Academic journal article Heritage Science

Consolidation of Porous Carbonate Stones by an Innovative Phosphate Treatment: Mechanical Strengthening and Physical-Microstructural Compatibility in Comparison with TEOS-Based Treatments

Academic journal article Heritage Science

Consolidation of Porous Carbonate Stones by an Innovative Phosphate Treatment: Mechanical Strengthening and Physical-Microstructural Compatibility in Comparison with TEOS-Based Treatments

Article excerpt

Authors: Gabriela Graziani [1]; Enrico Sassoni (corresponding author) [1]; Elisa Franzoni [1]

Introduction

Natural stones and mortars used in architecture and sculpture and exposed to outdoor conditions are affected by weathering phenomena hampering their cohesion and mechanical properties, thus making consolidation treatments necessary. Stone consolidation, however, needs careful designing and preliminary testing, as it is basically an irreversible intervention in most of the cases [1]-[4]. Moreover, consolidation might even result in an acceleration of materials decay [5],[6], if unsuitable materials or treatment conditions are selected. For these reasons, the study of stone consolidants is of primary importance.

Consolidation effectiveness is known to be influenced by a multitude of parameters [7],[8] and above all by the consolidant itself (in terms of active principle, solvent and concentration of the components), the substrate nature and weathering level, together with the application procedure and the environmental conditions, that might play a key role in on-site application.

The most used among stone consolidants is currently tetra-ethyl-ortho-silicate (TEOS) [1],[3], whose effectiveness derives from hydrolysis-condensation reactions, that lead to the formation of amorphous silica inside stone pores [5],[9]-[11]. The compatibility of the deposited silica gel with silicate substrates and its ability to form strong Si-O-Si bonds (that give the consolidant stability towards thermal weathering, solar light and oxidation, hence guaranteeing a high durability [5]) are the main advantages that make the use of this product so diffused. TEOS effectiveness, however, is known to be dependent on the presence of quartzitic fractions inside the substrate, allowing for chemical bonding. The reduced effectiveness of TEOS on carbonate substrates, compared to silicate ones, the temporary hydrophobicity of TEOS-treated stones and TEOS tendency to crack during drying are the main limitations of this consolidant when applied on carbonate stones [1]. TEOS efficacy is also linked to the solvent in which it is applied (that can be up to 25?wt% of the formulation), as solvent influences alkoxysilanes condensation reactions and hence their mechanical and physical properties. Solvent evaporation and gel syneresis (i.e. contraction due to condensation occurring between unreacted groups in the network [12]) during curing directly affect gel tendency to crack [13].

In order to overcome the limitations of TEOS in the treatment of carbonate stones, starting from 2010 [14] a new inorganic consolidant, based on the formation of hydroxyapatite (HAP) in the substrate, has been introduced and tested. HAP is formed inside the stone due to a reaction between an aqueous solution of diammonium hydrogen phosphate (DAP) and the calcite of the substrate. Experiments carried out so far on marble protection and porous stone consolidation have given very promising results, as HAP proved to be very effective on lithotypes with variable carbonate content [15]-[22]. HAP ability to develop high mechanical strength in just 48?hours curing, together with its application in aqueous solvent (non toxic), are further advantages of this treatment [14],[19].

In this study, the effectiveness of the HAP-treatment was tested and compared to that of TEOS on two carbonate stones, a limestone and a calcarenite with different mineralogical composition and microstructural features, namely Globigerina limestone (Malta) and Giallo Terra di Siena (Italy). Both lithotypes have been used in historical and modern buildings and, particularly in the case of Globigerina limestone, ethyl silicate is often used for consolidation of weathered elements (in spite of the reduced effectiveness of this consolidant on carbonate stones) mainly because of the lack of more suitable alternatives [3]. The HAP-treatment effects were evaluated in terms of mechanical effectiveness and compatibility with the substrate, and compared to those of TEOS, in order to determine whether HAP might be a valuable alternative to be employed for consolidation of these lithotypes. …

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