Effectiveness of a novel consolidant on granite: Laboratory and in situ results

Construction and Building Materials 76 (2015) 140–149

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Effectiveness of a novel consolidant on granite: Laboratory and in situ results Iván De Rosario a, Farid Elhaddad b, Aldara Pan c, Rosa Benavides d,1, Teresa Rivas a,⇑, María J. Mosquera b,⇑ a

Dpt. Ingeniería de los recursos naturales y medioambiente, ETSI Minas, Universidad de Vigo, Campus Lagoas Marcosende, Vigo 36310, Spain TEP-243 Nanomaterials Group, Departamento de Química-Física, Facultad de Ciencias, Campus Universitario Río San Pedro, Universidad de Cádiz, 11510 Puerto Real, Cádiz, Spain c Dpt. Física Aplicada, E.T.S.I. Industriales, Universidad de Vigo, Campus Lagoas Marcosende, Vigo 36310, Spain d Tomos Conservación Restauración, S.L. Rúa do Brasil, 37, 36204 Vigo, Spain b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 A new surfactant-synthesised

nanoconsolidant was applied on a romanesque church.  A previous laboratory evaluation was carried out on the granite from the original quarry.  Two popular commercial consolidants (acrylic resin and ethyl silicate) were also evaluated.  The new product effectiveness was demonstrated for laboratory and in situ conditions.  This work also defines application protocol for consolidation processes on granites.

a r t i c l e

i n f o

Article history: Received 28 January 2014 Received in revised form 11 October 2014 Accepted 26 November 2014

Keywords: Consolidation Nanomaterial Granite n-Octylamine Soluble salts Bees wax Peeling test Slake durability test

a b s t r a c t In this article, the protocol to identify the most suitable consolidant to be applied in a granitic Romanesque church form Galicia is presented. The study consisted in two phases: an initial laboratory evaluation during which the treatment with a nanoconsolidant never applied on granite is compared with the treatment with two commercial consolidants (an acrylic resin and ethyl silicate), evaluating several properties in order to determine the effectiveness and harmful effects on the stone. In this phase, the new nanomaterial was identified as the most suitable product in order to slow down the specific deterioration processes which affect the building. In a second phase, the evaluation of the effectiveness and harmful effects of the application of the nanoconsolidant in the building is presented. This article also provides new data about two procedures to asses effectiveness never applied in granites. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction ⇑ Corresponding authors. Tel.: +34 986811922; fax: +34 986811924 (T. Rivas). Tel.: +34 956016331; fax: +34 956016471 (M.J. Mosquera). E-mail addresses: [email protected] (A. Pan), [email protected] (R. Benavides), [email protected] (T. Rivas), [email protected] (M.J. Mosquera). 1 Tel.: +986 47 18 10. http://dx.doi.org/10.1016/j.conbuildmat.2014.11.055 0950-0618/Ó 2014 Elsevier Ltd. All rights reserved.

Consolidation is an intervention customarily applied in stone structures of architectural, artistic and archaeological interest whose goal is to restore the cohesion to the construction materials that has been lost as a result of various deterioration processes. In

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the case of the granitic stones used for such buildings and structures, consolidation is also the treatment commonly applied to mitigate the effects of salt crystallization processes which, in this type of rock, generate two serious deterioration forms affecting the rock cohesion: granular disintegration or sanding (associated with sodium chloride effect) and scaling (associated to the action of sulphates of moderate solubility) [1–4]. In the 1960s, a restoration program was undertaken to consolidate the granitic stonework of the architectural heritage of Galicia (NW of the Iberian Peninsula) based on the application of beeswax on the surfaces affected by granular disintegration and scaling [5]. Over time, the application of wax had very unfortunate consequences: once solidified, the wax blocked the pores of the stone and effectively prevented the water contained in the stone to be evaporated, with the result that the salts which remained into the stone tended to precipitate below the treated surface, creating an intense process of granular disintegration or detachment and seriously endangering the preservation of much of Galician architectural heritage. Furthermore, as the wax aged, particles of atmospheric dust and pollutants became embedded in the surfaces, turning them black in colour. Currently, interventions are being made for these surfaces treated with wax, with the object of eliminating the wax residues by mechanical or physical methods, such laser treatment [6], and of subsequently consolidating with suitable products. The Church of Santa María del Campo (A Coruña, Spain) is a clear example of the deterioration described above. This monument has a special historical importance and artistic interest, being representative of the Romanesque style of the Northwest of the peninsula. It is constructed entirely of granite and presents a severe deterioration in form of granular disintegration and superficial detachments. In an earlier study [7], the action of the salts from atmospheric origin (marine aerosol and anthropogenic emissions) has been associated with these severe deterioration forms which affect all of the building’s facades, especially the South-facing facade. Furthermore, during the 1960s the church was subjected to a consolidation with beeswax [5]; this treatment was applied principally to the sculpted surfaces, which are the most highly valued in historical and artistic terms. These areas in particular currently appear intensely eroded, with surfaces lacking cohesion and blackened in colouring. In the context of a recent restoration program [8], the wax removal was performed after which, and due to the intense loss of rock cohesion, the consolidation of the surfaces was considered necessary. The chosen product for the consolidation intervention should be effective enough to increase the cohesion of the rock but also have adequate resistance to the action of soluble salts; the remove of the salts from the deteriorated rock surfaces prior to the consolidation was discounted as not viable, due to the risk of losing even more surface material from the deteriorated surfaces by applying the method of extraction of salts (usually poultice based methods). To identify the consolidant most suitable for this situation, it was decided to carry out a prior laboratory evaluation of the treatment, on the same rock of the building, with two conventional commercial consolidants (an acrylic resin and a ethyl silicate) and a new nanomaterial, recently developed and commercially available under the corresponding exploitation patent [9]. The main components of this novel product are alkosylanes. As wellknown [10], these products polymerise in situ inside the pore structure of the disintegrating stone, through a classic sol–gel process, and significantly increase the cohesion of the material. Their advantages are well known: (1) The low viscosity of alkosysilane facilitates its penetration into the intergranular network of the stone and its ability to form siloxane bonds. This low viscosity

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prevents the dilution in organic solvents and, thus, a high consolidant matter is obtained. (2) No thermal processing is required in the synthesis processes developed. Thus, these products find good application on thermally-sensitive substrates, are more cost-effective and, particularly important, meet the requirements necessary for application in situ to exterior surfaces of buildings. The process is so simple that the consolidant is directly generated in the building, under normal outdoors conditions. The starting sol prepared in our laboratory consists simply of a silica oligomer and a surfactant. There are two fundamental reasons for adding a surfactant [11– 14]: (1) to prevent cracking by coarsening the pore structure of the gel network; and (2) to act as a basic catalyst of the sol-transition on the substrate surface. Finally, it should be stated that no volatile organic compound (VOC) is added to the sol. There are two important reasons for not incorporating volatile organic solvents: (1) to make ‘green’ conservation products; and (2) to increase the proportion of dry matter of the product that is applied onto the substrate, in order to improve its effectiveness [11]. The effectiveness of this simple product as a stone consolidant has been previously demonstrated [11]. Moreover, simple modifications of this route have produced hydrophobic materials [11] and photoactive products with self-cleaning properties [12,13]. The effectiveness of this low-cost and simple route has been already confirmed on different building stones. Moreover, other researches working in this field employed later our strategy (n-octylamine addition) to obtain crack-free products [15,16]. The effectiveness of these products has been tested in the studies cited above, in all cases of sedimentary and metamorphic rocks, but studies on its effectiveness in granitic rocks are scarce [17] and in no case direct application in granite monuments has been reported. This article describes the scientific procedure followed in order to perform a consolidation intervention on the South-facing facade of the church of Santa María del Campo; this part of the church is badly affected by the kinds of deterioration already described. The procedure was carried out in two phases: in the first one, performed entirely in the laboratory, the effectiveness and harmful effects on the stone of the novel consolidant was evaluated and compared with those obtained for the two commercial consolidants previously mentioned. Given the humid sub-tropical climate of the area in which the church is situated, it was considered of interest to also study the influence of different moisture contents of the stone on the uptake of the consolidants tested. Once identified the most appropriate consolidant during the laboratory study, the second phase consisted of the application of the product in the most deteriorate façade of the church and of the subsequent evaluation of its effectiveness and harmful effects in situ. This work also aims to make a contribution on methodologies for the evaluation of the effectiveness of consolidants in granitic rocks. The evaluation of the effectiveness of consolidation treatments in granites has two main problems. Firstly, the polimineral grained texture of this rock makes it difficult to apply laboratory techniques that provide consistent results in other rock types, such as microdrilling techniques [12,18]. So, in this article, we provide the first data on the applicability of the slake durability test to evaluate consolidation treatments. On the other hand, non-destructive tests based on the measure of bulk properties (such as ultrasonic wave velocity), commonly applied in situ, would not be applicable in granites since the alteration profile associated to granular disintegration is not very deep [3,4], and, so, hardly detectable by these techniques. So, in this study, we conducted an evaluation of the applicability of the peeling test to granitic rocks, an alternative protocol successfully applied in sedimentary rocks [19] but never tested in granitic architectural heritage.

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Fig. 1. (A) View of the main facade of the historic building in which areas affected by black crust derived from bee-wax application can be observed. (B) Areas to be treated: capital (Cap) and two adjacent ashlars (6H and 7H); the two zones of different colour in 7H (7H-b and 7H-w) are also shown. (C) Detail of the treated capital affected by a severe granular disintegration which has been eroded the anthropomorphic figures; in this picture, the two treated areas (C-A and C-B) are also shown.

2. Materials and methods

2.2. Rock sampling and characterisation

2.1. Description of the building

The leucogranite used in the construction of the Church of Santa María del Campo outcrops in an area a few km from the city of A Coruña, in the locality of San Pedro de Visma. The principal mineralogical characteristics of this rock are the low content of biotite (which confers a light colour to the rock) and the notable mineral orientation marked by the phyllosilicates [21]. It is composed of quartz, plagioclase, microcline and muscovite; among its accessory minerals are biotite, opaques and zircon. In a now-disused quarry at San Pedro de Visma, a sufficient quantity of granite was extracted for performing its physical characterisation; for this and for the laboratory phase of consolidation effectiveness evaluation, test pieces of variable dimensions were prepared. Water accessible porosity (open porosity) was determined in accordance with the RILEM standard [22] and water content was determined following ICR-CNR standards [23]. Five stone samples of 5  5  5 cm were employed in all the tests. Mercury accessible porosity and porosimetric distribution was obtained by mercury intrusion porosimetry (MIP). The assay was carried out on three stone specimens, each with a volume of around 1 cm3, using a PoreMaster-60 device from Quantachrome Instruments comprising two measurement units: a low pressure unit (Pascal 140), whose pressure range is between 0.69 kPa and 350 kPa, and a high pressure unit (Pascal 440) whose pressure range is between 0.1 MPa and 420 MPa. The use of two units enables the measurement of pores with diameter in a wide range, between 950 lm and 3.9 nm. Diffusivity to water vapour was

The Church of Santa María del Campo (Fig. 1a) is located in the city of A Coruña, situated in the Autonomous Region of Galicia (NW of Spain). According to the FAO’s agro-ecological zoning, Galicia has a humid sub-tropical climate with rainy winters (1200 mm annual rainfall) and warm temperatures (15 °C annual average) [20]. The church is located in the urban centre of the city, recently restricted to road traffic. It is built of a medium to fine grained leucogranite outcropping near of A Coruña. The building is affected by severe granular disintegration and scaling associated with the combined action of soluble salts, principally of atmospheric origin [7], and the previous application of bee wax. The south-facing facade is the part worst affected by these forms of alteration, especially its sculpted or carved areas: archivolt, columns, capitals and other sculptured elements (Fig. 1b and c). During the conservation intervention of the building, an initial phase of cleaning and removal of the wax was conducted by mechanical methods (using chisels) and, in some problematic areas, with laser (NdYAg, at 266 nm), which has been proven effective in bee wax removing in other buildings with the same problem [6]. After cleaning, the degree of loss of cohesion of the surfaces required the application of a consolidant. Furthermore, due to the impossibility of eliminating the source of soluble salts, finding an effective consolidant but also resistant to soluble salts crystallization was demanded.

I. De Rosario et al. / Construction and Building Materials 76 (2015) 140–149 determined by means of an automatic set-up previously devised [24] and based on the standard cup test [25]. The experiment was performed at constant temperature (20 °C) using three slabs of 4  4  1 cm for each determination. The slabs were each placed to cover an open-top cup (i.e. a receptacle with no top). The cup was suspended from a scale interfaced with a computer. Silica gel is placed inside the cup in order to maintain 0% relative humidity (RH), while the RH outside was maintained at 75% RH by means of a saturated NaCl solution. Changes in the mass of the cup were monitored by means of computer software, allowing the progress of vapour transport to be recorded continuously. The water vapour diffusion coefficient was calculated from steady flow data.

2.3. Consolidant products and application procedure The following three products were selected for testing: Paraloid B-82, a 100% acrylic resin based on methyl-methacrylate, marketed by the company Rohm and Haas. The product to be applied was prepared after dilution of the commercial solid product to 3% (w/v) in an ethanol:water solution (9:1 v/v). Estel 1000, a colourless liquid with a density of 0.97 kg/L at 20 °C composed on ethyl silicate dissolved in organic solvent (White Spirit D40) and marketed by CTS España. UCA-2o, a sol consists of a mix of Dynasilan 40 (from Evonik) and a surfactant (n-octylamine, from Aldrich). According to its technical data sheet, Dynasilan 40 is a mixture of monomeric and oligomeric ethoxysilanes with a SiO2 content of 40%. The synthesis route of UCA-2o was as follows: (1) an aqueous solution of n-octylamine with a concentration of the surfactant significantly higher than that corresponding to its critical micellar concentration (cmc), which is 0.010 M [26], was prepared by vigorous stirring. Specifically, a 1.57 M aqueous solution of n-octylamine was employed. Turbidity of solution due to the formation of micelles was clearly observed; (2) the aqueous solution of n-octylamine was mixed with Dynasilan 40 under stirring, in a mole ratio of Dynasilan to n-octylamine of 1:5
10 4. The sol was homogenised by high-power ultrasonic agitation (60 W cm 3) for 10 min and then it was ready to be applied on the stone samples without addition of any solvent. The consolidants were applied in prismatic pieces of varying dimensions depending on the procedures for determining the effectiveness and harmful effects. Two successive applications were made at a one minute interval.

2.4. Laboratory evaluation Uptake and dry matter content were determined on 4  4  2 cm test pieces and using five pieces by treatment. In this samples, treatments were applied in the face of the largest dimension. As stated in the objectives of the study, the influence on uptake and dry matter content of the presence of water on the rock substrate was evaluated, performing the treatment under two different conditions: (1) stone samples dried at 40 °C until constant weight; (2) stone samples with a water absorbed content of 0.63%, which corresponds to 50% of its saturation percentage. These contents were determined according the water absorption kinetic following [23] and knowing the quantity of water absorbed under vacuum according to [22]. For the two conditions evaluated, the products were applied under laboratory conditions (20 °C, 45% RH). The uptake of the consolidants was calculated by the difference of weight, expressed in % (w/w), after and immediately before the treatment. The treated test pieces were left to dry under laboratory conditions until reaching constant weight (20 days). At that point, the content of dry matter was determined, also by difference of weight, expressed in % (w/w). Surface fragments of each treated stone specimen and its untreated counterpart were visualised by Scanning Electron Microscopy (SEM) using a JEOL Quanta 200 Scanning Electron Microscope in order to observe the distribution and morphology of the coatings of the three consolidants on the surface of the granite. To evaluate the effectiveness of the consolidants, three determinations were applied which indirectly can give information about the ability of the products to bring back the cohesion to the rock: the reduction on mercury accessible porosity, the resistance to salt crystallization test and the resistance to alteration according the standard slake durability test. The reasons to select these tests are the following: previous studies on granites consolidation showed that the amount of consolidant absorbed for these stones is significantly low [27–29]. For this reason, in these studies the determination of bulk properties, such as mechanical strength or water accessible porosity, did not give reliable information about the improvement in the rock cohesion. As reported in the earlier study [29], mercury accessible porosity and artificial ageing tests (salt crystallization) give more realistic results about the effectiveness of consolidant products. So, the effectiveness of the treatments was indirectly evaluated following the three following methods, comparing the following parameters before and after the treatments:

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1. The mercury accessible porosity and the porosimetric distribution by mercury injection (MIP). The determination was made in triplicate from surface fragments of treated stone. 2. Resistance to salt crystallization cycles, evaluated following a test based on [30] and modified in [31]. The salt crystallization test was performed with cubeshaped test pieces with sides of 5 cm, treated in one face. Three test pieces per treatment and three untreated test pieces were used. The test is based on the repetition of a cycle of 24 h of duration that consisted of (1) a first phase of capillary absorption of a sodium sulphate solution at 14% (w/w) during 2 h, placing the test sample with the treated face facing upwards, (2) an oven drying at 40 °C during 8 h and (3) cooling at 20 °C and 45% RH during 14 h. Every 5 cycles the test pieces were washed in a bath of distilled water at 20 °C with continuous renewal for three days, after which time they were dried to constant weight. The resistance to the crystallisation of salts was measured from the loss of weight during the test. 3. Resistance to alteration according to the standard slake durability test [32]. This method is based on measurement of the resistance of the stone to alteration by impact and by friction of the test pieces against each other and against the walls of a perforated metal cylinder rotating at a constant velocity (20 rpm) inside a bucket filled with water. The amount of weight lost by the test pieces after a pre-determined number of cycles (each of 10 min duration) provides an evaluation of the stone’s resistance to physical alteration and to possible mineralogical alteration. The assay was performed with ten cube-shaped test pieces of 2.5 cm3 (2.5  1  1 cm) treated in all the faces with each product, and ten test pieces of the same dimensions of the untreated stone. The results are expressed as a percentage of material not altered (retained in accordance with the standard) and correspond to the index of durability (Slake index) of the material. Also, in order to determine the harmful effects of the consolidantes into the stone, the following determinations were made: 1. The vapour diffusion coefficient according to the procedure described in the preceding section. The slabs employed were cut from the top of the samples where the consolidants products were applied. 2. Modifications in the colour. Test pieces of 6  6  2 cm were used for this determination. On these test pieces, the consolidant products had been applied in the face with the largest dimension (6  6 cm2), following the procedure described previously. Colour was determined using a Minolta CM-700d/600d spectrophotometer. A total of 30 random measurements were made for each stone surface of 6  6 cm2, before and after each treatment. This number of measurements per surface has been taken following the recommendation for granitic stones proposed in [33]. In a previous study [34] it has been stated that, working with a number of measurements equal to or greater than that recommended following the granite texture, the factor of variation associated with the textural heterogeneity has a statistically no significant influence on the measurement of colour. This therefore allows a rigorous analysis to be made of the variations of colour associated with surface treatments on granitic stones. Colour was expressed in the CIE Lab and CIELCH colour spaces [35]. The measurements were made including the specular component (SCI mode), for a spot diameter of 8 mm, using illuminant D65 at observer angle 10°. CIE Lab DL⁄, Da⁄, Db⁄, DC⁄ and DH⁄ colour differences and total colour difference DEab were calculated, taking the original colour of the stone as the reference value. 2.5. In situ consolidant application and effectiveness evaluation The most suitable consolidant, according to the laboratory results, was applied in two different areas of the south-facing facade of the Church of Santa María del Campo, affected by scaling and granular disintegration. One area corresponded to two adjacent ashlars situated 4–5 m high, on the right of the entrance door (6H and 7H respectively) (Fig. 1b); on the 7H ashlar two zones of different colour have been differentiated, designated 7H-b (brownish in colour) and 7H-w (white). The other area to be treated corresponded to two anthropomorphic figures situated on one of the capitals: C-A (head of frontal figure) and C-B (head of lateral figure) (Fig. 1c). The procedure of application was the same as in the laboratory evaluation phase; also, the amount of product applied was the same as the uptake reached in laboratory conditions. Prior to the application of the consolidant, the following determinations were made in these two areas: 1. A peeling test, following the procedure described in [19], using TESAÒ 4962 double-sided adhesive tape. In each area to be consolidated three strips of tape were applied and removed consecutively. The peeled off material was determined as the difference between the weight of the tape after removal from the surface and the weight of the clean tape before application, expressing the results as the peeled weight of each individual strip of tape and the sum of the peeling off material of the three strip per area. 2. Measurement of colour using a Minolta CM-700d/600d spectrophotometer under the same conditions of measurement described previously in the laboratory treatment section. This measurement could only be made on the ashlars, because the sculpted surface of the capital prevented the correct contact with

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I. De Rosario et al. / Construction and Building Materials 76 (2015) 140–149 the spectrophotometer. On the ashlar 6H, thirty measurements were taken at random; on the ashlar 7H sixty measurements were made, thirty in the zone of brownish colouration (7H-b) and another thirty in the zone of pale colouration (7H-w). CIE Lab DL⁄, Da⁄, Db⁄, DC⁄ and DH⁄ colour differences and total colour difference DEab were calculated, taking the original colour of the stone as the reference value.

After these determinations, the consolidant was applied by brush; two consecutive applications were made, the second one minute after the first. One month after the treatment, the peeling test was performed again and the colour was evaluated again on the same surfaces.

3. Results and discussion 3.1. Laboratory study 3.1.1. Characterisation of the stone Presented in Table 1 are the data on the physical properties of the granite obtained from the San Pedro de Visma quarry. This granite had an important water accessible porosity (open porosity); however, the various hydric parameters revealed that, at atmospheric pressure, the stone absorbed only 60% of the water content absorbed under vacuum; the rock presented, consequently, only a limited communication between the pores in terms of water permeability. The difference between the value of the open porosity and of the mercury accessible porosity is also notable, indicating the existence of a considerable percentage of pores that are not involved in the water transport processes. Shown in Fig. 2 is the porosimetric distribution obtained by mercury injection. The stone presented a continuous distribution of pores in all the range of dimensions evaluated. About 70% of the total porous volume corresponded to pores with a diameter of access exceeding 1 lm. 3.1.2. Laboratory evaluation Table 2 gives the values for uptake and dry matter of the consolidants obtained after the treatment of samples of dry stone and stone with a 0.63% of water content. Also shown is the relationship between the dry matter and uptake. The results obtained demonstrated that the values of uptake and dry matter were higher when the products were applied in the rock with some water content. However, the relationship between these two parameters was higher when the consolidant was applied to dry samples; this finding indicates that, on the dry stone, the quantity of dry matter of consolidant is greater for a same uptake. In the light of this, it is recommended that when actual buildings are being treated with these consolidants, the products should be applied under conditions in which the stone is as dry as possible; it must be stated, nevertheless, that the dry matter amounts obtained in the wet samples are perfectly acceptable, taking into account the low capacity of absorption of granitic stones. In comparative terms, Paraloid B-82 presented values for uptake and dry matter significantly lower than the other two consolidants tested. The uptake of Estel 1000 was higher than that of UCA-2o, but this latter product presented a higher value for dry

Table 1 Physical parameters of the leucogranite from San Pedro de Visma quarry, used in the construction of the Church of Santa María del Campo. Property Open porosity (following [22]) (%) Water content under pressure [22] (% w/w) Water content under atmospheric pressure [23] (% w/w) Mercury accessible porosity (MIP) (%) Water vapour diffusivity (m2 s 1)

Value 3.18 ± 0.46 1.26 ± 0.19 0.75 ± 0.08 5.46 ± 0.19 2.53
10 7 ± 0.41
10

7

Fig. 2. Porosimetric distribution of the leucogranite from the San Pedro de Visma quarry. The distribution obtained for two samples of the rock is shown.

Table 2 Uptake (U, in % w/w), dry matter (DM, in % w/w) and dry matter/uptake ratio after the application in the laboratory conditions of the three consolidant; data for dry stone and for wet stone (with 0.63% of water content under atmospheric pressure) are presented. Uptake (% w/w)

Dry matter (% w/w)

DM/U

Dry stone Paraloid B-82 Estel 1000 UCA-2o

0.057 ± 0.02 0.312 ± 0.10 0.270 ± 0.20

0.036 ± 0.02 0.208 ± 0.11 0.226 ± 0.13

0.63 0.67 0.84

Wet stone Paraloid B-82 Estel 1000 UCA-2o

0.16 ± 0.04 0.77 ± 0.06 0.60 ± 0.05

0.05 ± 0.03 0.37 ± 0.08 0.38 ± 0.01

0.31 0.48 0.63

matter, and therefore also a higher relationship between dry matter and uptake. The explanation for this behaviour is that the product UCA-2o does not contain volatile organic components that would be evaporated during the process of polymerisation into the stone dragging active matter towards the surface. The consolidants Paraloid B-82 and Estel 1000 contain an organic solvent that, when it evaporates, could produce a reduction in the final dry matter of the product. Regarding the distribution and morphology of the consolidants in the rock, Fig. 3 shows SEM micrographs of the granite specimen surfaces treated with the products under study. In the untreated stone, features of typical granite-forming mineral surfaces can be seen. Paraloid B-82 created a thin film, apparently weakly adhered to the surface and showing hollows. Estel 1000 was shown as a dense coating on the mineral surfaces presenting fissures whereas the UCA-2o consolidant was shown as a crack-free and continuous coating. The formation of this crack-free coating has been previously explained as a consequence of the role played by the n-octylamine, which reduces the capillary pressure while the gel is drying, as explained in the introduction [11]. Presented in Table 3 are the values of mercury accessible porosity of the untreated and treated stone; also shown are the percentages of reduction of these parameters after the consolidation. Presented in Fig. 4 are the porosimetric distributions of the stone obtained by MIP after the application of the three consolidants, in comparison with the untreated stone. Paraloid B-82 was the product that produced the greatest reduction of the porosity of the granite and the consolidating UCA was the one that produced the lowest reduction in this property. With respect to the distribution of pores (Fig. 4), significant differences between the products were observed. After the consolida-

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Fig. 3. Scanning electron microscopy micrographs of the granite surfaces: untreated and treated with Paraloid B-82, Estel 1000 and UCA-2o.

Table 3 Mercury accessible porosity (P-Hg in %) and vapour diffusivity (m2 s 1) before and after the treatments. For each parameter the percentage of reduction with respect to the untreated stone is given. P-Hg (%) Untreated Paraloid B-82 Estel 1000 UCA-2o

% reduction

Vapour diffusivity (10 7) (m2 s 1)

% reduction

5.46 ± 0.19 3.01 ± 0.70

44.9

2.53 ± 0.41 2.26 ± 0.03

10.94

3.73 ± 0.35

31.7

1.91 ± 0.15

24.46

4.78 ± 0.08

12.5

1.73 ± 0.29

31.84

tion with Paraloid B-82, an increase was observed in the percentage of pores of larger access diameter (>10 lm), a substantial reduction in the percentage of pores of intermediate access diameter (1– 10 lm) and a notable increase in the percentage of micropores (<1 lm). Estel 1000 also produced a slight increase in the percentage

Fig. 4. Pore size distribution obtained by MIP of the granite samples treated with the three consolidants and their untreated counterpart.

of pores of larger access diameter (>10 lm) and a slight decrease in the percentage of pores of intermediate access diameter; however, unlike the treatment with Paraloid, Estel 1000 decreased slightly the pores of very small access diameter (less than 1 lm). The effect of UCA-2o on the porosimetry of the stone was evident principally in the pores of very small access diameter (less than 1 lm), which were considerably reduced in volume. The striking modification in the porosimetric distribution produced by Paraloid B-82 could be associated with the form in which the film of polymer is distributed on the mineral surface; the SEM images showed a discontinuous and very fissured film, in which hollows of different sizes were observed, explaining the increase found in the number of pores of larger and smaller sizes obtained by MIP. The film of Estel 1000 was more continuous although some crackings can be seen, which, by their dimensions, might explain the increase in the pores of larger size. In contrast, it can be observed that the consolidant UCA-2o formed a continuous and non-fractured film that did not contribute to the occurrence of pores of sizes not existing in the original stone. The UCA-2o product reduced the micropores because it effectively fills up these spaces. Also, these results demonstrated that there was no relationship between the reduction of the porosity and the values of dry matter obtained. Thus, the SEM observations may explain the fact that the product with the lowest dry matter (Paraloid B-82) promoted the highest reduction in porosity: this product poorly penetrated into the rock, accumulating on the surface of rock, where MIP analysis has been carried out. In addition, SEM observations confirmed that the increase in the percentage of pores of larger access diameter produced by Paraloid B-82 is related to the thin layer of this product that has been formed on the stone, which is affected by a higher degree of fissuration. Shown in Fig. 5 is the amount of weight loss by the untreated granite test pieces, and those treated with the three consolidants tested, during the salt crystallisation test. The untreated granite presented a weight loss of 2.89% (w/w) after 35 cycles. Treatment with Paraloid B-82 did not increase at all the resistance of the stone

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to salt crystallization; quite the opposite: the test pieces treated with this product suffered the greatest weight loss (3.21%). This occurred even though Paraloid B-82 reduced by almost half the mercury accessible porosity of the stone (Table 3). The test pieces treated with Estel-1000 and UCA-2o showed a resistance to salt crystallization notably higher than the untreated stone, despite these two consolidants did not reduce the porosity of the stone as much as Paraloid B-82. Of the two products, it was UCA-2o which induced a greater resistance to the crystallisation of salts: the final values of weight loss after 35 cycles were 0.17% for the stone treated with UCA-2o and 0.45% for the stone treated with ESTEL 1000. The differences in the resistance to salt crystallization test of the samples treated with the different consolidants can be related to the changes generated by these products on the porosimetric distribution of the stone (Fig. 4). The susceptibility of these materials to salt crystallisation processes is closely related to the access size of the pores; the existence of micropores in the material facilitates the deterioration, because the pressure of crystallisation is inversely proportional to the pore radius [36,37]. Thus, the lowest resistance to the salt crystallisation test was produced in the stone treated with Paraloid-B82, a consolidant that notably increased the proportion of pores with smaller diameter of access (Fig. 4). On the contrary, with UCA-2o, which notably reduced the range of pores with access diameter of less than 1 lm, the resistance of the rock to salt crystallisation notably increased. The Slake durability index of the untreated stone and that treated with each of the three consolidants after 10 cycles is presented in Table 4. The difference between the treated and untreated samples revealed that the three consolidants increased the stone resistance. By comparing the values obtained, a relationship between slake durability index and salt crystallization results was found: Paraloid B-82 is shown to be the least effective treatment against salt crystallization and also the treatment with the lowest Slake durability index. The stone treated with UCA-2o and Estel 1000 showed the highest Slake index values and the highest resistance to crystallization test. Regarding the harmful effects of the treatments on the stone, vapour diffusivity data are presented in Table 3. The product Paraloid B-82 hardly modified this parameter at all, whereas UCA-2o is the consolidant that generates the greatest reduction. These results could indicate the existence of a direct relationship between the dry matter and the vapour diffusivity. Moreover, the modifications caused by the consolidants in the porosimetric distribution may

explain these variations in the diffusivity. It is well known that the porosity of granitic stones is constituted by transgranular fissures (macro fissures) that are linked to each other by micro fissures. In an earlier study [38], it is reported that fissures of both types and size influence the behaviour of the granitic rock with respect to the transport of fluids. The disappearance of a proportion of the micro fissures of less than 1 lm in size after the treatment with UCA-2o could contribute to the stone’s vapour diffusivity being more reduced than after the other two treatments. For practical purposes it is important to mention that all of the products gave rise to a reduction of the permeability ranging from 11% to 32%. It highlights that breathability of the stone after consolidation was only slightly reduced, being these results acceptable for a consolidation intervention in cultural heritage [17]. Finally, the variations in colour provoked by the three treatments are presented in Table 5. Firstly, it is observed that the changes produced by Paraloid B-82 in the colour were qualitatively and quantitatively different from those produced by the other two products. Paraloid B-82 hardly altered the luminosity at all but it changed appreciably the b⁄ coordinate, which suffered a slight decrease in its value after the treatment causing a modification in the chroma. In consequence, the colour lost intensity, although the overall effect was small, taking into account the very low value of the total colour difference DE⁄ab. Estel 1000 and UCA-2o, to the contrary, reduced the luminosity and increased the chroma; the a⁄ and b⁄ coordinates were also increased, particularly the value of the b⁄ coordinate. In consequence, the original colour of the stone, after both treatments, is deepened. Between the two products, UCA-2o caused a quantitatively smaller change; the total colour difference DE⁄ab after this treatment was 2.97 whereas with Estel 1000 this parameter exceeded the value of 5. On that point, the treatment of this stone with Estel 1000 would correspond, according to the rate proposed by Delgado and Grossi [39], to an intervention with high risk of incompatibility. On the contrary, the risk of incompatibility of the treatment with UCA-2o would be considered zero. Equally, the colour change provoked by UCA2o may be considered not perceptible by the human eye as the total colour difference DE⁄ab did not exceed the value of 3 which is the threshold of colour perception by the human eye for a colour difference [40].

3.2. Selection of the consolidant for the application in the Church of Santa María del Campo The most suitable consolidant must be selected taking into account the results obtained in laboratory but also the conditions to which the stone in the building is subjected; these conditions are characterised by a significant deterioration risk related to salt crystallization process. From the results obtained in the laboratory, the application of Paraloid B-82 must be completely discounted: even though that is the consolidant that produced the greatest reduction in the mercury accessible porosity of the stone and the lowest colour changes, its presence on the stone increased the susceptibility to

Table 4 Slake durability index (Id 10, in %) of the stone untreated and treated with the three consolidants.

Fig. 5. Weight loss (%, mean value of 3 test pieces) during the salt crystallisation test of the untreated stone and the stone treated with Paraloid B-82, Estel 1000 and UCA-2o.

Product

Id10 (%)

Paraloid B-82 Estel 1000 UCA-2o Untreated stone

97.975 98.109 98.093 95.805

I. De Rosario et al. / Construction and Building Materials 76 (2015) 140–149 Table 5 Colorimetric differences (DL⁄, Da⁄, Db⁄, DC⁄ab and DH), and total colour difference (DE⁄ab) obtained after the treatments with Paraloid B-82, Estel 1000 and UCA-2o. Mean values (n = 60).

DL⁄ Paraloid B-82 Estel 1000 UCA-2o

0.02 4.70 2.62

Da⁄ 0.02 0.40 0.23

Db⁄ 0.39 2.07 1.38

DC⁄ab 0.40 2.10 1.40

DH 0.01 0.03 0.01

DE⁄ab 0.39 5.15 2.97

salt crystallisation, as is shown in Fig. 5. Also, the Slake durability test showed the worst result for this consolidant. The consolidants Estel 1000 and UCA-2o showed, to the contrary, clear effectiveness in increasing the durability of the stone to the predominant process of deterioration in this building, which is the salt crystallisation. Of the two products, UCA-2o gave to the stone a greater resistance to salt crystallization process and also a lesser colour change, below the critical thresholds defined in [39,40]. For these reasons, UCA-2o was selected as the consolidant to be applied on the monument. With respect to vapour diffusivity it is also evident that UCA-2o was the product that produced the greatest reduction (31%, Table 3). This effect may be explained by the reduction, after consolidation with UCA-2o, of the percentage of pores of very small access diameter. This feature has however a great advantage from a construction point of view. According to [38], a reduction of the percentage of pores of small access diameter (<0.1 lm) in granitic stones would reduce the capillary absorption capacity and thus the water transport capacity of the stone. In consequence, a more restricted movement of the soluble salts into the rock in the building is expected, also reducing the extent of the damage caused by soluble salts. On the other hand, we would like to remark, as reflected in [39], that the individual analysis of the properties considered to be compatibility indicators (among which the permeability to vapour is included) may have only limited or even no value at all if full account is not taken of the circumstances to which the stone of the particular building is subjected. Thus, according to those authors, the data regarding the change in any particular property must be interpreted jointly with data on changes in the other relevant properties, and always in the context of the external conditions (climate, pollution, previous treatments. . .) of the building. In the case of our study, the reduction in the diffusivity, which is a consequence of the almost complete elimination of the pores of

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very small access diameter, represents an advantage since it increases the resistance of the stone to the active deterioration process on the church, that is the crystallisation of soluble salts, especially taking into account that in this case salts derived from the atmosphere and are therefore impossible to remove [7]. Finally, UCA-2o product has another advantage: the product does not incorporate a solvent. This absence of solvent is, in all probability, the cause of the larger quantity of dry matter remaining after the polymerisation, in comparison with the Estel 1000 product. The DM/U ratio therefore indicates UCA-2o as the most efficient product in economic terms. 3.3. Results from the in situ application of the consolidant From the results of the tests performed in the initial laboratory phase, we concluded that the UCA-2o consolidant was the most suitable for application to the deteriorated stonework of the Church of Santa María del Campo. As has been commented in the Material and methods section, the consolidant selected in the phase of evaluation in the laboratory was applied by brushing onto two ashlars affected by arenization (7H-b, 7H-w and 6H, Fig. 1b) and on two parts of a deteriorated capital (C-A and C-B, Fig. 1c). In Fig. 6, pictures from effectiveness evaluation process are shown. The results of the peeling test after the consolidation of these areas are given in Table 6. In all the cases the weight of material stripped off by the adhesive tape was considerably reduced after the consolidation, particularly in the areas where the stone showed a high deterioration degree, the eroded head of the capital. This result clearly demonstrates the effectiveness of UCA-2o as a consolidant, even on surfaces contaminate with salts. The evaluation of the changes of colour after the treatment with UCA-2o in the ashlars form the South façade of the church (areas 7Hb, 7Hw and 6H) is shown in Table 7. Qualitatively, the colour changes provoked by the consolidation with UCA-2o in these areas were similar to that detected in laboratory: the luminosity decreased and the chroma increased slightly, which implies that the colour of the stone in the building became slightly deeper. The total colour difference DE⁄ab presented values similar to those obtained in the laboratory phase, that is, below 3, except for the treatment applied to the ashlar 7H-w which exceeds this value by only 0.05. It can be considered that, with respect to colour, the application of this consolidant to the granite of this church represented a zero risk of incompatibility [39], being also below the threshold at which it becomes detectable to the human eye [40].

Fig. 6. Evaluation of the effectiveness of the nanoconsolidant applied on the South façade of the church. (a) Picture of tape application during the peeling test carried out on the deteriorated capital. (b) Detail of colour measurement on the two ashlars of the façade.

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I. De Rosario et al. / Construction and Building Materials 76 (2015) 140–149 Table 6 Results of the peeling test on the areas of the South façade of the Church of Santa María del Campo treated with UCA-2o consolidant. The values shown are the amounts of loose matter (expressed by weight in g) stripped from the stone surface by each tape and the mean value before and after the treatment together with the percentage of reduction of lost materials after the treatment. Treated area

Pre-consolidation

Post-consolidation Total amount (g)

g/tape

Total amount (g)

7H-b

0.035 0.003 0.037

0.075

0.003 0.005 0.000

0.008

89

7H-w

0.023 0.010 0.000

0.033

0.009 0.001 0.003

0.013

60

6H

0.013 0.015 0.008

0.036

0.000 0.004 0.000

0.004

88

C-A

0.013 0.009 0.035

0.057

0.005 0.004 0.002

0.011

80

C-B

0.209 0.545 0.315

1.069

0.004 0.003 0.005

0.012

98

Table 7 Colorimetric variations DL⁄, Da⁄, Db⁄, DC⁄ab, DH and DE⁄ab before the treatment of 7H-b, 7H-w and 6H areas of the south façade of the Church of Santa María del Campo with UCA-2o. Mean values (n = 60). Treated area 7H-b 7H-w 6H

%

g/tape

DL⁄ 2.03 3.02 1.86

Da⁄

Db ⁄

DC⁄ab

0.09 0.28 0.42

0.02 0.38 1.22

0.04 0.43 1.27

DH 0.10 0.21 0.15

DE⁄ab 2.03 3.05 2.26

4. Conclusions The following conclusions can be drawn from the results obtained in this study: 1. With respect to methodology, the work presented defines the protocol that should be followed when the surface of a granite building needs to be consolidated. This proposed protocol is defined by a prior testing phase in the laboratory to identify from among the possible alternatives the most suitable product in function of its effectiveness and considering the conditions to which the stone in question is subjected on the building (i.e. humidity, types of deterioration, etc.). It is emphasised that the laboratory phase must concentrate on evaluating relevant properties such as permeability and resistance to the crystallisation of salts that cannot feasibly be measured in situ but that are essential for taking the correct decisions about treatment. 2. The results obtained in the laboratory confirmed the need to evaluate the modifications in the porous system of the material; in order to take the correct decision, it is not sufficient to determine only the improvement in the water or mercury accessible porosities. Thus, in this study, the product most appropriate for the circumstances of conservation of the building (UCA-2o) is not the product that specifically resulted in the greatest reduction in the porosity of the material; its suitability is based on the reduction of the proportion of pores with very small access diameter; this is the effect that leads to an increase in the resistance of the stone to the active deterioration process, that is, salt crystallization. This increase in stone resistance can be also associated to the absent of cracking in UCA-2o. A cracking material (ESTEL 1000) cannot effectively consolidate the granite under study.

3. With the aim of selecting the most suitable product, it is necessary to interpret the modifications that the product causes jointly in all the indicator properties (diffusivity, colour, porosity) while always taking into account the particular circumstances of the building or structure to be treated (environmental factors, types of deterioration,. . .). The product that, in this study, has the lowest impact on the diffusivity (Paraloid B-82) is, in turn, the one that gives the least protection to the stone against the main mechanism of deterioration that is acting on the building; therefore that product would not be the most suitable consolidant to apply in this particular case. 4. The effectiveness of the product UCA-2o as a consolidant of granite has been demonstrated both for laboratory and in situ conditions. The addition of n-octylamine contributes to preventing the occurrence of cracking during drying process; this in turn prevents the creation in the stone of pores of sizes which did not exist in the stone before being treated. The absence of solvent in its formulation also allowed higher ratios of dry matter/uptake to be obtained; this would be expected to give a greater depth of penetration because processes of migration of the active phase, due to mobility of the solvent during its evaporation, would not take place. In addition, the change of colour caused by this product is very slight and should not, therefore, compromise the intervention. 5. Finally, in this study two new approaches have been applied to evaluate the effectiveness of a consolidation in granitic stones: the Slake durability test and the peeling test, which have not to date been applied in the field of granitic stone conservation. In the case of both these test methods, the information provided has been very useful in the evaluation of treatment both in the laboratory and in situ conditions These findings are an encouragement to corroborate the usefulness of these tests in future studies.

Acknowledgements We are grateful for financial support from the Spanish Government/FEDER-EU (MAT2013-42934-R and Project GEOPETRA, Innpacto subprogram), Spanish Ministry of Science and Technology (Project CTM2010-19584) and the Government of Andalusia (Project TEP-6386 and Group TEP-243).

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