Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

1

COMPATIBILITY OF PHOTOCATALYTIC TiO2-BASED FINISHING FOR RENDERS

IN ARCHITECTURAL RESTORATION: A PRELIMINARY STUDY

Elisa Franzoni*″, Alberto Fregni*, Rossana Gabrielli°, Gabriela Graziani*, Enrico Sassoni*

* Dipartimento di Ingegneria Civile, Chimica, Ambientale e dei Materiali (DICAM)

Università di Bologna, Via Terracini 28, 40131 Bologna, Italy

° Leonardo S.r.l., Via S. Rocco 16, 40122 Bologna, Italy

ABSTRACT

Self-cleaning finishing treatments based on the use of TiO2 nanoparticles have been recently

proposed for the protection of materials in cultural heritage, with the aim of increasing the

durability of these materials in polluted environments. The results on stone seem encouraging, but

the experimental tests on different porous materials, such as bricks and renders used in historical

buildings, are still extremely limited.

In the present paper, the application of water dispersion of titania nanoparticles to the repair renders

of the former Del Corso hotel (XVIII cent., Bologna, Italy) was evaluated. The building and its

environment were first analysed for assessing the appropriateness of such finishing, then samples of

painted renders identical to those provided for in the intervention work were manufactured and

treated with the dispersion, both by brushing on the dry paint and by brushing on the wet paint (the

latter procedure in order to enhance the particles retention by exploiting the mechanism of fresco

paintings). The outcome of the different treatments was investigated mainly to evaluate its

compatibility with the substrate and its durability, which could be of interest also for the application

of photocatalytic finishing to renders in new constructions.

KEYWORDS

Historical buildings, render, pollution, nanoparticle, photocatalytic, titania, self-cleaning,

compatibility, rain.

″ Corresponding author. Ph. +39 051 2090329, fax +39 051 2090322, e-mail:

elisa.franzoni@unibo.it

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

2

1. INTRODUCTION

Although TiO2 (mainly in the rutile crystalline form) was introduced as white pigment in the

painting industry since the early years of 20th

century [1], its photocatalytic activity was highlighted

in the Twenties and investigated more extensively starting from the Fifties, when the photochemical

reactions occurring at the TiO2 surface (mainly in the anatase form) started to be elucidated [2]. The

progressive appraisal of these reactions disclosed a wide range of potential applications where

anatase could be usefully employed and boosted its industrial exploitation. Under UV light

exposure, TiO2 experiences both redox reactions of absorbed substances and hydrophilic conversion

[2], that can be jointly or singularly exploited for several functions, such as air purification of NOx,

SOx, VOCs and other pollutants [1, 3-4], self-cleaning and de-soiling [2], anti-fogging [2], anti-

microbial [5] and anti-biofouling [6]. Hence, in recent years photocatalytic titania, mainly in the

nano-sized form, has become an emerging material, gaining strong interest in different fields

including the construction sector [7-8], where it is used mainly in coatings, paints and cement-based

mortars/pavement blocks for air-purifying purposes [1, 3], anti-microbial paints [5], anti-microbial

tiles [3], and self-cleaning window glasses [4].

Despite the great potential acknowledged to photocatalytic titanium dioxide in the construction

sector, there are still many open questions on its performance in real applications and on its health

effect and environmental impact, hence a large amount of literature papers has been devoted to

investigate these aspects. The concern about the environmental impact of titanium dioxide is quite

logical, considering that its total demand in the world is about 3.7 Mt per year [4]. The generation

of SOx, sulphate waste and chloride waste during the production process was already pointed out in

1999, when the first Ecolabel Directive for indoor paints and varnishes fixed limits for these kinds

of waste during the production of TiO2-based pigments [9]. Recently, a study aimed at performing a

LCA analysis of nano-TiO2-coated window panes in comparison with uncoated ones highlighted

that, at least for the process considered, photocatalytic panes involve lower acidification potential,

eutrophication potential, air pollutants and smog formation potential, but higher global warming,

fossil fuel depletion, water intake, human health impact and ecological toxicity [4]. However, the

Authors conclude that, applying the multi-attribute decision analysis, the evaluation of nano-TiO2

coated window panes is positive. The high embodied energy and emissions connected with TiO2

manufacturing processes is highlighted also in [8]. However, titania can be also effectively used for

replacing large amounts of cement in structural concrete (due to its positive effect on the degree of

hydration) and for neutralizing gaseous pollutants, hence these aspects are suggested to improve

concrete sustainability [10]. Moreover, according to ongoing research [11], alternative production

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

3

processes may be used for manufacturing TiO2 nanoparticles with lower embodied energy with

respect to the current ones.

In terms of human health effects, literature shows extremely controversial opinions [12]. While

some Authors define it ‘not toxic’ [1, 13] or simply state that the one century long use of titanium

dioxide is a guarantee of its safety for humans [2], IARC cautiously classified titanium dioxide in

the Group “2B - possibly carcinogenic to humans” (as there is ‘inadequate evidence in humans’ and

‘sufficient evidence in experimental animals’), pointing out the urgent need for research in this

field. The concern about the risks associated to titania is quite widespread and many Authors

underlined its toxicity [7, 12], especially for workers involved in nanoparticles spraying and thus

exposed to inhalation risks [7]. Nevertheless, the cytotoxic effect of TiO2 was not evidenced in all

the studies that were carried out and it was finally assessed that this effect depends on many factors,

such as the crystalline polymorph and shape of TiO2, its functional groups, and many others [5, 12].

In terms of possible risks for aquatic life, results are controversial as well [14], but some Authors

evidenced a significant release of TiO2 nano- and micro-particles in aquatic environment from

painted façades under the effect of rain runoff [14], hence this aspect is worth of deeper

investigation.

Alongside the impact on health and environment, other aspects are currently actively investigated,

such as new possible applications where titania might represent a promising solution for currently

unsolved problems. Among these applications, photocatalytic TiO2 nano-particles have been

recently proposed also for the surface treatment of ancient materials in monumental buildings [13,

15-21]. Given the sensitivity of stones, mortars and bricks to pollution-related decay, the

application of titanium dioxide is intended to provide the materials with self-cleaning ability,

drastically reducing their degradation. Photocatalytic titania is also expected to be active against

bio-deterioration, which often threatens cultural heritage integrity [22]. Nevertheless, any

application in the cultural heritage field must accomplish many additional requirements with respect

to new buildings, such as compatibility (aesthetical, physic-mechanical, chemical, etc.), absolutely

necessary for avoiding defects occurrence and for achieving a good durability also in aggressive

environments (such as urban polluted areas).

Two main approaches were followed in proposing the application of TiO2 to historical building

surfaces. A first group of researchers suggested the dispersion of titania nanoparticles in

(traditional) water repellent protective organic treatments (silane/siloxane [6], polyalkylsiloxane

[15, 17]) and organic coatings (aqueous suspension of an acrylic polymer [18]), while a second

group suggested the direct application of self-cleaning dispersions of titania nanoparticles [13, 16,

19-21]. The two approaches appear antithetic, as in the first one materials surface is expected to

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

4

become super-hydrophobic, while in the second one surface is expected to become super-

hydrophilic.

In the papers pursuing the first route, TiO2 nanoparticles were firstly added, along with other kinds

of nanoparticles, to commercial poly-alkyl-siloxanes (water repellent coatings already widely used

for surface protection of stone in cultural heritage) and applied to different sandstone [15]. The

nanoparticles are here not photocatalytic, but they are meant to simply make the surface rougher

and hence water repellent. The nanoparticles made the static contact angle slightly increase with

respect to stone treated with plain poly-alkyl-siloxanes, while they caused an unacceptably high

colour change. Conversely, in [18], titania nanoparticles were dispersed in an acrylic polymer

matrix in order to achieve biocidal and hydrophobic features: the suspension was applied by

brushing to marble and limestone substrates. Despite the positive outcome of the treatment in terms

of biocidal and photodegradation efficiency, as well as in terms of colour variations, the possibly

competing behaviour of titania nanoparticles and polymer matrix was highlighted: on the one hand

the superhydrophilicity of the powder may interfere with the polymer, on the other hand titania may

even catalyse the degradation of the organic binder [18].

Following to the second approach, an aqueous colloidal suspension of TiO2 nanoparticles (1 wt%)

was prepared by sol-gel and hydrothermal combined process and applied by spraying to a highly

porous calcarenite [16], comparing its effect with a commercial suspension. Microcracked coatings

were observed when titania concentration exceeds 2 wt% and/or for high amount of solutions

sprayed (leading to film thickness greater than the critical one), but the 1% sol prepared led to no

cracks formation. Negligible differences in colour and water sorptivity were registered between the

treated and the untreated stones, while the methyl red decomposition test gave good results.

Encouraging results were found also with aqueous suspensions, again obtained by sol-gel and

hydrothermal method, used for spray-treating travertine [20]. Great lowering of the contact angle

and a slight reduction of the initial absorption coefficient were found as a consequence of the

treatment, but the long term water absorption by capillarity remained substantially the same. A

purposely designed test, involving the spraying of water by a manual nebuliser onto 80° inclined

samples (with and without UV exposure) was also performed in a further paper by the same

Authors [21]: under UV exposure, a clear decrease in water absorption was surprisingly registered,

which is in contrast with the decrease in contact angle and coefficient of water absorption. A

commercial nanotitania suspension, with particle size around 40-50 nm and two different

concentrations (0.5 and 1 wt%), was used for treating porous fired clay bricks [13]. Micro-cracked

and not homogeneously thick coating was produced by spray application and a subsequent quite

high colour change was registered, which however decreased after wetting-drying cycles. The

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

5

titania presence produced a slight decrease in the contact angle (unaffected by UV exposure,

differently from other findings on travertine [21], which was ascribed to different surface roughness

of the material under testing), which however was surprisingly high in the untreated brick (about

112°), and again the final water absorption by water spraying of the treated samples was

significantly reduced when exposed to UV. The application of wetting (by spraying) and drying

cycles resulted in the removal of fragments of the microcracked coatings.

Although some good experimental results were reported on the application of titanium dioxide to

cultural heritage, there are still many aspects to be fully elucidated, such as the performance of this

material on different substrates [13], the long-term behaviour, the actual self-cleaning effect in real

environments (self-cleaning ability is usually investigated by laboratory tests necessarily

simplifying the exposure conditions) and many others.

In the present paper, preliminary tests were carried out in order to evaluate the opportunity and

compatibility of a self-cleaning treatment to be applied onto the restoration renders in a XVIII

century building in Bologna, Italy. The treatment, consisting of a commercial aqueous dispersion of

nano-titania, was applied to samples of renders identical to those of the building, after preliminary

application of a lime-based paint on render surface. Nano-titania application was performed by

brushing, following two procedures: (i) after complete drying of the paint and (ii) directly onto the

fresh paint. The second procedure was intended to promote a better fixing of the nanoparticles on

the surface, by exploiting the same ‘encapsulation’ mechanism used for the pigments in the ancient

fresco technique. As the photocatalytic properties of titania nanoparticles-coated surfaces have been

already widely reported in the literature [18, 23], in the present study the photocatalytic efficacy of

the treatment was evaluated only in terms of degradation ability towards methylene blue under UV

exposure, which is one of the most common laboratory tests [23-26]. Conversely, the present study

is mainly focused to investigate the compatibility of the photocatalytic treatment, evaluated in terms

of morphology modification, alterations in static contact angle, time for water absorption, water

vapour permeability and colour change. Moreover, a preliminary assessment of the treatment

durability was achieved by exposing some samples to simulated rainfall by a purposely designed

laboratory equipment, in order to assess the possible release of titania from the surface.

2. MATERIALS AND METHODS

2.1 The building and the environmental context

The building under study is the so called “Ex-albergo del Corso” (Fig. 1), which was built in XVIII

century as a hotel for travellers. The building was designed by the architect Francesco Santini and

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

6

presently represents one of the most valuable examples of neoclassical private-housing buildings in

Bologna, integrating also features typical of local architecture, such as the portico. The façade is

mainly characterized by renders (with rusticated finishing at the first floor) probably dating back to

late XIX century, while sandstone elements and fired-clay decorative masks are only locally

present. Due to the severe degradation state affecting the existing renders, their replacement with

new ones having the same formulation was decided. Considering that the heavy vehicular traffic in

the frontage street is probably responsible for the bad conservation state of the old renders, the

protection of the new renders with a photocatalytic self-cleaning finishing was considered.

An analysis of the environment surrounding the building was firstly carried out, with the aim of

evaluating the actual usefulness of a photocatalytic self-cleaning treatment to be applied after the

restoration intervention. The analysis was performed, according to previously developed

methodologies [27-28], by using of air quality data (SO2, NO2, PM10 annual average

concentrations), collected by environmental monitoring stations of ARPA Emilia-Romagna

(Regional Agency for Environmental Protection). The analyzed data, collected for human health

safeguard, were registered by an environmental station located near the building (about 1 km), in a

street similar for traffic density conditions to the area of interest. Climatic data in Bologna were

provided by ARPA as well.

Moreover, for evaluating the effects of urban pollution on the building façade and hence the

degradation phenomena to which restoration materials will likely be subject, samples of the original

renders were collected from the pillars of the main facade (height of 0.5-2 m from the road). The

renders were characterised by the presence of an ochre painting and by significant soiling.

The identification of nature and state of conservation of materials was carried out by X-ray

diffraction (Philips PW 1840 40kV/20mA, Cu Kα radiation) and analysis of the soluble salts by

extraction in boiling distilled water, filtration and ion chromatography (Dionex ICS-1000).

2.2. Materials

Laboratory tests were carried out on plaster slabs (15×12×1.8 cm3), prepared using the same

materials used in the restoration intervention, namely natural hydraulic lime (NHL 2 according to

EN 459-1:2010, supplied by Brigliadori Fernando S.n.c., Italy), sand and ground brick powder, i.e.

the so called cocciopesto (Cremonini S.r.l., Italy). Mortars were prepared in a Hobart mixer (EN

196-1:2005), according to the following mixture proportions: hydraulic lime:aggregate:water

1:2:0.5 by volume.

After one week curing at room conditions (T=20 °C, R.H.=50%), the slabs were demoulded and let

curing for further three weeks at the same conditions. The plaster slabs were then painted with

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

7

exactly the same painting used in the building, namely a mixture of slaked lime (CTS, Italy),

inorganic pigments 5 wt% (Pure pigments for artists, CTS) and acrylic polymer 5 wt% (PRIMAL

B-60A, Rohm & Haas). The painting was applied in two layers by brushing wet-on-wet, as in

current on-site practice.

Finally, a commercial aqueous dispersion of nanoparticles of titanium dioxide in the anatase form

(3.4 wt%) with a 0.1% by weight of NaOH (Eco AT-01, Eco Coating photo-Catalyst S.r.l.) was

applied by brushing. Although spray application is recommended in the technical data sheet of the

product, brushing application was chosen to reduce the inhalation risk, as the health effects of TiO2

nanoparticles inhalation are not well known yet, as reported above.

In a first set of specimens, the application of the nanodispersion of titanium dioxide was performed

on the dried paint (i.e. one week the painting application), in one and two layers (samples labelled

1-TiO2-D and 2-TiO2-D respectively).

In order to promote a better incorporation of nanoparticles into the paint, other samples were

prepared by applying the titania dispersion immediately on the fresh dye, in one and two layers

(sample 1-TiO2-F and 2-TiO2-F), according to the same principle followed in the historical

technique of fresco paintings, where the pigments were applied on the fresh plaster to enhance their

incorporation during the carbonation process of the substrate. Moreover, it is noteworthy that the

presence of photocatalytic titania was found to accelerate the carbonation of lime, due to an

increase of the carbon dioxide concentration resulting from its photocatalytic oxidation of pollutants

[29]. All samples were then left to cure for 2 weeks.

In Fig. 2, some samples are shown. It can be noticed that the different applications of the TiO2

nanodispersion were carried out on the same slabs, in order to overcome problems due to possible

slight differences among the slabs. In this way, the untreated samples taken as reference for the tests

(labelled REF) always belonged to the same slabs of the treated ones, for comparison.

2.3. Methods

2.3.1 Influence of the application technique

To evaluate the effect of titania nano-dispersion and the differences among the various application

techniques, the surface morphology of untreated and treated samples was observed by a scanning

electron microscope (SEM), LEO "EVO 40XVP-M", Zeiss, coupled with an energy-dispersive X-

ray spectroscopy (EDS) analysis system (INCA Energy 250, Oxford Analytical Instruments Ltd.),

after a thin gold layer deposition.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

8

2.3.2 Photocatalytic effect and modification in substrate properties

The photocatalytic activity of the treatment was evaluated by observing the discoloration of

methylene blue applied to treated specimens, in comparison with untreated ones. Even if the

atmospheric pollutant oxidation ability and the de-soiling ability of titania are not necessarily

evaluated in a fully representative way by the methylene blue test, this test is one of the most

commonly used in laboratory testing. The experimental procedure, adapted from the literature [23-

24, 26, 30], consisted in applying 2 drops of a 50 mg/l solution of methylene blue in ethanol on both

untreated and treated specimens. Methylene blue was preferred over Rhodamine B as it is easier to

identify over the ochre painting, as suggested also for bricks [13]. After drying in the dark for 24

hours (to avoid any interference from absorption and exposure to daylight), all the samples were

photographed (time zero) and then irradiated with UV light working at 20 W/m2 for 5 hours [23-24,

26, 30], with a distance of 25 cm between the UV source and samples. Colour variations at the end

of irradiation were observed.

The compatibility of the treatment was assessed in terms of variations in: (i) static contact angle and

time for water absorption (as aqueous dispersions of titania nanoparticles are expected to make the

treated surface super-hydrophilic); (ii) water vapour permeability (as protective treatments should

not hinder the exchange of water vapour between the treated substrate and the environment) and

(iii) colour (as the aesthetic appearance of the substrate should not be significantly altered by the

treatment).

Contact angle was measured by a Contact Angle Measuring DSA30 – Krüss GmbH, in order to

determine the wettability and the time of absorption of the treated samples. A droplet of water (8 μl)

was dropped from a close distance, so that the kinetic energy of the droplet can be considered

negligible, onto at least three different points of each specimen. Droplet shape and absorption were

recorded with a camera and the contact angle was determined. The hydrophilic behaviour of the

treated surfaces was evaluated by comparing the results with those obtained for the untreated

surfaces. Although the determination of static contact angle for highly porous and rough surfaces is

not easy [31] and it is deeply influenced by surface irregularity [13], the results obtained provided at

least an indication about the wettability and absorption properties of the surface. All the contact

angle measurements were carried out in normal daylight conditions and not under artificial UV

radiation.

The water vapour permeability was evaluated by determining the water vapour diffusion resistance

coefficient of untreated and treated slabs (5×5 cm2), according to European Standard EN 1015-19

[32]. A saturated aqueous solution of KNO3 was used to maintain constant relative humidity inside

the cups, over which samples were placed (treated surface down) and sealed with silicon.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

9

The colour change after the treatment was assessed by spectrophotometer (Mercury 2000,

Datacolor). Chromatic values are expressed according the CIE L*a*b* notation, where L* is the

lightness/darkness coordinate, a* the red/green coordinate (+a* indicating red and −a* green) and

b* the yellow/blue coordinate (+b* indicating yellow and −b* blue). The total colour difference was

calculated by Equation (1):

ΔE* = (ΔL*2 + Δa*

2 + Δ b*

2)1/2

(1)

where the ΔE* value represents the colour change between the treated and the untreated surface.

The ΔL*, Δa* and Δb* values describe the change in brightness, red and yellow, respectively. For

each sample, the colorimetric test was performed in three positions (5 times for each position),

averaging the data obtained [33-36].

2.3.3 Durability

For samples that exhibited the most promising behaviour, due to the highest amount of titania nano-

particles (i.e., 2-TiO2-F samples, cf. 3.2.2), an evaluation of treatment durability was carried out.

The resistance to rain washout was assessed by a purposely designed experimental fixture, which

provides constant dropping of distilled water (with a rate of 350 ml/hour) onto samples rotated by

45° with respect to the horizontal reference. Considering that the annual average rainfall amount in

Bologna is about 80 cm and that the samples subjected to the dripping test had a surface area of 10

cm2, the test was run for a total of 4.5 hours. In this way, the total amount of water dropped onto

each sample (about 1575 ml) closely resembled the amount of rain expected for a field exposure of

2 years.

After the runoff test, SEM and EDS analyses were repeated, to check whether a significant washout

of titania nano-particles had occurred, and the methylene blue degradation test was repeated as well,

to check whether washout had significantly decreased the photocatalytic ability of the protective

treatment.

3. RESULTS AND DISCUSSION

3.1. The building and its environment

The data obtained from ARPA show that the last ten years have been characterized by a high

content of NO2, with average annual values ranging between 50 and 70 μg/m3 (Fig. 3), i.e. always

higher than the limit imposed by Italian law for air quality standard (40 μg/m3 [37]). The annual

average values of fine particles with size < 10 μm (PM10 in Fig. 3) are high as well: they are only

slightly below or sometimes even above the limit for human health safeguard (40 μg/m3 [37]). SO2

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

10

concentration in air is presently extremely low, to an extent that the ARPA control stations do not

register it any more. In terms of climatic features, rainfall is quite abundant (Fig. 4a), but the wind

rose for a typical year (Fig. 4c) highlights the scarce occurrence of wind blowing towards the

building façade (Fig. 4d), hence the incidence of wind driven rain seems quite limited. Temperature

values in Fig. 4b suggest the frequent occurrence of frost during the winter months.

XRD performed on plasters collected from the building façade indicate the presence of calcite,

quartz and feldspars, revealing the lime-based nature of the mortar and the presence of quartzitic-

feldspatic aggregate, typical of the area, with traces of gypsum owing to chemical attack by

atmospheric pollutants. The paintings, lime-based as well, exhibit extremely high amounts of

sulfates (SO4= about 55%), confirming the formation of black crusts in the past, and nitrate (NO3

- =

1.4-3.0%), probably due to both NOx presence in air and degradation of organic contaminants.

Given the air pollution data and the state of degradation of the existing renders/painting (clearly

pointing out some chemical attack), the application of a photocatalytic self-cleaning finishing was

hence considered worthy of testing.

3.2. Laboratory tests on the plasters with TiO2 finishing

3.2.1 Influence of the application technique

Before the tests, the TiO2 dispersion was observed by SEM, after spreading it on a glass surface and

letting it dry (Fig. 5). The mean particle size resulted around 20-50 nm, hence consistent with what

reported in the technical data sheet of the product.

The SEM observations showed that TiO2 nanoparticles are uniformly distributed on the surface of

the treated samples, significantly reducing the roughness of the painted surface (Figs. 6-8). From

the point of view of surface morphology, with reference to the comparison between Fig. 7 and Fig.

8, there are not significant differences between the surfaces treated with 1 layer or 2 layers of TiO2

dispersion, while the surfaces treated ‘a fresco’ showed a more pronounced reduction in roughness,

if compared to samples where treatment was applied on dry paint. This shows that the suspension of

titania is best retained on the surface if applied ‘a fresco’.

The EDS spectra show a higher Ti concentration in the samples with two applied layers respect to

one layer application (Fig. 7b, Fig. 8b), as expected, while for an equal number of layers, a higher

concentration of Ti was registered in the samples treated ‘a fresco’ (Fig. 8). This again confirms

that a higher amount of titania nanoparticles is retained by the ‘a fresco’ technique, resulting in a

higher smoothness of the treated surface.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

11

3.2.2 Photocatalytic effect and modification in substrate properties

The results of the methylene blue discoloration test are reported in Fig. 9. After exposure to UV

light for 5 hours, TiO2-treated samples exhibited a higher discoloration than the untreated reference,

as expected, and no substantial difference was visually observed among samples treated by different

techniques. Nevertheless, the discolouration was not as drastic as that usually reported in literature

for stone substrates. In the case of porous substrates such as the renders considered in this study, the

significance of the methylene blue discoloration test might be limited. Indeed, because of the

porous nature of the substrate, the methylene blue drop used in the test is partially absorbed into the

sample (where no titania is present) and this may alter the photocatalytic ability of the applied

treatment and, certainly, complicates its evaluation.

For a better assessment of the treatment performance in terms of photocatalytic activity, tests on the

decomposition of nitrogen oxides (having high concentrations in the surrounding of the building)

and antimicrobial activity of the treated renders are presently running.

In terms of modifications in contact angle and time for water absorption, the results are reported in

Fig. 10. The contact angle was greatly reduced by the application of titania, with no particular

differences between the samples treated ‘a fresco’ or on dry paint and no matter of the number of

layers. In particular, the contact angle was reduced by over 50%: from 43° in the reference sample

to 20° on average in samples treated with TiO2. Even more drastic was the reduction of the water

absorption time, which passed from 2.3 s in the reference sample to 0.7 s in the treated samples,

with a 70% reduction. Therefore, after the treatment, the hydrophilicity of the renders resulted

highly increased, which is an antithetical behaviour with respect to common “protective treatments”

for cultural heritage (usually hydrophobic). This aspect should be carefully considered, because,

even if hydrophilicity enhances the mechanism of removal of deposits and products by

photocatalytic action, water is also the main degradation factor for this kind of highly porous

materials. In particular, the occurrence of frequent freeze-thaw cycles characterizing the local

climate (Fig. 4b) may threaten the long-term integrity of façade materials when characterised by a

high moisture content. Moreover, the increase in the water uptake velocity must be carefully

considered as it may boost the absorption of the first rain drops by the material. The occurrence of a

higher capillary absorption coefficient was evidenced also by other Authors (for porous limestone

treated by titania aqueous suspensions [16]), while the same final total water absorption was found.

As titania nanoparticles do not penetrate into the substrate, it is quite reasonable that final water

absorption of the treated and untreated materials remains basically identical, but however it is

noteworthy that a higher initial rate of absorption of the first drops of rain in polluted environments

may be extremely harmful, as these drops are the most acid [28].

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

12

In the investigated building, however, the main direction of wind in the winter season is from the

quarter NW-SW, as resulted from the environmental analysis (Fig. 4), hence the amount of wind

driven rain supplied to the building façade is presumably quite limited.

In terms of water vapour permeability, in Fig. 11 it can be observed that, after titania application, no

significant alteration occurs. The only sample that exhibited a slightly different behaviour is 1-

TiO2-F, but this can presumably be attributed to some heterogeneity in render microstructure,

deriving from sample preparation, more than to the treatment effect.

As for aesthetic alteration after treatment, the determination of the colorimetric coordinates L, a, b

(Tab. 1) showed, for all the treated samples, an increase in the brightness value L, an increase of the

value of the parameter b (yellow-blue axis) and a substantial constancy of the value of the a

parameter (red-green axis) with respect to the untreated ones. The resulting ΔE values were higher

for the ‘a fresco’ application, compared with application on dry paint, as reported in Tab. 1. This is

consistent with the higher modification in sample morphology observed by SEM and the higher Ti

amount detected by EDS on samples treated ‘a fresco’. For these samples, the ΔE values amount to

4.35 for 1-TiO2-F and 5.22 for 2-TiO2-F, hence very close to what is commonly considered as the

acceptability limit for consolidation treatments on stone (ΔE = 5 [38]). Conversely, when titania

was applied on the dry surface, the ΔE values were lower than the threshold of visual detectability

(ΔE = 3Errore. L'origine riferimento non è stata trovata. [38]).

3.2.2 Durability

For the samples treated ‘a fresco’ with two layers of nano-suspension (2-TiO2-F samples), which

exhibited the highest amount of titania nano-particles, but still no dramatic alterations in water

vapour permeability and colorimetric parameters, a test of resistance to washout by simulated

rainfall was carried out. SEM images collected after the runoff test show that no particular

difference in sample surface morphology is present among samples analyzed before (Fig. 12b) and

after washing (Fig. 12c). Indeed, the results of EDS analysis confirmed the presence of Ti even after

the runoff test, although in a slightly lower amount than the non-washed samples.

Also in terms of photocatalytic effect, 2-TiO2-F samples proved to be fairly resistant to accelerated

durability test. Indeed, after exposure to simulated rain, the methylene blue discoloration ability of

the samples was not significantly diminished, as illustrated in Fig. 9.

4. CONCLUSIONS

The brushing application of an aqueous nano-dispersion of titanium dioxide on the repair renders of

the considered building resulted in a good amount of titania nanoparticles deposited on the surface,

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

13

especially when the application is carried out on fresh painting. In this case, nanoparticles seem

better retained on the surface thanks to the ongoing carbonation process of the fresh lime-based

painting. Even after a simulated prolonged rain, the treatment seems to remain on the surface

according to EDS analysis. Titania nanoparticles seem to be well retained on the render surface,

probably due to its roughness compared to other materials [13, 39], but further water run-off tests

are necessary for better assessing the effect of more prolonged rain action and of aggressive rain, as

well as for quantitatively evaluating the leached material.

The colour change was lower or, at most, only slightly higher than the threshold usually accepted

for surface treatments in cultural heritage. The colour change is mainly in terms of an increase in

the lightness and blue coordinates.

The water vapour resistance coefficient is basically unaltered after the treatment, for any kind of

application procedure, hence any film-forming or pore-clogging effect can be excluded.

In terms of behaviour of the treated renders towards liquid water, the presence of titania on the

surface decreases the contact angle and remarkably increases the initial water absorption rate, which

could be a drawback in façades subject to wind driven rain. Indeed, a faster rain absorption by the

render (which is already highly porous before the treatment) could possibly lead to frost damage

and/or chemical attack, the latter due to the low pH of the first rain drops.

The photocatalytic action of titania on the surface was investigated by the methylene blue test,

which however provided not so clear results. The use of a methylene blue drop on a highly

absorbent material involves its absorption by the material itself, hence the resulting blue stain is not

only on the surface but it affects a depth of about 1-2 mm in the material. As a consequence, the

effect of UV exposure is not so marked, as the active substance (titanium dioxide) is present only

on the surface. This suggest the need for more suitable testing procedures for the evaluation of the

photodegradation effect on highly porous materials.

ACKNOWLEDGEMENTS

Thanks to Dr. Stefano Gori for his active involvement in the tests. Dr. Martina Pedrini (Leonardo

s.r.l.) is gratefully acknowledged for the support during the restoration intervention. The Italian

Ceramic Centre, Bologna, Italy (Dr. Eugenia Rastelli, Dr. Francesca Prete and Dr. Daniele Naldi)

are gratefully acknowledged for the colorimetric and SEM analyses. Thanks to Dr. Laura Sisti for

her help in static contact angle measurement. The present study was performed under the auspices

of the Superintendence for architectural and landscape heritage in Bologna.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

14

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Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

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Table 1. Color variation of samples after nano-suspension application

L a b ΔE

REF 63.14 5.57 20.13 -

1-TiO2-D 63.37 5.57 18.58 1.57

2-TiO2-D 63.35 5.48 17.75 2.38

1-TiO2-F 64.46 4.90 16.03 4.35

2-TiO2-F 64.67 4.83 15.19 5.22

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

17

Figure 1. (a, b) The XVIII century building under investigation (former Del Corso hotel) in

Bologna and (c, d) some details of its materials and decay, before the restoration intervention.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

18

Figure 2. Some examples of the specimens with treated/untreated areas in the case of application

on (a) ‘dry’ and (b) ‘fresh’ paint.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

19

Figure 3. Annual values for NO2 and PM10 in Bologna since 2005.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

20

Figure 4. (a) Monthly rainfall (averaged from 2005 to 2013); (b) monthly temperature (with

absolute minimum and maximum temperatures) in a typical year (2013); (c) wind rose in relation to

the orientation of the building façade for a typical year (2012).

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

21

Figure 5. SEM image of the TiO2 dispersion, after drying (marker 100 nm).

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

22

Figure 6. SEM image and EDS spectrum for REF sample (marker 200 nm).

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

23

Figure 7. SEM images and EDS spectra for sample with: (a) 1 layer and (b) 2 layers of TiO2

applied on a dry paint (marker 200 nm).

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

24

Figure 8. SEM images and EDS spectra for sample with: (a) 1 layer and (b) 2 layers of TiO2

applied ‘a fresco’ (marker 200 nm).

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

25

Figure 9. Appearance of the samples in the methylene blue test: (above) before UV exposure;

(below) after 5 hours UV exposure.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

26

Figure 10. Static contact angle and time of absorption for the investigated samples.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

27

Figure 11. Water vapour permeability test results for the investigated samples.

Franzoni E., Fregni A., Gabrielli R., Graziani G., Sassoni E., Compatibility of photocatalytic TiO2-based finishing

for renders in architectural restoration: a preliminary study, Building and Environment 80 (2014) 125-135, DOI:10.1016/j.buildenv.2014.05.027

28

Figure 12. SEM images and EDS spectra for (a) REF sample; (b) sample 2-TiO2-F before the

washout test; (c) sample 2-TiO2-F after the washout test (marker 10 µm).