Laboratory assessment of a photoactive Gypsum-based repair plaster

Highlights

• Photoactive repair plaster based on CaSO₄·½H₂O and TiO₂ is described.

• TiO₂ admixture did not affect flexural strength of the specimens.

• TiO₂ had a slight impact on porosity, and, consequently, on compressive strength of the specimens.

• Good results have been obtained for NO photodegradation.

• The studied repair plasters showed no antimicrobial activity.

Abstract

Air pollutants, such as nitrogen oxides (NO and NO₂), have a negative impact on the built cultural heritage. In order to reduce it, a photoactive repair plaster for conservation of various gypsum decorations, in particular moulded elements, has been developed in compliance with the requirements of conservators-restorers international community. Hence, the composition of the repair plaster is based on traditional materials (plaster of Paris) with the admixture of a modern material – photoactive titanium dioxide (TiO₂) in different weight concentrations 0.5 %, 1 %, 1.5 %, 2 %. The paper presents the results of mechanical, photoactivity and microbiological tests along with the phase analysis and a study of the microstructure of the repair plaster specimens. Several analytical techniques were applied to characterize them, such as X-ray powder diffraction (XRD) and electron microscopy (SEM, TEM) coupled with energy dispersive X-ray spectroscopy (EDS). The results show that gypsum/TiO₂ repair plaster can be an effective solution for historical monuments struggling with high air pollution.

Introduction

Gypsum decorations [1] have been widely used in architecture throughout history, for executing both sculptural moulded ornaments and smooth surfaces often imitating stone finish (stucco marble), as well as for the transition areas from walls to ceilings. Since these architectural details are usually situated in places hard to access on a daily basis, dust and dirt particles may deposit there easily. High urban air pollution contributes to deterioration of built heritage. A photoactive repair plaster, potentially with self-cleaning effect, based on plaster of Paris (calcium sulphate hemihydrate) and novel highly photoactive 2D TiO₂ particles [2], [3] has been developed as a solution to reduce high levels of nitrogen oxides present in the built heritage environment.

Cultural heritage professionals are usually very sceptical about new conservation products but at the same time they look for new solutions to recurring issues. Hence, new materials must meet strict requirements. First of all, conservation and restoration of listed historic buildings are subject to international and national regulations. Their principles are based on the guidelines set out in the Venice Charter adopted by the International Congress of Architects and Technicians of Historic Monuments in 1964 [4]. The document takes into account the limitations of traditional techniques and encourages scientific developments while underlining the role of scientific evidence and experimentation (see article 10 of the Venice Charter). This strategy was followed by The Charter of Krakow [5] which stressed the influence of aging and introduced one of the most fundamental principles of art conservation methods, that is, reversibility. According to conservators-restorers, as even the most meticulous laboratory studies may not reveal the implications of natural aging processes, the optimal conservation product should be removable. The notion of removability was introduced by Barbara Appelbaum [6] who clearly demonstrated that some conservation treatments are by definition irreversible. Another important requirement is new products’ compatibility with the existing materials and the concern for authenticity. Thus, a conservation product must not change the appearance of the original matter; materials used for reconstruction should imitate the original ones in terms of physical, chemical and mechanical properties. Colour matching and surface finish should be taken into consideration as well.

New nanometric scale conservation materials have recently been successfully developed by a number of researchers. For example, a study of traditional techniques resulted in the development of lime (Ca(OH)₂) nanoparticles [7] and silicon-based strengthening agents (ethyl silicate consolidants) [8]. As for silicates, they have a long presence in art history and art conservation, as already in 1879 water glass (soluble silicates of alkali metals) was patented by Keim [9] and, what is most important for art conservation professionals, it withstood the test of time. Titanium dioxide (TiO₂) is another well-known painting material; from approximatively 1916, it was introduced as an alternative to lead, zinc and barium white or lithopone [10]. When TiO₂ is synthesized as large particles, it gives a white paint; while when the size of particle reaches nanometric scale a transparent layer may be achieved. For the purpose of this study, novel highly photoactive 2D TiO₂ nanoparticles [2] have been used. In 1967–72, Japanese researcher, Akira Fujishima [11] discovered the photocatalytic properties of nanosized TiO₂. Hence, TiO₂ NP started to be used for oxidative degradation of organic compounds in air and water cleaning systems and opened a new era in architecture with photocatalytic building materials such as, for instance, self-cleaning glass surfaces [12] for modern skyscrapers. Visible interest in research related to the use of TiO₂ as a self-cleaning transparent layer started in mid-1990s. Studies show that TiO₂ NP coatings reduce the levels of airborne pollutants (VOCs, NOx etc.) [13], [14], [15], [16], [17], [18]. So far, the problem in the application of photoactive paints has been that TiO₂ NP reacted with organic matter, present not only in pollutants, but also in the substrate or the paint itself, e.g., the acrylic [19] or vinyl [20] binders used in commercial paints. Moreover, with each paint application and durability of the coating require consideration. Therefore, to avoid the risk of self-photodegradation of the layer, the photocatalyst has been incorporated into the structure of inorganic matrix.

Photocatalytic mortars with air lime, cement, and gypsum as binder for degradation of pollutants have already been studied [21], [22], [23], [24], [25]. It was observed that TiO₂ NP can have an impact on hydration acceleration and properties of cement mortar [21], [26]. Surprisingly, experiments showed that lower concentrations of TiO₂ (below 5 wt%) were better for photocatalytic properties – probably as a result of better dispersion of the particles in the matrix and partial deactivation of the photocatalyst when used in big quantities [22], [23]. Therefore, low concentrations of TiO₂ (0.5 wt%, 1 wt%, 1.5 wt%, 2 wt%) have been chosen for the experiment.

Studies of gypsum-based repair plasters have already been undertaken, but they usually addressed mixtures of gypsum and lime in different proportions [27]. The choice was based on analyses of real samples taken from historic monuments located in a given region [28]. Considering the different local traditions and the use of local sources of raw materials, the recipes may vary depending on geographical region. In the literature, much attention has been devoted to ancient, Islamic and Southern European gypsum plasters [28], [29], [30], [31], [32], while the subject of the use of gypsum in artworks in Central and Eastern European countries seems less studied [33], [34], [35]. The experience of the authors shows that many decorative architectural elements for interior and exterior use in Poland were executed in pure gypsum. A Danish study confirms that pure gypsum moulded elements were used on building facades in Scandinavia [36].

This research focuses on development of a photoactive pure gypsum repair plaster for moulded decorations that can also be potentially used as repair putty, if less water is added to the paste. In response to the Venice Charter’s call for scientific evidence, a number of laboratory tests of raw materials (CaSO₄·½H₂O, TiO₂) and final specimens of repair plaster were conducted. Electron microscopy was used to establish the morphology and homogeneity; X-ray diffraction was applied to monitor the phases formed after curing. Furthermore, mechanical, physical and photocatalytic properties of mixtures with 0.5 wt%, 1 wt%, 1.5 wt% and 2 wt% admixture of TiO₂ were investigated. Additionally, considering the frequently reported antimicrobial properties of crystalline TiO₂ NP [37], microbiological tests were carried out.