuable property for such products; it is important to prevent the penetration of water because water is the main vehicle by which the agents of decay enter the pore structure of the stone. We report the development of a hydrophobic crack-free nanomaterial with application to stone restoration. Specifically, organically modified silicate (ormosil) has been synthesized by the co-condensation of tetraethoxysilane (TEOS) and hydroxyl-terminated polydimethylsiloxane (PDMS) in the presence of a nonionic surfactant (n-octylamine). The role played by the surfactant in the assembly of the organic-inorganic hybrid silica gel was investigated. We also prepared a crack-free material using the same synthesis but without adding PDMS to the starting sol. Finally, the effectiveness of the nanomaterials synthesized as a consolidant and hydrophobic protective treatment was evaluated on a particular widely used monumental stone. The high hydrophobicity of the organic-inorganic hybrid product synthesized in our laboratory is discussed as a function of the surface roughness of the material.

Simple strategy for producing superhydrophobic nanocomposite coatings in situ on a building substrate.

Numerous superhydrophobic materials have been developed in recent years by using a combination of two strategies: reducing the surface free energy and roughening the surface. Most of these procedures have the serious drawback of involving tedious multistage processes, which prevent their large-scale application, such as on the external stone and similar material surfaces of buildings exposed to the weather. This paper describes an innovative synthesis route for producing superhydrophobic surface coatings. The coating can even be produced, outdoors, on the building by a low-cost process. We demonstrate that the addition of silica nanoparticles to a mixture of organic and inorganic silica oligomers in the presence of a surfactant produces a coating of closely packed particles. The effect of this is to trap air beneath the water droplets, thus significantly minimizing the contact area between droplet and surface. The organic component reduces the surface free energy of the material, resulting in a high static contact angle. This has the effect of repelling water because the water droplets that form simply roll rapidly down the coated surface. The surfactant plays a valuable role, acting as a sol-gel transition catalyst and, by coarsening the pore structure of the gel network, prevents the coating material from cracking.

Stress During Drying of Two Stone Consolidants Applied in Monumental Conservation

The object of this paper is to evaluate behaviour during drying of two stone consolidants: Wacker OH and Tegovakon V, containing tetraethoxysilane. During drying, the gel network contracts due to capillary pressure generated by solvent evaporation. When the consolidant dries inside the stone porous structure, the shrinkage is constrained in all three dimensions. In these conditions, the dried gel suffers a high stress that could cause it to crack. When there is a free surface, as for a consolidant layer on the surface of a pore, the stress can relax in the direction normal to the surface. In this case, the stress is controlled by network rigidity.The rigidity of the gel network has been evaluated by mercury porosimetry, while pore size, which controls capillary pressure, has been determined by nitrogen adsorption. The shrinkage of gels under mercury pressure is characterised by high moduli. This fact suggests a high rigidity of the networks. The small pore radii found in the network (<3 nm) indicate that high capillary pressures are generated within the gel network.

Application of mercury porosimetry to the study of xerogels used as stone consolidants

Abstract Alkoxysilanes, low-viscosity monomers that polymerize into the porous network of stone by a sol–gel process, are widely used in the restoration of stone buildings. We have used the mercury porosimetry technique to characterize changes in microstructure of three granites following their consolidation with two popular commercial products (Wacker OH and Tegovakon V). The suitability of this technique is questioned because a surprising increase of stone porosity is observed. In order to investigate the feasibility of porosimetry, we analyze the behavior of xerogels prepared from the two commercial products, under mercury pressure. Gels are basically compacted and not intruded by mercury. Thus, the increase of stone porosity after consolidation can actually be associated with gel shrinkage. Mercury porosimetry, therefore, has been found unsuitable for characterizing the microstructure of consolidated rocks. However, it can be employed usefully to evaluate shrinkage of gels under mercury pressure, which permits the behavior of a consolidant during the process of drying in stone to be predicted. It is a key factor because many problems of consolidants are related to their drying process within the stone. Gels under study exhibit a high rigidity and an elastic behavior, as consequence of their microporous structure. Finally, the reduction in the porous volume of gels after the porosimetry test demonstrates that the shrinkage mechanism is based on pore collapse.

New Nanomaterials for Protecting and Consolidating Stone

The sol-gel process has been found to be successful in applications for the conservation and restoration of stone. However, a well-known drawback of the materials obtained by this process is their tendency to crack during drying inside the pores of the treated stone. In this article, we present an overview of our current research centred on producing crack-free sol-gel materials for consolidating and protecting building stone. A novel synthesis, in which a surfactant acts as a template to make the pore size of the gel network coarser and more uniform, is shown to provide an effective alternative for preventing the cracking of consolidants. We also highlight an alternative pathway, in which we add an organic component to the silica precursor in the presence of the surfactant. The hybrid organic-inorganic gel prepared in our laboratory provides excellent waterproofing to the stones under study.

Producing lasting amphiphobic building surfaces with self-cleaning properties

Nowadays, producing building surfaces that prevent water and oil uptake and which present self-cleaning activity is still a challenge. In this study, amphiphobic (superhydrophobic and oleophobic) building surfaces were successfully produced. A simple and low-cost process was developed, which is applicable to large-scale building surfaces, according the following procedure: (1) by spraying a SiO2 nanocomposite which produces a closely-packed nanoparticle uniform topography; (2) by functionalizing the previous coating with a fluorinated alkoxysilane, producing high hydrophobicity and oleophobicity. The formation of a Cassie-Baxter regime, in which air pockets could be trapped between the aggregates of particles, was confirmed by topographic study. The building surface demonstrated an excellent self-cleaning performance. Finally, the surface presented lasting superhydrophobicity with high stability against successive attachment/detachment force cycles. This high durability can be explained by the effective grafting of the silica nanocomposite coating skeleton with the substrate, and with the additional fluorinated coating produced by condensation reactions.

Producing superhydrophobic/oleophobic coatings on Cultural Heritage building materials

Abstract Water is the main vehicle of decay agents in Cultural Heritage building materials exposed to weathering. In this work, a simple method to produce superhydrophobic/oleophobic coatings building materials, including under outdoors conditions, has been developed. In addition, a study of the behavior of the developed coatings on different substrates (limestone, granite, concrete and wood) is reported. The addition of 40 nm-SiO2 nanoparticles to a fluoroalkylsilane reduces surface energy and produces a Cassie-Baxter surface in all the materials evaluated. It promotes high static contact angle values of around 160°, and a contact angle hysteresis of around 3°, giving rise to repellence. The building surfaces also demonstrate an excellent self-cleaning performance. The coatings maintain the building materials esthetics as required in the Cultural Heritage field. Finally, the coating presents a long-lasting performance due to condensation reactions producing effective grafting to the four building materials evaluated.

Producing new stone consolidants for the conservation of monumental stones

A customary procedure in the protection of monumental buildings is to consolidate decaying stone by applying commercial products containing tetraethoxysilane (TEOS). These products polymerize within the porous structure of the stone, significantly increasing the cohesion of the material. However, TEOS-based consolidants have practical drawbacks, such as cracking during the drying phase, and blockage of the rock pores. In order to address this problem, the authors increased the porosity of the product by including colloidal particles in the initial sol state. Colloidal silica particles were added to TEOS-based sols resulting in a stone consolidant with improved properties. The addition of ethanol increased the products viscosity in order to better permeate into the stone structure. The newly formulated TEOS-based consolidant is compared with the commercial consolidants, Tegovakon V 100, using graphs and tables. -- ICCROM