TEOS based consolidants for Maltese Globigerina Limestone : effect of hydroxyl conversion treatment

Abstract

Lime-based consolidants are widely used in Malta where the main building material is a sedimentary limestone. Over the past few years, commercial alkoxysilane consolidants have also infiltrated the local market, yet the effects of these materials on local Globigerina Limestone is still not well documented. Whereas it is well established that alkoxysilane consolidants form strong bridges between the deteriorating surface and healthy underlying stone in sandstones, the same is not necessarily true for porous limestone. A hydroxyl conversion treatment (HCT) has been proposed to improve bonding between silica-based consolidants and limestone (Wheeler 2005). In this work, Globigerina Limestone test samples were treated respectively with a laboratory-prepared consolidant based on tetraethylorthosilicate (TEOS), and the same consolidant loaded with 35 nm silica particles. Another set of stone samples were first pre-treated with an ammonium tartrate solution known as HCT before application of TEOS and nanoparticle-loaded TEOS. The HCT converts the stone surface into calcium tartrate, offering a hydroxylated surface onto which the alkoxysilane can anchor. Ammonium tartrate was preferred to tartaric acid on grounds of conservation ethics. The effect of the surface conversion treatment was evaluated by an infra-red technique. Untreated limestone samples were employed for comparative purposes. Consolidant- stone interactions were observed at high magnification under the electron microscope. Alteration to stone colour was followed by a colorimetric technique. The physical properties of the consolidated stone were assessed by a hardness test, water absorption by capillarity and resistance to sodium sulphate crystallisation. Results showed that HCT improved the consolidation properties of the TEOS and possibly those of the alkoxysilane containing nanoparticles. The ammonium tartrate (AMT) pre-treatment was successful even though chemical formation of calcium tartrate was not favourable according to solubility equilibria. The tartrate treatment however led to some discolouration, probably resulting from mobilisation of intrinsic iron in the limestone.