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Publicly Available Published by De Gruyter May 10, 2017

Modification of water transport properties of porous building stones caused by polymerization of silicon-based consolidation products

Ioannis Karatasios, Anastasia Michalopoulou, Maria Amenta and Vassilis Kilikoglou

Abstract

This work studies the polymerization process of four different commercially available silicon-based polymers and their consequent effect on surface tension and water transport properties of natural stones used in architectural monuments, essential for modeling and predicting the durability of natural stones against weathering action of aqueous solutions. The four products studied consisted of two ethyl-silicate based materials, an alkyl-alkoxy siloxane oligomer with hydrophobic agents and finally, a silane/siloxane emulsion. In all cases, the morphology of the amorphous material deposited into the pore network of stones was examined by electron microscopy (SEM). The polymerization process was studied by infrared spectroscopy (FTIR) on both inert surface and porous stone substrate. The treated stone specimens were further tested after polymerization in terms of determination and comparison of water absorption coefficient, open porosity, pore size distribution (mercury intrusion porosimetry) and surface tension (contact angle measurements) properties. According to the analysis, the modification of the wetting properties of the stone surface should be related rather with the chemistry than the microstructure of the xerogels alone.

1 Introduction

Silicon-based polymers offer a variety of very effective materials for strengthening the weathered layers of sandstones and argillaceous limestones, due to their ability to form Si–O–Si bonds with the substrate and thus, improving chemical compatibility and mechanical properties of building stones used in architectural monuments [1], [2], [3]. In a different approach, hydrophobic materials change the surface tension of the stone substrate and thus, they produce a water-repellent surface for keeping away water-based solutions and soluble salts [4].

Today, tetra-ethoxy-ortho-silicate (TEOS) products [(Si(OC2H5)4)n] provide a wide range of commercially available conservation products with various strengthening properties and hydrophobic/hydrophilic characteristics. These products are applied in liquid state, they are absorbed and penetrate into the stone mass by capillary absorption. After application, environmental humidity replaces the –C2H5 groups with –OH groups leading to the hydrolysis of the consolidation products and deposition of the amorphous silica (SiO2). In a very simplified form, the hydrolysis reaction of TEOS could be expressed by the following equation:

(1) Si(OR) 4 + 2H 2 O SiO 2 + 4ROH

However, this is a very slow process that may take several weeks to be completed [5]. After hydrolysis, the oligomers condense and form the amorphous silica gel that is deposited inside the stone matrix, developing covalent bonds with the silanol groups on the surface of the stone [6].

Conservation practice requires extensive laboratory studies and performance evaluation before field application on stone monuments [7]. Although the majority of laboratory tests focus on performance evaluation, durability and compatibility of consolidation and surface protection materials [8], [9], [10], [11], [12], [13], time and condition of curing have not been studied thoroughly. For example, the hydrolysis and condensation of alkoxysilanes depends on environmental parameters such as relative humidity, pH, catalysts, etc [14] and thus, different curing conditions may lead to different properties and performance characteristics [4]. However, what is usually followed during laboratory tests is a standard curing time for 3–4 weeks at laboratory conditions. This practice is based on the technical data sheets of the commercial products, which usually simplify the application requirements [5], [15].

The goal of this paper is therefore to highlight the important role of curing time, by studying the deposition mechanisms of the materials into the stone microstructure. It is shown that the main parameter that affects the performance characteristics of consolidation materials is the degree of polymerization or otherwise, the completion of the transformation process of oligomers to amorphous silica and the elimination of any residual unreacted silane groups on the stone surface. The study aims to contribute towards the development of reliable laboratory evaluation of results that may prolong the service life of field conservation interventions.

2 Materials and methods

2.1 Consolidation materials

Four commercial products were used in this study which are grouped in two main categories, according to their use. In particular, the first category included two consolidation/strengthening products, namely: Wacker BS OH-100 [tetra-ethyl silicate (TEOS), with an active content ~99%] and Remmers KS 300HV (TEOS, with active content ~95% and additives). Aiming to highlight the “fake” hydrophobic behavior of the non-hydrophobic consolidation products (due to incomplete hydrolysis), the second category included for comparison reasons two water-repellent products, namely: Remmers Funcosil SL (alkyl-alkoxy silane with additives, with active content ~7%) and Wacker SILRES BS 1001 (silane-siloxane emulsion, with active content ~10%, diluted to 1:4 in de-ionized water). The two water repellents were therefore used as a reference material for their hydrophobic properties.

2.2 Test specimens

Two types of substrates were selected for studying the polymerization process of consolidation products; (a) Si-wafers, as an inert surface and (b) natural stone specimens. The stone samples were consisted of marly limestone, being representative of the majority of stones used in a wide range of architectural monuments in the Mediteranean basin. The mesoporous structure of the specific stone and the presence of a relative high percentage of open pores create a set of ideal microstructure characteristics for test specimens. Stone samples belong to the coastal sedimentary Neogene formations, from the area of Piraeus coast, in Greece.

Mineralogical analysis of stone samples was carried out by X-ray powder diffraction (XRD), on a Siemens D-500 diffractometer, using the Cu-Kα radiation (λ=1.5406 Å) with a graphite monochromator in the diffraction beam, at 1.2 kW (40 kV, 30 mA). Spectra were collected in the range of 2–60° 2θ, with a step of 0.03°/s.

Petrographic examination of thin-sections was carried out under the petrographic microscope, in crossed and parallel Nichols, aiming to describe their fabric, microstructure and mineralogy. Finally, cross sections and freshly fractured surfaces of treated stone specimens were also examined under scanning electron microscope (FEI – Quanta Inspect) coupled with energy dispersive X-ray analyser (SEM/EDX).

According to the requirements of standard tests for the characterization of the microstructural properties, plate-like (4×4×1 cm) and cubic specimens (4×4×4 cm) were shaped and treated with the selected consolidation products. Since the aim of this study was the monitoring of the hydrophobic behavior of the treated specimens and the consequent modification of their water transport properties, rather than the evaluation of the consolidation/surface protection efficiency, the test specimens were not weathered.

The stone samples were initially dried at 60±5°C, until constant mass. In order to provide a uniform application procedure and avoid potential performance differences and/or questions due to different method of applications, capillary absorption was used in all treatments. Therefore, each side of the specimens was immersed in the solution to a depth of 2±1 mm and left to absorb through capillary pores for a period of 30 min. In this way an homogeneous application of all the treatments was ensured. In addition, three drops from each different consolidation materials were spread on the Si-wafers surface. The use of Si-wafers was aiming to the better monitoring of polymerization in an inert substrate, without any interference of the stone material, e.g. Si–O and C=O bands from quartz and carbonates, respectively. For both type of substrates, a reference specimen was used for comparison. During curing, all specimens were kept under controlled environmental conditions (25±1°C, 50–55% RH), with a continuous control of the weight change.

2.3 Analytical methodology

In order to study the effect of consolidation materials on the modification of the pores space network and water transport properties of stone specimens, the polymerization process of the silane-based materials was monitored at preset time intervals (between 20 h and 12 months) by FTIR analysis, while in addition, the microstructural characteristic of xerogels and the subsequent effect of their deposition on the stone material were examined by scanning electron microscopy (SEM) [16].

The polymerisation process was studied by FTIR spectroscopy in uniform thin films (1–3 μm) on silicon wafer, left for 20 h to set before the first measurement. FTIR measurements were used for providing detailed information on the chemical structure of consolidation materials and monitoring changes in chemical bonding during time [17]. Spectra were collected on a Bruker-Tensor 27 spectrometer, in transmission mode [18], in the range of 4000–400 cm−1 (resolution 4 cm−1). Transmission mode allowed collection of average information across films, representing changes in the entire body of each material. The spectra were processed on Bruker OPUS software, version 4.2; baseline correction was applied with special care not to affect any particular absorption. No smoothing or normalization was applied.

SEM images were recorded using a FEI Quanta Inspect instrument operated at 25 kV. All specimens were sputter-coated with gold prior to examination. The petrophysical characterization of both untreated and treated stone samples was based on the determination of water absorption coefficient (C), according to the procedure described in EN 1925 [19], open porosity and pore size distribution determined by mercury intrusion porosimetry (MIP). Representative samples of approximately 1 cm3 cut from treated areas of the stone specimens (sampled from the external 5 mm of the treated cubes) were analyzed in a PoreMaster 60 (Quantachrome) mercury intrusion porosimeter, in both low and high pressure.

Finally, the hydrophobic characteristics of treated and reference surface characteristics were studied through continuous contact angles measurements between water droplets (10 μL) and the stone surface, via the sessile drop method. The contact angles were measured with a Contact Angle Meter (CAM) 100 (KSV Instruments, Ltd.) equipped with a horizontal microscope and a protractor eyepiece.

3 Results and Discussion

3.1 Mineralogical properties of stone substrates

Mineralogical analysis of natural stone substrate resulted the presence of calcite (CaCO3) and quartz (SiO2) as primary constituents along with secondary phases such as dolomite, illite/montmorillonite and chlorite.

The petrographic examination in crossed pollars (XPL) indicated a porous structure where micritic calcite crystal is the dominant phase and act as inorganic binder for clastic phases of quartz, illite/montmorillonite and other aluminum–silicate phases (Fig. 1). Bivalve fragments and casts are abundant within the limestone matrix, where in some cases the shell moulds have been filled with large calcite crystals. The presence of silicate minerals favors the chemical compatibility of the silicon-gel formed during polymerization and the stone substrate due to the developed bonding between the methyl groups and the hydroxyl sites of the substrate [1], [6].

Fig. 1: 
            Petrographic characteristics of stone substarte in (a) optical (XPL) and (b) SEM microscopes, exhibiting the messoporous structure formed by micritic calcite and argillaceous phases.

Fig. 1:

Petrographic characteristics of stone substarte in (a) optical (XPL) and (b) SEM microscopes, exhibiting the messoporous structure formed by micritic calcite and argillaceous phases.

Moreover, SEM examination in backscattered mode (Fig. 1) indicated the clastic texture and open pore structure of stone matrix in messo-scale that allows the absorption and transport of consolidation materials inside stone matrix. Quartz and feldspar particles are surrounded and connected by fine calcite particles.

The study of natural stone mechanical properties provided the the strength values for 3-point bending (7.6 MPa), unconfined compressive strenght (16.3 MPa) and E modulus (7.5 MPa).

3.2 Microstructure examination and properties

The evolution of the polymerization progress was initially monitored through the weight change of the treated stone specimens (Fig. 2). The initial strong reduction attributed to the evaporation of the solvent from the stone matrix and it is followed by the slower polymerization of the active material and the deposition of the amorphous silica. At the same time, although solvent-free, weight decrease of TEOS is due to formation and evaporation of ethanol during ethyl-otrthosilicate polymerization, according to the following reaction:

Fig. 2: 
            Weight change of the treated stone specimens, at different curing times.

Fig. 2:

Weight change of the treated stone specimens, at different curing times.

(2) Si(OC 2 H 5 ) 4 + 2H 2 O SiO 2 + 4C 2 H 5 OH ( )

The most notable weight reduction for all specimens is observed after the first 24 h. The water repellent materials (Funcosil SL and SILRES 1001) follow a very fast rate resulting a weight reduction of about 7% w/w within 3 days. After 7 days the graph follows an asymptotic trend, indicating the end of solvent evaporation. In contrast, the consolidants (BS OH 100 and KSE 300 HV) present exactly the same behavior to each other and follow a considerably slower evaporation rate. After the first week of curing the weight reduction is about 4% w/w. However, tetra ethyl orthosilicates (TEOS) reduce further their weight, indicating that the evaporation, condensation and gelatination process are active and can be monitored by mass changes at least until 28 days, exhibiting an asymptotic trend after 2 months. The effect of the application of the four products on the water block ability of the stone, related with the curing period is presented at Fig. 2.

After 28 days of curing the microstructure of the treated specimens were examined in SEM in order to study the morphological characteristics of the material deposited within the pores. The penetration depth was estimated by SEM/EDX examination of polished sections to 1.5–2 cm for consolidation materials and to 2–3 mm for water repellent materials. SEM examination (Fig. 3) revealed the way that different materials are deposited and the form of xerogels on the stone substrate: the two ethyl silicates (BS OH and KSE 300 HV) form a dense thick film up to 3 μm that covers the grains surface. In the case of the silane/poly-siloxanes (Funcosil SL and SILRES 1001) the active component tends to deposit between the grains of the substrate forming non-continuous films of irregular morphology. A common observation among the four materials is the uneven distribution of the material within the total mass of the porous substrate and their presence in layer between 3 and 10 mm below the surface of the specimens.

Fig. 3: 
            SEM photomicrographs of fractured stone surfaces depicting the deposited materials into the pores surface of stone substrate, acquired 5–10 mm below the surface (a) BS OH (b) KSE 300 HV (c) Funcosil SL and (d) SILRES 1001. The hydrophobic components (c and d) tend to form a non-continuous films of irregular morphology.

Fig. 3:

SEM photomicrographs of fractured stone surfaces depicting the deposited materials into the pores surface of stone substrate, acquired 5–10 mm below the surface (a) BS OH (b) KSE 300 HV (c) Funcosil SL and (d) SILRES 1001. The hydrophobic components (c and d) tend to form a non-continuous films of irregular morphology.

The effect of the deposited material on the surface characteristics of the treated stone specimens was determined through static contact angle measurements, calculating the angle between a water droplet and the substrate surface. The measurements carried out at preset time intervals between 20 h and 10 months curing time. The results (Fig. 4) indicated an intense hydrophobic behavior of all specimens, especially during the first 2 months. Although the hydrophobic behavior was expected for the two water repellents (Funcosil SL and SILRES 1001) it is quite strange for the two consolidants (BS OH and KSE 300 HV). The measurement of the contact angle exhibited a sharp reduction after 35 days, whereas, the values of the two consolidants BS OH and KSE 300 HV came in accordance with the expected ones far after 2 months. It is also noticeable that, after 10 months none of the materials presented hydrophobic properties, while the water drop was absorbed immediately.

Fig. 4: 
            Variations of hydrophobic surface properties of treated stone specimens, reflected on contact angle measurements. Due to high hydrophylicity and enhanced open porosity of stone substrate, the contact angle could not be measured in the reference specimens.

Fig. 4:

Variations of hydrophobic surface properties of treated stone specimens, reflected on contact angle measurements. Due to high hydrophylicity and enhanced open porosity of stone substrate, the contact angle could not be measured in the reference specimens.

The intense water repellency discussed above (until the end of curing time) had a great impact on the measurement of open porosity as determined by the hydrostatic method, through water saturation by capillary absorption (Figs. 5 and 6). Capillary absorption is associated with water transport properties, as well as the pore space characteristics (open porosity and pore size distribution) (Figs. 5 and 8), the surface tension between the water and the pores and, finally, with the weathering phenomena and the physico-chemical reactions inside the porous substrate. The impact of hydrophobic characteristics on the porosity values is reflected on the great difference between the measurements carried out at 28 days (the common period used among conservators for the determination of water absorption and porosity) and then at 12 months. The values obtained at 28 days by water absorption are very low and quite different from those determined by mercury porosimetry at the same age, since the later eliminates any surface tension effect. Moreover, these values are not supported by the open structure that is described by pore size distribution results (Fig. 8). However, the open porosity values determined by water absorption after 12 months curing are very close to those of MIP determined at 28 days (Fig. 6). Overall, KSE 300 HV had the greater impact on porosity values, reducing the open porosity up to 10%, followed by BS OH 100 (~6%) and Funcosil SL (~4%).

Fig. 5: 
            Physical properties of stone specimens related to the water transport properties of natural stone samples (both treated and untreated), determined by MIP and water absorption/saturation.

Fig. 5:

Physical properties of stone specimens related to the water transport properties of natural stone samples (both treated and untreated), determined by MIP and water absorption/saturation.

Fig. 6: 
            Open porosity values of treated stone specimens measured at 4 weeks and 12 months by water absorption and mercury intrusion porosimetry. The differences observed between the values determined by water absorption and MIP are due to the evolution of the hydrophobic properties of the materials applied (see also Fig. 5), highlighting the importance of MIP measurements.

Fig. 6:

Open porosity values of treated stone specimens measured at 4 weeks and 12 months by water absorption and mercury intrusion porosimetry. The differences observed between the values determined by water absorption and MIP are due to the evolution of the hydrophobic properties of the materials applied (see also Fig. 5), highlighting the importance of MIP measurements.

A very similar trend is observed on the capillary absorption coefficient, where initially all specimens present very low values and consequently very low water accessibility (Fig. 7), independently of their pore space characteristics (Fig. 8). The absorption behavior is very different after 10 weeks of curing, where there is an obvious differentiation between hydrophobic and non-hydrophobic treatments. The water absorption coefficient values depend on both surface hydrophobic/hydrophilic character and on pore space properties (open porosity and pore size distribution values) of the treated stone specimens. The results of BS OH 100, KSE300 HV and SILRES 1001 specimens (Fig. 7) exhibit an increase. This is in agreement with contact angle values (Fig. 4) and the slight shift/alteration of the pore size distribution (Fig. 8) towards to higher amount of capillary pores.

Fig. 7: 
            Modification of water absorption coefficient of treated specimens, at different curing periods.

Fig. 7:

Modification of water absorption coefficient of treated specimens, at different curing periods.

Fig. 8: 
            Comparison of pore size distribution between treated and reference stone specimens (a) high pressure and (b) low pressure.

Fig. 8:

Comparison of pore size distribution between treated and reference stone specimens (a) high pressure and (b) low pressure.

Since the hydrolysis and condensation of TEOS inside the stone pores involves the reaction of ethyl silicate with atmospheric moisture [14] they are environmental depended processes. Consequently, the treated stone surfaces remain hydrophobic for several weeks after application [6].

The hydrophobic behavior of treated stone specimens in all the above cases should be attributed to the incomplete hydrolysis and condensation of TEOS inside stone pores and the residual ethoxy groups in the deposited gel, which is documented by FTIR results (Fig. 11a). The relevant absorption bands of ethoxy groups are marked at 2980 cm−1 (CH3) and 2930 cm−1 (CH2) [15].

The interpretation of pore size distribution results indicates that the deposited material manly influences the upper and the lower limit of the mesopore area, as well as the lower part of the macropores (Fig. 8). The treated stone specimens exhibit an increase of the pore volume below 0.01 μm and appear a small peak at the pore radius range of 0.002–0.01 μm. This pore volume should be related to the porosity of the deposited consolidation material and the formation of the silica gel matrix. The main pore volume is slightly shifted to pore-radii above 1 μm, due to the deposition of consolidants inside walls of larger pores. For the same reason, the amount of pore volume below 10 μm is increased.

Finally, concerning the modification of porosity and pore size distribution, a significant result during SEM examination was the presence of extensive gel cracking of the two TEOS-based materials (Fig. 9). This is a well known phenomenon [17], associated with the formation of the gel during condensation of the resin and the evaporation of the solvent. In particular, the cracking effect results from the differential capillary pressure between solvent and gel that occurs inside the drying body resulting to a non non-uniform contraction of the network [18].

Fig. 9: 
            SEM photomicrograph exhibiting extensive gel cracking of TEOS-based KSE 300 HV material after condensation and gel formation.

Fig. 9:

SEM photomicrograph exhibiting extensive gel cracking of TEOS-based KSE 300 HV material after condensation and gel formation.

3.3 FTIR analysis

Aiming to explain and support the above experimental results by chemical modification of the conservation materials after treatment of stone specimens, the monitoring of polymerizations was carried out by FTIR analysis at different time periods, in Si-wafers. For comparison purposes, FTIR spectra of treated stone samples are presented in Fig. 11c, indicating the evolution stage of polymerization after 10 months.

The FTIR spectra of organo-silicons after 20 h setting time defined the characteristic vibration bands related to the different categories of the products (Fig. 10).

Fig. 10: 
            FTIR spectra of organo-silicons after 20 h setting time defined the characteristic vibration brands related to the different categories of the products.

Fig. 10:

FTIR spectra of organo-silicons after 20 h setting time defined the characteristic vibration brands related to the different categories of the products.

Around 970 cm−1, the peak associated with the stretching vibration of Si–OH it is observed. The marked bands at 1168 cm−1 (faint shoulder), 1127 cm−1 and 1038 cm−1 are attributed to the non-hydrolyzed ethyl silicate groups (Si–O–C) and the condensed silica (Si–O–Si), respectively. The band at 970 cm−1 is attributed to silanol (Si–OH) groups and the Si–O free broken bonds [20]. The broad band at 3200–3600 cm−1 and the faint peaks at 1400–1800 cm−1 could be associated with the residual OH groups and the water/humidity effect.

In the case of polysiloxane-based water repellents, besides the evaporation of water and organic solvent, clearly the modification of the spectra is less obvious, mainly related to the shape of the peaks in the band of 1000–1200 cm−1, where it is recorded the stretching of different (Si–O) groups (the band at 1075 cm−1 increases with respect to the 1270 cm−1, which corresponds to the C atom attached to Si) [5].

The evolution of polymerization process can be therefore monitored by following the evolution of Si–O–C and Si–O–Si groups (Fig. 11a,b). According to the literature [5], [20], during the evolution of the polymerization process the methyl/ethyl groups (C–H) tend to disappear (in particular there is a reduction of the intensity of C–H stretching around 2900 cm−1 and almost disappearance of the related bands in the area between 1200 and 1500 cm−1), while there is an increase in the intensity of Si–O–Si groups (corresponding to the silica gel formed) and the consequent intensity increase of Si–O between 1035 and 1080 cm−1.

Fig. 11: 
            Spectra of representative FT-IR bands at different curing periods, indicating the evolution of polymerization process: (a) KSE 300 HV on Si-wafer, (b) Silres 1001 on Si-wafer, (c) BS OH 100 on stone.

Fig. 11:

Spectra of representative FT-IR bands at different curing periods, indicating the evolution of polymerization process: (a) KSE 300 HV on Si-wafer, (b) Silres 1001 on Si-wafer, (c) BS OH 100 on stone.

Similarly, in stone samples (Fig. 11c) measured after curing for 10 months, the Si–O–Si band (1080 cm−1) is remarkably more pronounced, while the faint Si–O–C band (1169 cm−1) that is detected in 28 days is barely detected in 10 months, suggesting that polymerization has proceeded but still not completed.

Overall, the continuous monitoring of the polymerization process by FTIR indicated a continuous change of the siloxane (Si–O–Si) and methyl (C–H) bands for periods far beyond 4 weeks or 2 months.

4 Conclusions

The present work has demonstrated the importance of curing time in the efficiency of consolidation in the conservation practices, as well as the importance of monitoring the water related properties of the cured substrates, in order to evaluate effectively the procedure.

Based on the above conclusions, laboratory tests for performance evaluation should be carried out during a curing period of at least 8–10 weeks, considering data of both FTIR and contact angle results. This is important since common practice is the evaluation of such procedures 4 weeks after the application of the consolidation materials, at laboratory conditions.

Alkoxysilane based treatments may attribute to stone surface a fake hydrophobic character for several weeks after application, until the hydrolysis–condensation reactions are completed.

Considering the main evaluation approaches followed in practice, weight monitoring presents considerable limitations, since it provides only indications on the evaporation of solvent(s) and not on the state of hydrolysis–condensation reactions. Instead, the detailed study of polymerization by FTIR spectroscopy on both inert and stone substrates offers a more robust and reliable tool.

An alternative tool for polymerization monitoring is the the study of the time-depended hydrophobicity of TEOS-based products, which is very easily evident by the reduction of the contact angle through time.

The microstructure of the silica gel is characterized by the non-uniform distribution and by the presence of cracks. This gradual gel-cracking strongly affects porosity, water transport properties and performance of stone-consolidant system.

The polymerization of silicone based conservation products is a time, humidity and pH depended process. Consequently, the determination of the physicochemical properties of the treated porous substrate prior to the completion of polymerization reflects only the specific “moment” of the performance characteristics and the service-life of the material-substrate system.


Article note

A collection of invited papers based on presentations at the 16th International Conference on Polymers and Organics Chemistry (POC-16), Hersonissos (near Heraklion), Crete, Greece, 13–16 June 2016.


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Published Online: 2017-05-10
Published in Print: 2017-10-26

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