Лако-красочные материалы — производство

N. Maravelaki, C. Kapridaki, E. Lionakis
and A. Verganelaki

Technical University of Crete, School of Architectural Engineering,

Greece

Abstract

In this work nano-titania of anatase form has been added to mortars containing (a): binders of either lime and metakaolin or natural hydraulic lime and, (b): fine aggregates of carbonate nature. The aim was to assess the adhesive performance of the above binders for reassembling fragment porous stones and more specifically to explore the effect of nano-titania in the hydration and carbonation of the derived mortars. The nano-titania proportion was 4.5-6% w/w of binders. The physicochemical and mechanical properties of the nano-titania mortars were studied and compared to the respective ones of the mortars without the nano-titania addition, used as reference. DTA-TG, FTIR, SEM and XRD analyses indicated the evolution of carbonation, hydration and hydraulic compound formation during a period of one-year curing. The mechanical characterization indicated that the mortars with the nano-titania addition showed improved mechanical properties over time, when compared to the

Corresponding Author address: Email: pmaravelaki@isc. tuc. gr.

specimens without nano-titania. The results evidenced carbonation and hydration enhancement of the mortar mixtures with nano-titania. The hydrophylicity of nano-titania enhances humidity retention in mortars, thus facilitating the carbonation and hydration processes. This property can be exploited in the fabrication of mortars for reassembling fragments of porous limestones from monuments, where the presence of humidity controls the mortar setting and adhesion efficiency. A specifically designed mechanical experiment based on direct tensile strength proved the suitability of mortars with nano-titania as a potential adhesive means for restoration applications. The rapid discoloration of methylene blue stains applied to mortars with nano-titania supported the self-cleaning properties of mortars with nano-titania presence. Based on the physico­chemical and mechanical characterization of the studied adhesive mortars with nano-titania, binders of metakaolin-lime and natural hydraulic lime have been selected as most appropriate formulations for the adhesion of fragment porous stones in restoration applications.

Keywords: Adhesive mortars, nano-titania, metakaolin-lime, hydraulic lime, mechanical properties

Introduction

The effective adhesion of mortars to fragments of archaeological stone or other building materials results in substantial structural integrity and prevents further decay. Treatment options include the application of adhesives and grouts, as well mechanical pinning repairs. Commonly used adhesives, such as epoxy, acrylic and polyester resins and cement mortars demonstrate excessive strength, high irreversibility and, if improperly applied, their removal may be more damaging to the historic fabric (Amstock 2000).

In this research work, two different kinds of stone from Piraeus, namely Aktites and Mounichea stones, corresponding to a hard dolomitic limestone and a marly limestone of the area respectively, were selected due to their common employment as main construction materials for the Athenian Acropolis building during the Archaic period. The preferred treatment strategy for the reassembling and adhesion of these fragments was addressed through designing bonding mortars compatible to these stones. Repair criteria were as follows: (a) physico-chemical and mechanical compatibility between repair materials and stone, (b) adequate strength to resist tensile and shear forces, (c) retreatability, (d) longevity, (e) affordability and (f) ease of installation.

The design of adhesive mortars was based on binders of either hydrate lime-metakaolin or natural hydraulic lime, with the aim of formulating a complex system characterized by the highest compatibility. Nowadays, both hydrate lime-metakaolin and natural hydraulic lime mortars are widely used in the field of restoration and conservation of architectural monuments, due to their capability to enhance the chemical, physical, structural and mechanical compatibility with historical building materials (stones, bricks and mortars) (Rosario 2009). This compatibility is a very critical prerequisite for the optimum performance of conservation mortars, considering the damage caused to historic monuments during the past decades, due to the extensive use of cement-based composites.

In this framework, in the specially designed mortars consisting of binders of either lime and metakaolin or natural hydraulic lime and fine aggregates of carbonate nature, nano-titania of anatase (90 per cent) and rutile (10 per cent) form has been added (4.5-6% w/w of binder). The aim was to study the effect of nano-titania in the hydration and carbonation of the above binders and to compare the physico-chemical properties of the nano-titania mortars with those mortars without nano-titania, used as reference. Thermal analysis (DTA — TG), infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses were performed to investigate the evolution of carbonation, hydration and hydraulic compound formation during a six-month curing period. Furthermore, the stone-mortar interfaces, the adhesion resistance to external mechanical stress, relative to the physico­chemical characteristics of the stone-mortar system and the role of the nano — titania as additive, were reported and are discussed in this chapter.