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Development of innovative nano-composite lime-based materials for the consolidation of porous building materials and the protection of built heritage

Αναστασια Μιχαλοπουλου

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Summary

The consolidation of porous building materials aims at the re-establishment of the internal cohesion among the decayed stone particles and contributes to the sustainability of architectural monuments and protection of their aesthetical and historical values. A breakthrough point on the preservation of architectural heritage and the potential materials science in the consolidation and protection field was the introduction of nanotechnology in the synthesis and development of new materials Among the vast range of the products applied for the consolidation of calcareous stones, of the most effective consolidants are considered the nanolime (Ca(OH)2) dispersions. Nanolimes are colloidal dispersions of calcium hydroxide nanoparticles and their effectivity is due to the physical and chemical compatibility with the calcareous substrate and the enrichment of the active component, especially when compared with the traditional saturated solutions of Ca(OH)2. The replacement of water by alcohol has improved the colloidal stability and the penetration ability of the dispersions of Ca(OH)2, but, it has proven to creates limitations on the consolidation efficiency through the incompletion of the carbonation reaction and the migration of the Ca(OH)2 nanoparticles to the layers of the substrate that are close to the surface during the fast evaporation of the alcohol. In addition to this, the lack of control of the morphological characteristics (size and shape) of the Ca(OH)2 nanoparticles leads to an uncertainty concerning the morphological characteristics of the final product, thus, reducing the consolidation effectiveness.Aiming to contribute on the optimization of the nanolime (Ca(OH)2) dispersions, this study aims the development of aqueous nanolime dispersions. In particular, this study focuses on the control of the morphological characteristics of the Ca(OH)2 nanoparticles, thought the determination of the parameters affecting the surface modification of the nanoparticles. In addition to this, this study also focuses on the use of either aqueous dispersion medium or mixed polar dispersion medium, as a means to achieve the completion of the carbonation process. The experimental outline was based on the comparative evaluation of the synthesis implemented via the two main synthetic routes: top-down and bottom-up. The first parameter being studied concerned the experimental conditions. The use of high-energy sonication led to the dissolution of the agglomerates (top-down) and to the reduction of size of the Ca(OH)2 nanoparticles (bottom-up). The combined use of high-energy sonication and implementation of the experimental procedure under inert conditions (He) underlined the production of plate-like Ca(OH)2 nanoparticles characterized by homogeneity in terms of size (bottom-up). The second parameter studied in this research was the possibility of the modification of the surface of the nanoparticles through the addition of three surfactants in the aqueous dispersions: the non-ionic Triton X-100 and n-octylamine and the cationic amylamine. The comparative evaluation among those three surfactants revealed that the addition of Triton X-100 led to the reduction of the size of both the nanoparticles (top-down and bottom-up) and the crystallites (bottom-up) and to the homogeneity of nanoparticles in terms both size and shape (bottom-up). This was attributed to the steric stabilization achieved by the addition of the non-ionic surfactant during the synthesis of the nanoparticles.The third parameter being studied was the modification of the dispersion medium. The first modification concerned the innovative use of water enriched with O2 nanobubbles. This aqueous dispersion medium led to the synthesis of hexagonal-plate like nanoparticles Ca(OH)2 and to the absence of aggregates, achieving the steric stabilization. Also, the combined use of water enriched with O2 nanobubbles with Triton X-100 and 2-propanol was proved to be effective. The second modification concerned the study of the effect of the addition of 2-propanol. The addition of 2-propanol led to the production of spherical nanoparticles and to the increase of the specific surface area.Furthermore, the determination of the effect of the modifications on the colloidal stability of the nanodispersions was implemented. The addition of the two non-ionic surfactants on the aqueous dispersions led to the increase of the colloidal stability of the aqueous dispersions for the first 60 min. The addition of 2-propanol led to the increase of the colloidal stability of the dispersions, through the achievement of steric stabilization and the lower sedimentation rates.Finally, the newly synthesized dispersions of Ca(OH)2 nanoparticles were applied into different inorganic porous substrates. The application of the dispersions on three sintered glass filters of different and pre-determined porosity underlined their diffusion inside the filters without the phase separation of the dispersions. The application on the dispersion on three porous building materials (limestone, sandstone, mortar) underlined the key role of the correlation between the microstructural characteristics of the substrate with the characteristics of the nanolime dispersions.Overall, the synthesis of aqueous dispersions of Ca(OH)2 nanoparticles was achieved. In addition to this, the innovative use of water enriched with O2 nanobubbles was proved to be a successful replacement of water.

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