Bioinspired Alkoxysilane Conservation Treatments for

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Bioinspired alkoxysilane conservation treatments for building materials based on amorphous calcium carbonate and oxalate nanoparticles Alejandro Burgos Cara, Carlos Rodriguez-Navarro, Miguel Ortega-Huertas, and Encarnacion Ruiz-Agudo ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00905 • Publication Date (Web): 18 Jul 2019 Downloaded from pubs.acs.org on July 23, 2019

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ACS Applied Nano Materials

Bioinspired alkoxysilane conservation treatments for building materials based on amorphous calcium carbonate and oxalate nanoparticles A. Burgos-Cara, C. Rodríguez-Navarro, M. Ortega-Huertas & E. RuizAgudo* Department of Mineralogy and Petrology, University of Granada, 18071 Granada, Spain. Keywords:

bioinspired

treatment,

nanoparticles,

surface

protection,

hydrophobicity, alkoxysilane *corresponding author: [email protected] ABSTRACT The weathering of building and sculptural stone due to increasing air pollution and salt damage results in the loss of invaluable cultural heritage artworks. This has prompted the design and application of novel and effective conservation treatments. Here, we study the effect of amorphous calcium carbonate (ACC) and oxalate (ACO) nanoparticles synthesized through an emulsion-assisted precipitation on alkoxysilane-based products, commonly used for the conservation of cultural heritage. Mimicking the mechanisms by 1 ACS Paragon Plus Environment

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which biominerals form via amorphous precursors and natural surfaces achieve high hydrophobicity through surface roughness (e.g., lotus leaf), alkoxysilane gels dosed with ACC and ACO nanoparticles were applied on different nonsilicate substrates (marble, calcarenite and gypsum). Our results show that this bioinspired approach for enhancing the performance of alkoxysilane-based treatments could be suitable for the protection of the aforementioned type of materials, as it has a limited aesthetic impact and negligible effects on hydric and water-vapor transport properties, while it foster compatibility with nonsilicate surfaces, preventing drying crack development, enhancing resistance to acid-attack and increasing hydrophobicity due to nanoparticle-induced surface roughness. Best results were obtained using ACO nanoparticles.

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1. INTRODUCTION

2

The built and sculptural heritage is affected by a range of weathering

3

processes that endanger its survival. These include, but are not limited to, (i)

4

freeze/thaw cycles that generate stress inside porous materials, (ii) mineral

5

dissolution which is accelerated by the presence of air pollution-derived acids,

6

and (iii) mobilization of salts resulting in their punctual accumulation and

7

subsequent in-pore precipitation inducing stresses inside the material.

8

Numerous conservation treatments, including hydrophobic protective coatings

9

and/or consolidants that fill cracks or cement loose grains, have been proposed

10

and implemented over the last decades aiming at strengthening the building

11

material and halting or reducing the impact of such deleterious processes,

12

which typically involve the presence of water or aqueous solutions1.

13

Most protective/consolidant treatments are based on the application of either

14

inorganic (e.g., nanolimes, oxalates and phosphates among others2–5) or

15

organic and silico-organic products (e.g., epoxy, silicon and acrylic resins and

16

alkoxysilanes among others1,6) to damaged stone, bricks, and/or mortar and

17

plaster surfaces. In particular, alkoxysilanes have found extensive application in

18

the consolidation of stone monuments and other artworks, also presenting

19

protective

20

shortcomings associated with the application of alkoxysilanes in heritage

21

conservation10. One is the fact that alkoxysilanes do not form primary bonds

22

between the Si-OH groups and non-silicate surfaces1. The second drawback of

23

alkoxysilanes is associated with the pervasive cracking that takes place during

24

the drying of the silica gel. Drying cracks strongly diminish the mechanical

hydrophobicing

properties7–9.

There

are,

however,

several



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(strengthening) properties of alkoxysilane consolidants. Finally, although alkyl-

26

alkoxysilanes have shown promising effects as protective coatings, their

27

hydrophobicity is limited9. Several approaches have been proposed to

28

overcome these important limitations. These include the use of additives (e.g.,

29

polydimethylsiloxane, n-octylamine, and nanoparticles)11–14 that could play a

30

dual role as anticracking agents15 and as hydrophobic functionalities, or even,

31

as hydrophobic enhancers8,12,16 (not necessarily related to chemical effects, but

32

to increased surface roughness, see below). In the absence of PDMS and n-

33

octylamine drying cracks systematically forms following sol-gel transition and

34

the resulting cracked surface layer loses any protective or consolidating

35

effect11–15. If such additive(s) could also act as coupling agent between the

36

silicate gel and non-silicate substrates (e.g., limestone or marble), some of the

37

above mentioned drawbacks would be overcome. This, however, remains a big

38

challenge.

39

Nature can be a source of inspiration for the search of solutions to the

40

problems faced by conservators. Over the last decades, research on biological

41

surfaces has shown that they display multiple unusual properties and

42

functionality, such as water-repellency, high and adjustable adhesion, and non-

43

reflective properties, among others17. The structure-function characteristics of

44

such natural surfaces can be an inspiration in the development of novel, more

45

efficient materials and treatments for the conservation of cultural heritage.

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Biological surfaces have been the focus of research aimed at elucidating how

47

nature solves a range of engineering problems and to use these solutions in the

48

design of novel materials and surfaces with selected properties17. In this 4 ACS Paragon Plus Environment

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context, attempts to mimic natural structure-property relationships and designs

50

for the generation of weathering-resistant, water-repellent surfaces, not only for

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conservation purposes but also with industrial applications, have been

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reported18.

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The systematic study of the hierarchical microstructure of water-repellent

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biostructures has shown that micro-roughness leads to the non-wetting and

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self-cleaning properties of many living organisms17. It has been observed that

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micro-roughness changes the contact angle from a Young-Laplace to a Wenzel

57

and Cassie-Baxter regime19–21, leading to superhydrophobicity, as occurs in the

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case of the lotus leaves22 or the mosquito eyes23. Based on this gained

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knowledge, it has been hypothesized that increasing the surface roughness of

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building materials at the nano- and micro-scale will ultimately lead to a

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hierarchical two-scale surface roughness that would result in enhanced

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hydrophobic behavior16,19,23. This can be achieved by applying nanoparticles to

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the surface of the material to be protected. Nanoparticles have been already

64

used for conservation purposes, including TiO2 nanoparticles, which display

65

self-cleaning properties8,24,25, nanosilica (SiO2)26–28, nanoalumina (Al2O3)29 or

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even tin oxide (SnO2) nanoparticles30, as well as calcium oxalate monohydrate

67

(wewhellite)31 and mixtures of amorphous and crystalline calcium oxalate

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nanoparticles16. These nanoparticles have been applied in combination with a

69

wide range of alkoxysilane-derived formulations or products and all of them

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have shown to be effective anticracking additives30,32 and, in some cases,

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hydrophobic enhancers16,19,29,30. Despite the foreseen potential of these

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treatments and the reported effects of nanoparticle addition to alkoxysilanes, 5 ACS Paragon Plus Environment


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their applicability and effectiveness for the protection of building materials needs

74

to be fully elucidated. Moreover, additional, improved functional characteristics

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of the nanoparticle-based treatments, such as better bonding to non-silicate

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substrates, and resistance against chemical weathering (e.g., acid-attack), are

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desirable aspects that have not been explored yet.

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Amorphous calcium carbonate and oxalate (ACC and ACO, respectively)

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nanoparticles are known to play a key role on several biomineralization

80

processes33–35 as well as many other crystallization processes36, as they are

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key metastable precursor phases preceding the development of crystalline

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mineral phases which form following nonclassical nucleation pathways35–39. In

83

this context, they are tested here as a feasible and effective bioinspired

84

approach to overcome some of the aforementioned limitations of alkoxysilane-

85

based conservation treatments (i.e., development of drying cracks and poor

86

bonding to non-silicate substrates), and to confer additional functionalities, such

87

as enhanced hydrophobicity linked to their micro-roughness effects. Amorphous

88

inorganic phases are receiving an increasing research interest, as they are

89

present in almost all biomineralization processes34. For conservation purposes,

90

the higher solubility of amorphous phases compared to their crystalline

91

counterparts may help to introduce larger amounts of precursors inside stone

92

materials compared to other frequently used treatments such as limewater40,41.

93

It will also include a reactive (soluble material) that would eventually dissolve

94

and reprecipitate forming a stable crystalline phase. The molar volume, VM, of

95

amorphous phases is larger than that of their corresponding crystalline phases

96

(e.g., VM calcite ≈ 36 cm3mol-1 vs. VM ACC ≈ 73 cm3mol-1

42).

This is advantageous 6

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from a conservation point of view because during the amorphous-to-crystalline

98

conversion porosity would be generated in the treatment layer, thereby resulting

99

in enhanced water vapor permeability. The latter is a key parameter to take into

100

account in all conservation interventions because the formation of impervious

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treatment layers systematically leads to enhanced substrate damage1. On the

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other hand, if the amorphous phases used in these treatments convert into

103

crystalline phases with a crystal structure similar or very similar to that of the

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minerals constituting the materials to be treated, a strong cohesion (epitaxy)

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between the treatment and the substrate is expected. This is the case of ACC

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and calcitic or gypsum substrates43, as well as ACO and calcitic substrates. In

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this way, ACC and ACO (upon their crystallization into stable, crystalline

108

phases) might act as coupling agents between silica gels and non-silicate

109

substrates after its recrystallization into mineral phases with an epitaxial

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relationship to the calcitic substrate2,44,45.

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The main goal of our study was to investigate the properties that both calcium

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carbonate and calcium oxalate, applied as amorphous nanoparticles, confer to

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Si-coatings based on standard tetraethoxysilane (TEOS) and pre-polymerized

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TEOS (Dynasilan® 40) currently used in cultural heritage conservation. We

115

evaluated the effectiveness of these treatments following their application on

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common non-silicate building materials such as marble, porous calcarenite and

117

gypsum plaster. In particular, we studied the treatment-induced changes on

118

surface texture, hydrophobic behavior, resistance against acid-attack, material

119

color changes, hydric properties and mechanical properties of treated supports



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in order to quantify the efficacy and durability of the bioinspired nanoparticle-

121

based treatments.

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2. MATERIALS AND METHODS

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2.1. Synthesis and characterization of amorphous nanoparticles

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Several methods for the synthesis of amorphous nanoparticles have been

125

reported46–48.

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precipitation46,49,50 lead to amorphous nanoparticles covered by the emulsifying

127

agent. In this work, nanoparticles were produced by mixing 100mM CaCl2

128

solution and 100 mM H2C2O4 or Na2CO3 solution in a 1/1 vol. ratio in order to

129

obtain a precipitate of amorphous calcium oxalate (ACO) or amorphous calcium

130

carbonate (ACC), respectively. Polydimethylsiloxane (PDMS) was added to all

131

the aforementioned solutions at a 1/100 (v/v) ratio. As PDMS is immiscible with

132

water, vigorous stirring and sonication were applied to solutions in order to

133

obtain a homogeneous PDMS/solution emulsion and to promote the

134

subsequent emulsion-assisted nanoparticle precipitation50. Afterwards, the

135

above-mentioned emulsions were quickly mixed and, in less than 3 s, the

136

reaction between both CaCl2 and Na2CO3 or H2C2O4 emulsions was stopped by

137

adding isopropanol in a 10/1 (v/v) ratio under vigorous stirring. Isopropanol

138

induced a reduction in water activity that favored the stabilization of amorphous

139

phases51,52.

140

transformation) was also favored by the presence of PDMS, which tended to

141

cover the precipitated nanoparticles (see below) thereby inducing steric

142

hindrance

Among

these,

Stabilization

that

(i.e.

prevented

those

based

inhibition

aggregation.

of

on

emulsion-assisted

amorphous-to-crystalline

Afterwards,

suspensions

were 8


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centrifuged at 1200 rpm for 10 min, and solids were separated from supernatant

144

solution, washed with isopropanol and centrifuged again before storage. All

145

reagents were purchased from Sigma-Aldrich with a purity ≥99% according to

146

ACS Reagent Grade and solutions were made using ultrapure water (type I+,

147

resistivity 18.2 MΩcm, Milli-Q®). Formation of amorphous phases was confirmed

148

by powder X-Ray Diffraction (XRD) using a X’Pert PRO diffractometer

149

(Panalytical, Eindhoven, The Netherlands). The following working conditions

150

were used: radiation CuKα (λ = 1.5405 Å), voltage 45 kV, current 40 mA,

151

scanning angle (2θ) 3–60º and goniometer speed 0.1 °2θ s–1. Nanoparticles

152

were examined by transmission electron microscopy (TEM) using a Titan (FEI,

153

Oregon, USA) and a CM-20 (FEI-Philips, Amsterdam, The Netherlands) in the

154

case of ACC and ACO nanoparticles, respectively. Nanoparticles were re-

155

suspended in ethanol and collected from the reaction media using directly the

156

TEM grid. To analyze the distribution of PDMS on and/or within ACC

157

nanoparticles, maps of Si and Ca present in PDMS and ACC/ACO,

158

respectively, were obtained on the Titan TEM using high-angle annular dark-

159

field (HAADF) images collected in STEM mode and energy dispersive x-ray

160

spectroscopy (EDS) analyses.

161

2.2. Synthesis of sols

162

Sols were prepared using either a commercial monomeric tetraethyl

163

orthosilicate (TEOS) with reagent grade quality (>98%) which had a silicon

164

content of ~28 wt%, calculated as mass percent of SiO2 upon complete

165

hydrolysis, or a partially pre-polymerized TEOS (Dynasilan 40, Evonik) with a



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repeating unit between 5 and 7

167

content of about 40 wt% also expressed as SiO2 present following complete

168

hydrolysis. Hydroxyl-terminated PDMS with a viscosity of ~ 25 cSt and an

169

average molecular weight of 550 g mol-1 was used as hydrophobic agent and as

170

a promoter for the sol-gel transition due to its role in reducing gelation times14.

171

n-Octylamine (>99% purity) was employed as an anti-cracking agent and as a

172

basic catalyst28,53. All reactants were purchased from Sigma-Aldrich except

173

Dynasylan 40 that was kindly provided by Evonik. The different treatment

174

groups were noted as TPO and DPO in reference to their composition, and

175

were prepared using very similar molar ratios as those previously reported12,54:

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TPO-group treatment included TEOS/PDMS/H2O/Ethanol/n-Octylamine in

177

molar

178

Dynasylan®40/PDMS/n-Octylamine in molar ratios 1/0.04/0.00052. While the

179

first treatment (TPO) is based on standard TEOS formulation with a lower silica

180

content that impart a lower viscosity (thereby enhancing penetration), it has the

181

drawback of producing a less dense amorphous silicate protective layer. In

182

contrast, the second alkoxysilane is a commercial and widely available

183

conservation treatment (Dynasilan 40) for stone protection. By including a

184

higher silica content, the product is more viscous and usually presents a lower

185

penetration. The positive part of this commercial formulation is that, in theory, it

186

would produce a denser and stronger silica gel. By selecting these two products

187

as the TEOS base for our treatments, we wanted to evaluate the role of silica

188

content and associated sol viscosity and density on the performance of our

189

modified TEOS-based protective coating. Note that the silica content and pre-

ratios

1/0.04/4/4/0.00052

groups and a higher silicon

and

DPO-group

treatment

included


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polymerization degree of TEOS are critical parameters controlling viscosity,

191

penetration, sol-gel transition and final product density9. Reactants were added

192

to a beaker placed in an ultrasonic bath (Ultrasons 150W, JP SELECTA,

193

Barcelona, Spain) in the same order than written. Gels were stirred and

194

sonicated for 10 min after the last reactant was added in order to promote the

195

homogenization of the mixture. Amorphous nanoparticles, prepared as

196

described above, were added to beakers of the different synthesized sols

197

placed in the ultrasonic bath (i.e., TPO and DPO) in a 5%w/v and noted as –Ca

198

or –Ox in the case of ACC and ACO, respectively (i.e., for TPO-group: TPOCa

199

and TPOOx; for DPO-group: DPOCa and DPOOx). Such a nanoparticle

200

concentration was selected based on previous results by Manoudis and co-

201

workers30, who found that SiO2 and TiO2 nanoparticles added to TEOS in such

202

a concentration led to an optimal enhancement of static contact angles (i.e.,

203

hydrophobic behavior).

204

2.3. Preparation of selected building materials

205

Some of the most common building materials used worldwide since ancient

206

times for construction and ornamental purposes were selected to evaluate the

207

performance of the aforementioned protection/consolidation treatments: white

208

marble, calcarenite (biomicritic limestone), and gypsum plaster. Each of them

209

presents differences in their porous network. White marble from Macael

210

(Almería, Spain), frequently found in monuments of Southern Spain55 has the

211

lowest porosity (~ 1.8 %)2,56 of the selected materials. Santa Pudia calcarenite

212

has been widely used to build the most significant historic buildings in the city of

213

Granada (Spain)57. Nonetheless, its bioclastic nature, high porosity (~ 30 %), 11 ACS Paragon Plus Environment


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coarse pore size, and low degree of cementation limit its durability58. Finally,

215

gypsum has been used in several historic buildings, as exemplified by the

216

plasterworks in the Alhambra59 (Granada, Spain). Gypsum plaster shows the

217

highest porosity (up to 50 %) of all materials tested in the present study and it is

218

the most prone to deterioration.

219

Test cubes (side = 2 cm) and test slabs (4.5 x 4.5 x 1 cm) were prepared for

220

all three substrates. Marble samples were prepared from raw quarry blocks

221

from Macael (Almería, Spain). Calcarenite samples were quarried at La

222

Escribana (Granada, Spain). Gypsum samples were prepared from a

223

commercial plaster of Paris by mixing thoroughly 1kg of gypsum powder with

224

750mL of water for 5 min. Then, the paste was cast on a 20 x 30 x 5 cm plastic

225

container and let to set and harden for 3 days before preparation of the gypsum

226

test samples with the above-mentioned dimensions.

227

2.4. Application of the gels containing amorphous nanoparticles

228

Freshly prepared sols were sprayed (~250µL/cm2) over all sample surfaces

229

(except the bottom ones) and left to dry and harden at room conditions (21±2ºC,

230

~50%RH) for 1 month prior to further testing. Immersion and pouring application

231

methods were also tested in preliminary experiments and similar results as in

232

the case of spray application were found in the final products regarding their

233

hydrophobic behavior. However, thicker and less homogeneous coatings were

234

observed following immersion or pouring application. Therefore, spraying was

235

finally selected not only because of its versatility as an application technique but

236

also because it enabled better control on the amount of treatment product 12 ACS Paragon Plus Environment

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237

applied over the sample surfaces. Prepared gel viscosities were evaluated at

238

25.0 ± 0.1 ºC using a rheometer R\S+ (Brookfield AMETEK, Massachusetts,

239

USA) and the software Rheo® 2000 v28.

240

2.5. Evaluation of treatment effectiveness

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One month after treatment application, grazing angle (2 º2θ) XRD analyses

242

were performed on treated sample surfaces (using the same working conditions

243

described above) in order to investigate the recrystallization of the amorphous

244

nanoparticles. Morphology, texture examinations and microanalysis with energy

245

dispersive X-ray spectroscopy (EDX) were done using a Field Emission

246

Scanning Electron Microscope (FESEM) model Auriga (Carl Zeiss SMT).

247

Samples were carbon coated and secondary electron (SE) images were

248

acquired by FESEM using the SE-inLens detector at an accelerating voltage of

249

3

250

perpendicularly to the treated surface) were prepared in order to evaluate

251

treatment penetration within the different substrates. Cross-sections were

252

carbon coated and examined using the backscatter electron detector of an

253

environmental SEM (ESEM) model Quanta 400 (FEI). Point microanalyses and

254

element distribution maps were performed with an EDS XFlash detector

255

(Bruker) at an accelerating voltage of 20 kV.

kV.

Additionally,

cross-sections

of

the

treated

samples

(i.e.,

cut

256

To evaluate the acid resistance capacity of the samples after treatment,

257

sample probes were immersed in 25 mL hydrochloric acid solution with a pH of

258

4 at room conditions and the pH was monitored using a Titrando 905 system

259

(Metrohm) coupled to a pH-meter (Electrode Plus mod. 6.0262.100, Metrohm) 13 ACS Paragon Plus Environment


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260

and controlled by a computer with the software Tiamo v2.5 for continuous data

261

log (Figure S1). pH measurements enabled to determine the neutralization rate

262

of the solution as a result of the dissolution of the samples' surface. After acid

263

attack tests, samples were carbon coated and SEM images were collected to

264

evaluate damage.

265

Color changes were determined according to CIEDE200060 using a

266

spectrophotometer Minolta CM-700 d, with the standard illuminant D65 and

267

observer at 10°. Color coordinates L* (luminosity or lightness which varies from

268

black with a value of 0 to white with a value of 100), a∗ (which varies from

269

positives values for red to negatives values for green) and b∗ (which varies from

270

positives values for yellow to negatives values for blue) were measured. The

271

color change ΔE*ab was calculated using the equations proposed by Sharma et

272

al. (2005). At least 20 measurements on every sample were done before and

273

after treatment.

274

Modification of the pore access size distribution upon treatment application

275

was determined by means of mercury intrusion porosimetry (MIP) using a

276

Micromeritics Autopore III 9410 equipment with a maximum injection pressure

277

of 414 MPa. Three replicates per treatment (as well as untreated samples) were

278

done. Samples with mass between 1 to 2 g were used.

279

The permeability to water vapor (WVP) was determined according to

280

European Norm (EN) 15803:201061 using the wet cuvette method, test samples

281

45 x 45 x 10 mm in size and the test device (see Figure S2) proposed by the

282

EN. First, test samples were placed in a climatic chamber model KMF 5.2 14 ACS Paragon Plus Environment

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283

(BINDER GmbH, Tuttlingen, Germany) at 23 ºC and a relative humidity (RH) of

284

50 % up to constant weight. Afterwards, a saturated KNO3 solution was placed

285

inside the cuvette to maintain a constant 93% RH and the cuvette was closed

286

with a lid that has embedded the stone/plaster slab (Figure S2). Three

287

replicates of each sample were placed in the climatic chamber at 23 ºC and 50

288

% RH weighting the assay devices periodically for a month.

289

To evaluate the mechanical properties and the consolidation behavior of the

290

applied gels on treated samples, drilling resistance tests were performed using

291

the drilling resistance measurement system (DRMS, SINT Technology, Firenze,

292

Italy). 10 mm-deep holes were done using a 5 mm diameter drill-bit with flat

293

diamond head. Specific test conditions for each substrate were selected based

294

on extensive testing aimed at disclosing optimal drilling and penetration rates

295

for maximum signal to noise ratio. For marble samples, a rotation speed of 600

296

rpm and penetration rate of 10 mm/min were used; in the case of gypsum, a

297

rotation speed of 200 rpm and penetration rate of 20 mm/min were selected and

298

for calcarenite a rotation speed of 300 rpm and penetration rate of 10 mm/min

299

were chosen. At least 6 measurements per treatment and substrate type were

300

done.

301

For the evaluation of the surface hydrophobicity, static contact angle

302

measurements were done according to EN 15802:200962. Water drops of 7.5

303

µL were deposited on the samples surface and a picture of every drop was

304

acquired with a Nikon® D5300 digital camera before 6 seconds after drop

305

deposition. Samples were placed between the camera and a light source and

306

fixed to the vertical central point of the acquired images. Subsequent image 15 ACS Paragon Plus Environment


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processing for static contact angle determination were done with the software

308

Autocad® 2014. The contact angle of at least 20 drops deposited per every

309

treatment and substrate type was determined


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311 312

3. RESULTS

313

3.1. Characterization of nanoparticles

314

Figure 1 shows that synthesized calcium carbonate nanoparticles were

315

amorphous (ACC) as reflected by the broad hump between 15 and 25 º2 in the

316

X-ray diffraction pattern63 and the lack of diffraction peaks corresponding to any

317

of the crystalline calcium carbonate polymorphs. In the case of calcium oxalate,

318

it was found that most of the precipitate was amorphous (ACO) but trace

319

amounts of calcium oxalate monohydrate (whewellite) and calcium oxalate di-

320

hydrate (weddellite) were detected. In the absence of PDMS, and under our

321

synthesis conditions, all the amorphous nanoparticles rapidly (within minutes)

322

underwent transformation into crystalline phases as observed in previous

323

works36,64. Conversely, nanoparticles synthesized in the presence of PDMS

324

were stable for several weeks. Note that the latter ones were those used in the

325

tested treatments.

326

TEM imaging (Figure 2a) showed that only ACC nanoparticles with no well-

327

defined shape and with an average size below 100nm formed in the calcium

328

carbonate system. The diffuse haloes in the SAED pattern (inset in Figure 2a)

329

confirmed the lack of crystallinity of the nanoparticles. A large amount of ACO

330

nanoparticles with an average size ≤50 nm formed along some well-defined

331

crystals in the case of the calcium oxalate system (Figure 2b). In both cases, a

332

low-contrast veil corresponding to PDMS covering the nanoparticles was



Page 18 of 55

333

observed in HAADF images (Figure 2c). EDS analysis (Figure 2c) confirmed the

334

presence of silicon in the veil area, as well as in the bulk ACC nanoparticles.

335 336

3.2. Evaluation of treatment effectiveness.

337

Figure 3 shows that in the case of the treatments containing ACC

338

nanoparticles (i.e., TPOCa and DPOCa), ACC recrystallized to rhombohedral

339

calcite crystals with size up to ~1 µm (see also Figure S3 and Figure S4) which

340

left a limited number of nanoparticles present on the samples' surfaces. In the

341

case of the treatments containing ACO nanoparticles (i.e., TPOOx and

342

DPOOx), the size of the crystalline phases detected using grazing angle XRD

343

analysis (whewellite, Figure 4) was smaller (between 100 – 250 nm) as

344

compared to the treatment containing ACC nanoparticles, as seen in FESEM

345

images (Figure 3). The morphology of the whewellite particles formed in our

346

experiments did not match the common growth habit reported for this phase

347

(short prisms elongated along [001])65. In the case of the DPO-group

348

treatments, the gel covering the surface of the different substrates was

349

apparently thicker than that formed in the case of the TPO-group treatments.

350

The latter showed a more uniform surface coverage. Additionally, in the case of

351

marble samples a very limited penetration of both TPO- and DPO-group

352

treatments was demonstrated by BSE imaging of samples cross sections

353

(Figure 5) and the thicker surface coatings (Figures S4 and S5).

354

Backscattered electron microscopy (BSE) images of cross sections (Figure 5)

355

also showed a lower penetration depth in the case of DPO-group treatments as 18 ACS Paragon Plus Environment

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356

compared to the TPO-group treatments. In the latter case, a more homogenous

357

distribution of the product within the porous system of the samples was

358

achieved (Figure 5b).

359

Grazing angle (2 º2θ) XRD patterns performed on samples with a contrasting

360

mineralogical composition between the substrate and the nanoparticles showed

361

that amorphous nanoparticles recrystallized to calcite and to whewellite, in the

362

case of the treatment with ACC and ACO, respectively (Figure 4). Note that

363

grazing angle XRD favors diffraction of x-rays from the surface of a sample (i.e.,

364

intact test cubes, directly analyzed with no prior grinding). Because marble and

365

calcarenite samples include calcite, treatments with ACC nanoparticles leading

366

to conversion into calcite showed no change in their XRD patterns. This is why

367

they are not shown in Figure 4.

368

Only in the case of gypsum samples, application of the treatments led to a

369

distinct increase in dilling resistance, which was more pronounced in the case of

370

DPO-group treatments (Figure 6b). For gypsum and within a particular

371

treatment group (TPO- or DPO-group), no significant differences in drilling

372

resistance along the depth profiles were observed (Figure 6a), except in the

373

case of the gypsum samples treated with the DPO-group (Figure 6b). In this

374

later case, a significant increase in the drilling resistance along the first ~2 mm

375

of the depth profile was observed regardless of wether they contained

376

nanoparticles or not, which suggests that this treatment concentrated in such a

377

near-surface area where nanoparticles present at the sample surface did not

378

apreciably contribute to an increase in drilling resistance. Apparently,

379

nanoparticles detached from the sample surface as soon as the drill bit touched 19 ACS Paragon Plus Environment


Page 20 of 55

380

them. Due to the larger pore size and heterogeneity of the calcarenite samples,

381

a significant dispersion in drilling resistance values was found (Figure S6a),

382

which precluded drawing any conclusions regarding the depth of treatment

383

penetration or the relative increase in drilling resistance. In the case of the

384

marble, a much higher resistance to drilling was observed66 (Figure S6b), which

385

together with the low penetrability of the treatments associated with its low

386

porosity67 made impossible to unambiguously measure differences in drilling

387

resistance after treatments.

388

Regardless of the treatment type, marble samples presented a significant

389

color change mainly due to a decrease in the luminosity component (ΔL*), while

390

the changes in the Chroma component (ΔC*) were almost negligible. Overall,

391

this resulted in relatively high ΔE*ab values (4.5 – 7, Figure S7) leading to a

392

"plastic" or "wet" appearance of the surface. In the case of TPO treatments

393

applied on gypsum and calcarenite substrates, the color changes were

394

negligible, with ΔE*ab ≤ 3

395

were observed when applied onto gypsum and calcarenite substrates.

396

However, the measured changes, which varied within the range 5 ≥ ΔE*ab ≥ 3,

397

could be catalogued as detectable by the human eye but still acceptable for

398

conservation purposes68. Both gypsum and calcarenite treated with the DPO

399

treatments group showed a reduction in the luminosity component (negative

400

ΔL* values), associated with a slight darkening effect of the coating, together

401

with an increase of the color saturation given by positive ΔC* values69.

60.

Regarding DPO treatments, perceptible changes

402

The results of acid resistance tests showed a significant increase in the time

403

required for neutralization of the hydrochloric acid solution (initial pH 4) in all 20 ACS Paragon Plus Environment

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404

treated substrates (Figure 7). Neutralization times were systematically higher in

405

the case of all substrates treated with DPO treatments compared with TPO

406

treatments. Within the DPO-group, the application of the DPOOx treatment,

407

initially containing ACO nanoparticles, resulted in the longer times for

408

neutralization of the acid solutions. In the case of the calcarenite and gypsum

409

substrates, the time required to reach the different reference pH values (5,

410

6,and 7) was significantly longer in the case of treated samples (irrespectively of

411

the treatment applied), as compared with untreated ones (blank). That was also

412

the case of the marble substrate, with the exception of the samples treated with

413

TPOCa.

414

FESEM images of gypsum samples subjected to the acid resistance test

415

(Figure 8, see also Figure S8 and Figure S9) showed that the silica gel covering

416

the surface was damaged by the acid. Shrinking of the gel as denoted by the

417

occurrence of gel-free areas was observed, especially in the case of the DPO-

418

group treatments. It was also observed that some of the nanoparticles were left

419

exposed. In the case of the treatments containing ACC nanoparticles and for all

420

substrates, dissolution of the recrystallized calcite was observed (see Figure

421

8a). This agrees with the lower neutralization time (see Figure 7a) for TPOCa

422

and DPOCa treatments, because calcite was easily dissolved under acidic

423

conditions. Interestingly, marble samples treated with TPO and DPO treatments

424

without nanoparticles and subjected to acid resistance tests showed massive

425

cracking and shrinking (crack width ~200-400 µm) of the silica gel coating

426

(Figure S9a), as well as its complete detachment in some areas which exposed

427

the marble surface fully pitted and corroded by the acid-promoted CaCO3 21 ACS Paragon Plus Environment


Page 22 of 55

428

dissolution (Figure S9b). These results demonstrate that there was very little

429

adhesion between the silica coating and the marble surface. Conversely, the

430

surface of marble treated with TPO and DPO containing nanoparticles

431

presented small lacunae (~10-20 µm in diameter) (Figure S9d and S10c) and

432

narrow cracks (crack width  20 µm) that exposed the treatment-marble

433

interface at their dead-end indicating limited gel shrinking (Figure S10d-e). The

434

cracks typically displayed a wedge- or V-shaped profile (normal to the treatment

435

surface) showing that no detachment of the treatment layer occurred at the

436

treatment-substrate interface. Indeed, at the narrow dead-end of the cracks, the

437

treatment was still observed to be attached to the substrate. Altogether, these

438

results demonstrate that the nanoparticles enabled a stronger bonding of the

439

silica gel to the non-silicate substrates and enhanced the resistance to acid

440

attack.

441

In the case of the calcarenite and gypsum substrates changes in the porosity

442

were evaluated (Figure S11) but not in the case of the marble due to its limited

443

porosity (~1.8 %) and the lower penetrability of treatments. It was observed that

444

both TPO- and DPO-group treatments reduced systematically the open porosity

445

of both gypsum and calcarenite substrates. No differences were found within a

446

particular treatment group (i.e., with and without nanoparticles). Note that

447

nanoparticles tended to remain on the sample surfaces, therefore not

448

contributing to a reduction in the porosity of bulk samples. In the case of the

449

calcarenite, the amount of coarse pores (i.e., r > 1 µm) decreased while the

450

amount of small pores (i.e., r < 1 µm) increased. The latter is in agreement with

451

the silicon maps of cross-sections where a partial filling of pore spaces by 22 ACS Paragon Plus Environment

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452

silicon was observed (Figure 5). The largest reduction in the amount of coarse

453

pores was observed in the TPO-group treatments. In the case of gypsum

454

substrates, TPO-group treatments led to a slight reduction in the volume of

455

coarse pores while DPO-group treatments induced a reduction in the volume of

456

pores with size 0.01 < r < 0.1 µm. Changes in the internal porosity of any

457

material due to filling of empty spaces typically lead to an increase in its

458

mechanical strength, but also to a reduction in its permeability. The latter may in

459

turn lead to undesirable effects related to impermeabilization towards gases.

460

However, in the case of the marble and calcarenite substrates, no significant

461

changes were found in the water vapor permeability, WVP (Figure S12). Only in

462

the case of gypsum samples a slight reduction in WVP from 0.013 ± 0.005

463

g/m2hPa in the untreated samples to ~0.012 g/m2hPa in both the TPO- and

464

DPO-group treatments was observed (Figure S12). However, such a reduction

465

in WVP was within error of the measurements.

466

All treatments could be regarded as hydrophobic as deduced from the

467

measured static contact angles of the different substrates after treatment

468

application (Figure 9). Regardless of the substrate type, the highest static

469

contact angles were obtained following TPOOx treatment. However, in the case

470

of TPOCa treatments, non-hydrophobic contact angles were obtained. On the

471

contrary, in the case of DPO treatments, hydrophobic contact angles were

472

obtained systematically, but no significant differences in contact angle between

473

nanoparticle-free and nanoparticle-inclusive treatments were observed.

474

4. DISCUSSION



Page 24 of 55

475

Our results show that nanoparticles synthesized by mixing Ca2+ and CO3= or

476

C2O4= emulsions, using PDMS as emulsifying agent, lead to stable ACC and

477

ACO nanoparticles, respectively, with an average size below 100nm (Figure 2).

478

The use of PDMS as emulsifying agent also makes the amorphous

479

nanoparticles suitable to be integrated in the hybrid matrix of the gel during the

480

sol-gel transition of the alkoxysilane treatments used here. This should be

481

fostered by the fact that the nanoparticles are covered by PDMS, as

482

demonstrated by our HAADF-EDS analyses. Such a surface layer would enable

483

a direct bonding with the silica gel matrix. Moreover, our results suggest that

484

PDMS was pivotal for a lasting and effective stabilization of amorphous phases,

485

which enables application of the nanoparticle-based treatments in their original

486

(amorphous) state. PDMS is an elastomer whose presence in TEOS-based

487

polymers not only influences the chemical properties of the coatings (making

488

them more elastic70–72) but also their surface morphology and hydrophobic

489

character favoring a dual-scale roughness which enhances the hydrophobic

490

behavior of Si-coatings11–14,73,74. Among the advantages of using PDMS, it has

491

been shown that some PDMS compounds as the one selected here with

492

hydroxyl-terminated functional groups can also act as coupling agents between

493

the gel network and non-silicate substrates12. Ultimately, PDMS facilitates

494

condensation reactions in the sol-gel process, is perfectly integrated into the

495

inorganic gels network28 and due to its low surface energy, it reduces the pore

496

size of the gel network. The latter effects result in a reduction of gel-cracking

497

that also provides toughness and flexibility. It has also been reported that

498

nanoparticles contained in gels, together with surfactants (e.g., n-octylamine)

499

prevents gel cracking during drying and/or curing12. This is why, n-octylamine 24 ACS Paragon Plus Environment

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500

was also used here to prevent gel cracking by acting as a template during the

501

formation of the gel network and as a basis catalyst28,75.

502

All these positive effects brought about by the amorphous nanoparticles in

503

combination with PDMS and n-octylamine are in good agreement with our

504

results as rough surfaces (Figure 3), increased static contact angles (Figure 9)

505

and the absence of cracks in the gel layer were obtained. These results are

506

comparable but superior to those obtained by Manoudis and co-workers16 using

507

TEOS dosed with SiO2 nanoparticles. Taking 90º as the threshold contact angle

508

considered as minimum for stone protection76, all the tested treatments, with the

509

exception of TPOCa, could be considered as effective hydrophobicing agents

510

for stone protection. In particular, TPOOx treatments displayed a significant

511

increase in the values of static contact angle for all substrates as compared with

512

reference TPO treatments (i.e., without nanoparticles). Conversely, DPO-group

513

treatments with and without nanoparticles showed similar static contact angles

514

(within error). This is likely due to the fact that DPO treatments prevented the

515

development of surface rugosity (vide infra). Overall, the higher static contact

516

angles of TPO-group treatments could be attributed to the lower viscosity of this

517

treatment that facilitates the product distribution along sample surfaces,

518

ultimately resulting in a higher micro- and nano-rugosity of the surface. This is in

519

agreement with the hierarchical surface topography observed in SEM (Figure

520

3). In the particular case of the TPOCa treatment, ACC nanoparticles used in

521

the initial TPOCa dispersion resulted in a final homogeneous distribution of

522

partially gel-embedded rhombohedral calcite crystals with an average size

523

~1µm. ACC transform into calcite via dissolution-precipitation, likely fostered by 25 ACS Paragon Plus Environment


Page 26 of 55

524

the presence of water in the initial solution sprayed onto the surfaces and/or

525

moisture from the environment. Such a microstructure prevented the

526

development of a Casey-regime, and being such crystals hydrophilic, they also

527

increased the wettability of the treated substrate. In this case, and for the

528

different tested substrates, these calcite crystals acted as sacrificial material

529

during acid weathering as observed using FESEM (Figure 8) and in agreement

530

with the lower neutralization times observed during the acid-attack test (Figure

531

7). However, a poor hydrophobicity was achieved. This is not necessarily a

532

negative or undesirable phenomenon in terms of stone protection. Those

533

rhombohedral calcite crystals will dissolve before any other material from the

534

substrate surface behind the gel, and therefore will protect the original surface

535

against dissolution. In the case of DPOCa treatments and despite the fact that

536

some calcite crystals were also present, the hydrophobic contact angles

537

measured could be attributed to the greater thickness of treatment product

538

remaining on the sample surfaces. However, such a higher amount of silica gel

539

with embedded amorphous ACC or ACO (or their crystalline product) smoothed

540

the surface thereby preventing the development of a Wenzel and Cassie-Baxter

541

regime19–21. As a result, the contact angles where lower than those achieved

542

with the less viscous TPO-group treatments (with the exception of TPOCa, as

543

indicated above). A rougher initial surface in the case of the very porous

544

gypsum plaster and the calcarenite substrates also explains why their contact

545

angles after TPO-group treatment application were systematically higher (again,

546

with the exception of TPOCa) than those measured in the case of the smooth

547

(non-porous) treated marble surface.


Page 27 of 55 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

548


Conversely, it is also known that nanoparticles may affect the aesthetic

549

appearance

of

surfaces,

as

also

occurs

with

many

other

surface

550

treatments2,77,78. In general, the higher the nanoparticle concentration is, the

551

greater its aesthetic effect would be30. However, in our case, the foregoing

552

effect was within acceptable values for conservation purposes68. Indeed, color

553

differences between treatments with and without nanoparticles were within error

554

(Figure S7). Nonetheless, the colorimetric results indicate that, in general, both

555

TPO- and DPO-group treatments are suitable for the conservation of porous

556

substrates. Results from colorimetric measurements seem to be related to

557

differences in viscosity of treatment products, which play an important role on

558

the penetrability of the products within the porous substrates. Results from

559

elements distribution performed with the EDS detector on BSE images of cross-

560

sections (Figure 5a) suggest that the higher the viscosity of the treatment

561

product, the more limited the penetration depth. A higher viscosity (3.7 ± 0.5

562

mPa·s), and, hence, lower penetrability of the product, in the case of the DPO-

563

group treatment resulted in ~2 mm thick Si-rich surface rim, as was clearly

564

observed in the case of the gypsum samples (Figure 5a). The latter explains the

565

higher drilling resistance (Figure 6b – DPO-group) found along the first ~2 mm

566

along the depth profile of treated gypsum samples. In the case of the

567

calcarenite, and due to its heterogeneous nature and pore size distribution, the

568

DPO-group treatments penetrated in a more uniform way as shown by the

569

element distribution maps of BSE images (Figure 5c,e). However, in this latter

570

case (calcarenite samples), the higher viscosity of DPO-group treatments

571

resulted in localized product accumulation within the cavities and holes of the

572

calcarenite, where a silicon-rich rim with an average thickness of ~1 µm was 27 ACS Paragon Plus Environment


Page 28 of 55

573

observed (Figure 5c,e). Such an accumulation of the treatment product was

574

likely responsible for the observed greater aesthetic differences (i.e., higher

575

ΔE*ab values). In the case of the TPO-group treatments, which presented a

576

lower viscosity (1.2 ± 0.2 mPa·s), a more homogenous distribution of the

577

product within the porous system of the samples was achieved. Indeed, the

578

largest reduction in the amount of coarse pores in porous samples after

579

treatment with TPO-group can be also related to a more homogeneous

580

distribution of the treatment within the porous substrate due to the lower

581

viscosity of these treatments as compared with the DPO-group. In the case of

582

marble, the limited penetrability of both types of treatments was not unexpected

583

due to its very low porosity. For gypsum samples, the higher treatment

584

penetration can explain the significant strengthening effect observed, as shown

585

by the increased drilling resistance measured here. The limited differences

586

found within a particular treatment group could be attributed to the presence of

587

ACC or ACO nanoparticles within the bulk substrate, as they tended to

588

accumulate on the first µm of the sample depth. In the case of the marble, the

589

very high drilling resistance of the substrate and the negligible penetration of the

590

treatments prevented to disclose any significant strengthening effect of the

591

different treatments from drilling resistance measurements (Figure S6b). In the

592

case of the calcarenite, the heterogeneous nature of this stone and the high

593

dispersion in the values of drilling resistance also prevented us to disclose any

594

possible strengthening effect of the different treatments (Figure S6a).

595

The resistance against acid-attack following treatment was related to the type

596

of nanoparticles used. ACO nanoparticles led to a drastic increase in the 28 ACS Paragon Plus Environment

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597

resistance against acid-attack, regardless the type of treatment group (i.e.,

598

TPOOx or DPOOx). The highest required times for neutralization of the acid

599

solutions obtained for the DPOOx treatment (Figure 7) are attributed to two

600

main effects: (i) the limited wettability of the surface because of the hydrophobic

601

character of the treatment, and (ii) the presence of a less soluble phase (i.e.,

602

calcium oxalate) within the surface coating that may also act as a protective

603

layer when the substrate is subjected to dissolution. In contrast, the use of

604

relatively soluble ACC nanoparticles (and the calcite crystals resulting from the

605

amorphous-to-crystalline phase transformation), provided the same resistance

606

against acid-attack (within error) than that of treatments without nanoparticles.

607

In the case of the marble, shorter neutralization times where observed for both

608

TPOCa and DPOCa treatments which could be attributed to the higher amount

609

of precipitated micron-sized calcite aggregates (formed after ACC) on the

610

surface. This is likely due to the low penetrability of the treatments, leading to a

611

surface accumulation of ACC (or calcite) that could be easily dissolved thereby

612

acting as a sacrificial material.

613

Acid tests also disclosed that the presence of nanoparticles, specially ACO-

614

containing treatments applied on marble, significantly reduced loss of surface

615

silica gel coating and its cracking and shrinking (Figures S9 and S10). In the

616

presence of nanoparticles, micrometer-sized holes and narrow cracks with a V-

617

shaped cross-sectional profile developed after acid attack, but they did not

618

affected the full treatment profile. Indeed, our FESEM observations showed that

619

the treatment was still attached to the substrate both at the bottom of holes/

620

lacunae and cracks. Apparently, the nanoparticles limited treatment loss, 29 ACS Paragon Plus Environment


Page 30 of 55

621

cracking, and shrinking, because they enabled a stronger bonding between the

622

non-silicate substrate and the alkoxysilane-based treatment. Chemical bonding

623

between gel components and nanoparticles in our study can be inferred from

624

FTIR. Figure S13 shows spectra of individual gel components and the gels

625

containing

626

nanoparticles. It can be observed a shift from the band corresponding to Si-OH

627

bonds, from 1620cm-1 to 1640cm-1 that could reflect the interaction between

628

ACO and ACC nanoparticles and PDMS The 860cm-1 band in PDMS is also

629

shifted due to bonding with nanoparticles within the gel. Moreover, in the case

630

of treatment with ACC nanoparticles, the calcite peak at 875cm-1 corresponding

631

to out-of-the-plane C-O bending shows a shift to ~850cm-1 that suggests a

632

reaction among Si-O-Si and carbonate groups.

both

calcium

carbonate

and

calcium

oxalate

amorphous

633

Likely, this was due to an epitaxial crystallization of calcium carbonate or

634

calcium oxalate crystals (formed after the amorphous nanoparticles) on the

635

treated substrates, enabling crystallographic continuity and a strong bonding

636

between the substrate and the treatment layer as schematically depicted in

637

Figure S10f. Note that an obvious epitaxy, or more exactly, self-epitaxy, exists

638

between calcite (present both in the tested marble and calcarenite) and calcite

639

formed after ACC, as both have the same crystal structure. Calcium oxalates

640

have been also shown to grow epitaxially on calcite crystals2,44,45. Moreover,

641

calcite (formed after ACC) and gypsum have been shown to display an epitaxial

642

relationship43,79. Finally, it is worth mentioning that despite the fact that

643

hydroxyl-terminated PDMS could enable the bonding between the silica gel

644

treatment and the calcitic and gypsum substrates, such a bonding does not 30 ACS Paragon Plus Environment

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645

seem to be very strong or durable, as suggested by the extensive cracking,

646

shrinking and massive detachment of the silica gel coating in samples treated

647

with TPO and DPO treatments without nanoparticles and subjected to acid

648

attack (Figure S10a and f).

649

Consequently, and considering the tradeoff between the use of different

650

types of nanoparticles and additives which help to balance a compromise

651

between a maximum water repellency, crack prevention, strengthening of the

652

bonding between the applied gels and the substrates, treatment durability,

653

(acid) weathering resistance, and minimum aesthetic impact, both TPO and

654

DPO treatments with ACO nanoparticles could be considered optimal for

655

conservation purposes. Nonetheless, treatments containing ACC nanoparticles

656

should not be discarded because of their relatively high solubility and

657

preferential dissolution during the action of weathering agents (e.g., acid

658

solutions or rain water), as such a sacrificial effect will help protect and preserve

659

the substrate underneath. They will also be optimal to ensure a good bonding

660

between the marble or limestone calcitic substrate and the silica gel.

661

In any case, both ACC and ACO will eventually transform into crystalline

662

phases with a lower molar volume. This enables the generation of porosity,

663

especially within the surface coating formed on top of the treated substrates.

664

The latter can help reduce one of the main drawbacks of the use of

665

alkoxysilanes in stone conservation: i.e., the formation of impervious layers.

666

Here, why show that indeed the reduction in water vapor permeability 2 months

667

after treatment application, when calcite and weddellite have formed after ACC

668

and ACO, respectively, is negligible. Moreover, the formation of both crystalline 31 ACS Paragon Plus Environment


Page 32 of 55

669

CaCO3 (calcite rhombohedra) and CaC2O4·H2O (whewellite) did not result in

670

any detrimental effect on the integrity of the silica gel coating. Indeed, their

671

formation led to no detrimental effect during acid resistance tests. Actually, a

672

significant improvement in the silica gel acid resistance as compared with

673

reference treatments without nanoparticles was observed (Figure 7 and Figures

674

S8-S10).

675

ACC and ACO nanoparticles thus provide an effective bioinspired solution for

676

overcoming the main drawbacks of the use of alkoxisilanes in stone

677

conservation.

678

5. CONCLUSIONS

679

Our results show that the bioinspired approach based on the use of

680

amorphous nanoparticles in combination with alkoxysilane-based gels, could be

681

suitable for the protection of the aforementioned type of stone or plaster

682

materials. It has been found that the viscosity of the initial sprayed solutions has

683

a marked influence on the strengthening and protective effects of the

684

treatments. The higher the initial viscosity of the solution, the greater the

685

consolidation and protective effect of the product is, but the less homogeneous

686

the distribution of the treatment is along samples´ depth profile and the higher

687

the color changes are. The applied nanoparticles contributed to overcome the

688

main drawback of alkoxysilanes: gel cracking during drying and curing, while

689

enhancing the hydrophobic behavior of the coating due to the transition from a

690

Young-Laplace to a Wenzel and Cassie-Baxter regime due to the observed

691

changes in surface topography. Moreover, amorphous nanoparticles, especially 32 ACS Paragon Plus Environment

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692

calcium oxalate ones, confer an enhanced acid resistance (~45%) to the

693

applied treatments, presenting limited impact on aesthetic and hydric properties.

694

Addition of the nanoparticles fostered the bond of the alkoxysilane treatment to

695

non-silicate substrates, especially in the case of the marble. This was likely due

696

to an epitaxial crystallization on the substrates of calcium carbonate or calcium

697

oxalate crystals (formed after the amorphous nanoparticles) that enabled the

698

anchoring to the substrate of the silica gel formed upon alkoxysilane sol-gel

699

transition.

700

ACKNOWLEDGMENTS

701

Authors acknowledge funding from European Commission (European

702

Regional Development Fund, CGL2015-70642-R), Junta de Andalucía (P11-

703

RNM-7550,

704

Competitividad (CGL2015-70642-R, CGL2015-73103-EXP) and Universidad de

705

Granada (UCE-PP2016-05).

research

group

RNM-179),

706

SUPPORTING INFORMATION.

707

Figures S-1 to S-13.

708

Ministerio

de

Economía

y

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(79) Ruiz-Agudo, E.; Álvarez-Lloret, P.; Ibañez-Velasco, A.; Ortega-Huertas, M. Crystallographic Control in the Replacement of Calcite by Calcium Sulfates. Cryst. Growth Des. 2016, 16 (9), 4950–4959. https://doi.org/10.1021/acs.cgd.6b00522.



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1007


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FIGURE CAPTIONS Figure 1. Powder X-Ray diffraction patterns of the synthesized nanoparticles. Whe: Whewellite (calcium oxalate monohydrate); Wed: Weddellite (calcium oxalate dihydrate). Figure 2. TEM images of the synthesized nanoparticles: a) amorphous calcium carbonate (ACC) and corresponding SAED pattern of the blue-circled area, and b) amorphous calcium oxalate (ACO) and the corresponding SAED pattern of the bluecircled area. The yellow-circled area shows well-formed whewellite nanocrystals c) HAADF image and Ca-Kα and Si-Kα EDS maps of ACC nanoparticles. Figure 3. Representative FESEM images of the samples after treatment application. Yellow arrows show rhombohedral calcite crystals. The scale bar applies to all photomicrographs. Figure 4. Grazing angle XRD patterns of samples with differences between substrate and nanoparticle composition. Color lines mark the most intense Bragg peaks of the mineral phases: green-calcite, red-gypsum, blue-whewellite. C calcarenite, G gypsum, M marble. Figure 5. BSE images with element maps from selected cross-sections. a) gypsum treated with DPOCa. b) gypsum treated with TPOCa. c) calcarenite treated with DPOCa. d) marble treated with DPOCa. e) calcarenite treated with TPOCa. Color code: blue - Ca-Kα, yellow – S-Kα, Pink – Si-Kα. White arrows indicate the surface of the samples. 43 ACS Paragon Plus Environment


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Figure 1. Drilling resistance measurements for gypsum a) TPO-group b) DPOgroup. Error bars show ±2σN (N≥6) Figure 7. Required time for reaching different pH values (5 - red, 6 - green and 7 blue) during acid-resistance tests for (a) marble, (b) gypsum and (c) calcarenite. Inset in (a) shows an enlarged view of the control and TPO-group treatments (t-3h). Error bars show ±2σN Figure 8. Selected FESEM images of gypsum treated sample surfaces after acidattack (AAA) tests showing: a) corroded (partially dissolved) calcite (area marked by the blue circle), and general loss of the polymer gel; b) partial surface loss of the silica gel coating which also shows crack development (yellow arrows); c) calcite rhombohedra still protected by the silica coating. However, cracking and holes (arrow) are observed in the gel; d) absence of corrosion in the substrate or in calcium oxalate, and incipient development of holes in the polymer gel Figure 9. Static contact angles of treated and untreated samples. Error bars show 2σN

FOR TABLE OF CONTENTS ONLY


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Figure 1. Powder X-Ray diffraction patterns of the synthesized nanoparticles. Whe: Whewellite (calcium oxalate monohydrate); Wed: Weddellite (calcium oxalate di-hydrate). 223x125mm (300 x 300 DPI)


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Figure 2. TEM images of the synthesized nanoparticles: a) amorphous calcium carbonate (ACC) and corresponding SAED pattern of the blue-circled area, and b) amorphous calcium oxalate (ACO) and the corresponding SAED pattern of the blue-circled area. The yellow-circled area shows well-formed whewellite nanocrystals c) HAADF image and Ca-Kα and Si-Kα EDS maps of ACC nanoparticles. 176x154mm (300 x 300 DPI)



Figure 3. Representative FESEM images of the samples after treatment application. Yellow arrows show rhombohedral calcite crystals. The scale bar applies to all photomicrographs.


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Figure 4. Grazing angle XRD patterns of samples with differences between substrate and nanoparticle composition. Color lines mark the most intense Bragg peaks of the mineral phases: green-calcite, redgypsum, blue-whewellite. C calcarenite, G gypsum, M marble. 223x168mm (300 x 300 DPI)



Figure 5. BSE images with element maps from selected cross-sections. a) gypsum treated with DPOCa. b) gypsum treated with TPOCa. c) calcarenite treated with DPOCa. d) marble treated with DPOCa. e) calcarenite treated with TPOCa. Color code: blue - Ca-Kα, yellow – S-Kα, Pink – Si-Kα. White arrows indicate the surface of the samples.


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Figure 6. Drilling resistance measurements for gypsum a) TPO-group b) DPO-group. Error bars show ±2σN (N≥6) 224x97mm (300 x 300 DPI)



Figure 7. Required time for reaching different pH values (5 - red, 6 - green and 7 - blue) during acidresistance tests. Inset in (a) shows an enlarged view of the control and TPO-group treatments (t-3h). Error bars show ±2σN 89x215mm (300 x 300 DPI)


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Figure 8. Selected FESEM images of gypsum treated sample surfaces after acid-attack (AAA) tests showing: a) corroded (partially dissolved) calcite (area marked by the blue circle), and general loss of the polymer gel; b) partial surface loss of the silica gel coating which also shows crack development (yellow arrows); c) calcite rhombohedra still protected by the silica coating. However, cracking and holes (arrow) are observed in the gel; d) absence of corrosion in the substrate or in calcium oxalate, and incipient development of holes in the polymer gel



Figure 9. Static contact angles of treated and untreated samples. Error bars show 2σN 223x157mm (300 x 300 DPI)


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For Table of Content only 82x44mm (300 x 300 DPI)


Bioinspired Alkoxysilane Conservation Treatments for

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