TEOS Nanocomposites for the Consolidation of Carbonate Stone: The Effect of Nano-HAp and Nano-SiO2 Modifiers.

Alexandra Rodrigues,Susana Piçarra,Maria De Fátima Montemor,Ana Paula Ferreira Pinto,Bruno Sena Da Fonseca

doi:10.3390/ma15030981

Alexandra Rodrigues, Susana Piçarra + Show 3 more

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https://doi.org/10.3390/ma15030981

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Abstract

This study aimed at evaluating the effect of hydroxyapatite (HAp) nanosized structures and nanoparticles of hydrophilic silica as modifiers of both acid- and alkaline-catalysed tetraethoxysilane (TEOS)-based products for the consolidation of carbonate stones. Their initial effectiveness and some compatibility aspects were assessed in a porous limestone (sound and artificially aged Ançã stone samples) and two types of treatment (capillary absorption and brushing). The studied products were examined by scanning electron microscopy (SEM) and micro-Raman spectroscopy. Their depth of penetration and strengthening effect were evaluated through drilling resistance. Their action on the substrate was also further assessed by non-destructive methods based on colour variation and Shore-D hardness. Treated stone samples were dissimilarly affected by the tested treatments and exhibited a significant increase in strength with a low risk of over-strengthening. Adequate in-depth penetration patterns, as well as colour compatibility with the substrate were obtained with some of the prepared formulations through two types of treatment, both in sound and aged stone samples. The potential most effective treatments with the lowest colour change were obtained with the acid-catalysed TEOS-based products modified with HAp nanosized structures.

Highlights

Carbonate sedimentary stones are a well known and highly representative group of materials in cultural heritage, built heritage
The fact that not all xerogels were completely colourless or without cracking has relative importance, as the authors have observed in previous works [47], since flasks represent a different gelling environment when compared to the stone pores [44], and positive results can still be obtained within the carbonate-medium [45,47]
The potential of a set of novel formulations was investigated for the consolidation of soft carbonate stone through the modification of acid and alkaline TEOS-based formulations with newly developed HAp nanorods and nano-SiO2

Summary

Introduction

Carbonate sedimentary stones are a well known and highly representative group of materials in cultural heritage, built heritage. Despite being historically widely applied (e.g., Portuguese Ançã stone, Maltese Globigerina stone, Dutch Maastricht stone, French Tuffeau stone, among others) [1,2,3,4], high porosity limestones are sensitive to outdoor environments, suffering several decay processes that may impair their integrity [1,5] These materials often demand active conservation actions to prevent or slow down material losses [6]. Consolidation of decayed carbonate stone has been intensively studied to find appropriate treatments to address the fragile surfaces of these stones (e.g., [7,8,9]). Alkoxysilane-based products are still the most well-established and applied stone consolidants, due to recognised advantages, such as: (i) low viscosity, allowing for in-depth penetration into the stone pores, (ii) versatility and the possibility of modification and adaptation to a wide range of stone substrates, as to other porous materials,

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