Characterization and reactivity of silicatic consolidants

Abstract

In this study some results on the reactivity of silicatic consolidants employed for the conservation of valuable artistic and cultural porous materials are discussed. In particular, a colloidal suspension of silica made by particles with an average diameter of 10–15 nm, a sodium silicate and an ethyl silicate consolidant are studied. The chemical–physical characterization of xerogels achieved from the examined products were carried out determining the superficial area (BET method) and the average dimension of particles (SAXS) while the chemical structure of xerogels were investigated through NMR MAS spectroscopy of the ²⁹Si nucleus. The investigation on reactivity between silica and stone support was executed by mixing consolidants with calcite and quartz powder to simplify as much as possible the system otherwise very complex. On silica produced by the three examined consolidants structural differences are revealed and these diversities can be responsible for the dissimilar reactivity of the silicatic systems towards the stone, in particular with regard to the carbonatic component.

Introduction

The porous materials, as stones and plasters, exposed to the external environment are subjected to chemical–physical decay processes that produce the loss of the consistence of the material. The phenomenon is of particular importance when it involves valuable architectural and artistic works, like masonry of historical buildings or mural paintings. The conservation project of these materials includes a consolidation intervention for restoring the physical–mechanical characteristics of the support and in particular the cohesion between material particles and their adhesion to the non-deteriorated support. Usually this operation is carried out applying polymeric compounds in organic solvent or in inorganic products [1], [2], [3].

In general the polymeric products in organic solvent have demonstrated to own good consolidant properties but also some limits related above all to the different behaviour towards the water of the consolidated material compared to the original one. Besides colour variations of the treated support are often observed and they are not always acceptable in the restoration field.

The inorganic consolidants and most of all the consolidants with a basis of silica react reducing the porosity of the material which becomes more compact and less susceptible to the water action. They have less cohesive property than the organic consolidants but they are most compatible with the support that shows a less heterogeneous behaviour in relation with water both in liquid and in vapour state. In particular sodium silicate, ethyl silicate and a colloidal suspension of silica made of particles with an average diameter of 10–15 nm [1] are the most preferably used among the inorganic consolidants made by silica. The application of these products causes after many reactions the formation of amorphous silica that acts as a consolidant and it is made by a disorderly and continuous lattice of silica tetra-coordinated tetrahedrons that make rings with 3, 4, 5, 6, 7 and 8 atoms of silicium. This structure contains numerous silanolic Si–OH groups, which however give hygroscopicity to the system, in particular where an interruption of lattice structure occurs [4], [5].

Initially the sodium silicate, the ethyl silicate and the colloidal silica, that are the objects of this study, form a gel of amorphous silica that afterwards transforms itself in xerogel because of the evaporation of solvent and assumes characteristics strictly correlated with the condition of the solvent evaporation process. It could be put forward the hypothesis that the consolidant action is exercised by the aerogel filling the accessible porous of stone [6] but it is not excluded that it could be influenced by the silicatic structures reactivity with support [7]. There is not much research on the subject but the knowledge of interactions between consolidant and support could be of great applicative interest; in fact it could allow to modify the entity of this interaction in relation to the conservation state of the support and its chemical nature.

In this study, some results on the reactivity of the sodium silicate, ethyl silicate and colloidal silica with calcium carbonate and quartz, that are the principal compounds of the stone used in historical buildings, are reported. In the first part of the research the xerogels obtained from the three silicatic systems are characterized; afterwards the chemical reactivity of xerogels are studied mixing them with calcite and quartz because they are system well-known and recognizable for the employed techniques and facilitate the interpretation of the results.

In this work, the data obtained by ²⁹Si MAS spectroscopy on samples coming from the support–consolidant reactivity are discussed. This technique has already given interesting results in studies on the interaction between binder and aggregate of mortars with fragments of crushed bricks and on the stone decay after consolidant treatment [8], [9].

Tests correlated to some properties of the products were carried out in order to evaluate applicative aspect. The capacity of the products to penetrate into real samples and how they distribute on supports were considered determining at different depth the porosity and the porous distribution in function of cumulative volume. The cohesive capacity was measured by a method set up in laboratory and designed in expectation of an employment in building yards.