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Resistance to simulated rain of hydroxyapatite- and calcium oxalate-based coatings for protection of marble against corrosion
Corrosion Science 127 (2017) 168–174
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Resistance to simulated rain of hydroxyapatite- and calcium oxalate-based coatings for protection of marble against corrosion
MARK
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Gabriela Graziania, Enrico Sassonia, , George W. Schererb, Elisa Franzonia a b
Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), University of Bologna, Via Terracini 28, 40131 Bologna, Italy Department of Civil and Environmental Engineering (CEE), Princeton University, Eng. Quad E-319, Princeton, NJ 08544, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Marble B. Calcium phosphates C. Acid corrosion C. Acid inhibition C. Atmospheric corrosion
Dissolution in pure and acid rain is among the main environmental degradation processes of marble. Coatings based on hydroxyapatite (HAP) have been proposed to prevent marble corrosion, providing encouraging results. In this paper, the resistance to dissolution was investigated on marble treated with different HAP-treatment formulations and compared to commercial ammonium oxalate. The ability of the coating to prevent corrosion was evaluated by performing wetting/drying cycles, by a custom-designed apparatus simulating rain. When ethanol is added to the solution used to form HAP, a significant reduction in calcium ions leached from marble is found, which is indicative of reduced corrosion.
1. Introduction Marble is among the most used materials in historic and modern architecture, but when exposed to the outdoor environment, it experiences severe decay, mainly because of the characteristics of calcite, its main mineral constituent [1,2]. Calcite has a relatively high solubility in water (which leads to the surface recession of marble elements) [1–7] and an unusual anisotropic thermal behavior (which leads to marble sugaring at the micro-scale and bowing of marble slabs at the macro-scale) [8–10]. Reaction products that can form as a result of calcite interactions with the environment, such as gypsum and calcium nitrate, also have very high solubility, thus further causing surface recession and threatening the conservation of marble artifacts. Dissolution and consequent surface recession of marble can occur in both acidic and neutral environment [11]. In particular, there are three main processes leading to dissolution of marble on site. i) Dissolution in pure rain in equilibrium with atmospheric CO2: pH in these conditions is around 5.6, but might span from 5 to 7 in European environments [6]; ii) Dissolution in an acidic environment, where the acidic pH results from the presence of atmospheric pollutants, such as SO2 and NOx, that can push the pH down to 4; iii) Dry deposition of gaseous atmospheric pollutants, such NOx and, mostly, SO2, that cause conversion of marble into more soluble products, such as gypsum and calcium nitrate, which are easily washed away by rain [12–14].
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According to current data and to predictions for this phenomenon in European climates in the near future, dissolution in pure rain (the socalled Karst effect [6]) is the main mechanism. It accounts for about 50–90% of the total surface recession of carbonate stones [6]. The rate and mechanism of calcite dissolution fall into different regimes, depending on the rain pH: i) acid solutions, ii) neutral and alkaline solutions, and iii) transitional regime. The boundaries between the regimes are at about pH 4 and 5.5 for KC1 solutions at 25 °C, but they might vary slightly. What changes substantially in each regime is the dependence of dissolution rate upon H+ concentration and presence of Ca2+ in solution [11]. No significant differences are expected for a pH change within the ambient range, while dissolution rate rises significantly as pH drops in the acid range and dissolution kinetics may vary significantly when switching from one regime to another. Dissolution has caused the corrosion of several millimeters from historical surfaces in the past centuries, resulting in the damage of architectural and sculptural details, and further recession is to be expected in the future, with the consequent loss of precious material. Surface recession depends on several parameters and can be modeled by different equations (for example, according to the Lipfert equation [15], surface dissolution is dependent on the solubility of calcium carbonate in water in equilibrium with CO2, the amount of precipitation per year, the H+ concentration, the deposition velocity of SO2 and HNO3, and the concentrations of SO2 and HNO3). For this reason, current data and estimates for the near future vary to a very high extent, depending on the geographical location and on the rain pH: values between 4 and 130 μm/year surface recession are predicted by Lipfert
Corresponding author. E-mail address: [email protected] (E. Sassoni).
http://dx.doi.org/10.1016/j.corsci.2017.08.020 Received 13 March 2017; Received in revised form 15 August 2017; Accepted 18 August 2017 Available online 26 August 2017 0010-938X/ © 2017 Elsevier Ltd. All rights reserved.
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[15] for carbonate stones, while values of about 14 μm/year are indicated in [6]. In the latter case, 96–99% of recession is because of the Karst effect. For this reason, the search for a suitable protective treatment that might retard or prevent dissolution is one of the main goals in cultural heritage preservation. However, none of the currently used protectives can be considered satisfactory, because they either lack efficacy (typically, in the case of traditional inorganic products [16–18]) or efficacy and/or durability (typically, in the case of organic products [19–22]). To overcome these limitations, treatments based on hydroxyapatite (Ca10(PO4)6(OH)2, HAP) have been proposed for marble protection [23–25], in view of the very low solubility and dissolution rate of the mineral HAP [23,26]. Together with low solubility and slow dissolution rate, HAP has shown an excellent compatibility with calcite substrates and is non-hazardous for the operators and the environment [10,27–30], which also pushed towards its use for stone conservation. HAP can be obtained by reacting a solution of diammonium hydrogen phosphate ((NH4)2HPO4, DAP) with calcium ions, coming from the substrate and/or externally supplied by adding CaCl2 to the DAP solution [23–26]. Notably, no unreacted DAP, chlorides (when CaCl2 is added to the DAP solution) or reaction by-products (such as ammonium carbonate [2]), all potentially harmful to the stone, remain after rinsing with water at the end of the treatment, as verified in previous studies by several techniques, i.e. SEM-EDS [10,26–29,32], FT-IR [24,27–29], Raman [10], and GI-XRD [32]. The advantage of the low solubility of HAP was first suggested in [23] and later evaluated in [24] and [25]: tests carried out so far on both marble powders and massive samples confirmed that HAP treatments can increase the resistance of marble to dissolution. In particular, tests performed on marble powders [24] indicated that, by adding ethanol (EtOH) to the DAP solution, it is possible to significantly enhance surface coverage while reducing cracking and porosity, because ethanol influences the hydration shell of ions in the solution, thus favoring HAP formation [24,31]. This, in turn, boosts the resistance of the samples against corrosion [31]. Moreover, superimposed layers formed at low DAP concentration were found to have a higher efficacy than one layer formed at high DAP concentration, because in this latter case the HAP layer is cracked [24]. The results of acid resistance tests carried out so far on marble powders [24] still need to be validated on massive specimens, i.e. samples of at least a few square centimeters surface, so as to get closer to the conditions of marble buildings and statues on site. In fact, the formation of HAP coatings and their resistance to dissolution in acid may differ between powders and massive specimens: in addition to the different specific surface area, in powders the coverage of each particle and its resistance to dissolution is basically independent of the behavior of the other particles; on the contrary, in massive samples each calcite grain, having a different orientation, may positively or negatively influence the surrounding grains. Indeed, on the one hand, grains with the most favorable crystallographic orientation may promote HAP nucleation and spread to the adjacent grains; on the other hand, grains with the least favorable orientation and hence poor coverage by HAP may allow acid to reach the marble substrate and trigger dissolution at the calcite-HAP interface also in adjacent grains, in spite of their good initial coverage with HAP. Therefore, in this paper the resistance to dissolution was investigated on massive samples of Carrara marble, treated with different concentrations of the DAP solution, with and without ethanol addition, and by performing single and double applications. The efficacy of the DAP-based treatments was compared to that of ammonium oxalate (AmOx), which is currently one of the most used inorganic protectives for marble [16,17,33–36]. To better reproduce the real conditions experienced by marble on site, resistance to dissolution was evaluated with a custom-designed apparatus able to simulate rain instead of using an apparatus with a finite volume of acid, as in references [24,25]. Unlike dissolution tests carried out by exposing samples to acid solutions in a beaker, which are
very useful for screening the most promising treatments, this apparatus prevents accumulation of calcium ions (that originate from marble dissolution) near the marble surface, which would reduce the dissolution rate and hence possibly influence the reliability of the results [11,24,37]. Moreover, by means of this rain simulating apparatus, it is also possible to prevent any influence deriving from the stirring rate of the solution, which might also have an impact on the dissolution rate [11], and also to reproduce the slight mechanical action of rain drops hitting the surface of the stone [38,39]. Finally, differently from references [24,25], calcium and phosphate ion concentrations were determined to measure the dissolution of the substrate and the coating, instead of pH shift over time. 2. Materials and methods 2.1. Materials Specimens (30 × 30 × 20 mm3) were sawn from a single slab of Carrara marble (supplier: Imbellone Michelangelo s.a.s.). DAP (> 99%, Sigma Aldrich), calcium chloride (assay > 99.0%, Sigma Aldrich), ethanol (Fisher-Scientific) and ammonium oxalate (≥99.99%, Sigma Aldrich) were used for the treatments. Prior to treatment and characterization, prisms were rinsed with water and ethanol to remove possible surface impurities and dried in an oven at 40 °C until constant weight. 2.2. Treatments For each treatment, duplicate samples were used. The number of samples for each treatment was considered sufficient based on preliminary tests carried out on marble powders and 1 × 1 × 1 cm3 cubes, for which no significant differences in surface coverage or in coating morphology, composition and acid resistance were found among different samples treated with the same procedure [24]. The conditions used are summarized in Table 1 and described in the following. Two reference samples were left untreated (Samples “UT”). Samples “0.1MED” were treated twice, the first time by total immersion for 24 h in an aqueous solution containing 0.1 M DAP + 0.1 mM CaCl2 + 0.5 wt% EtOH, and the second time (after drying) by total immersion for 24 h in an aqueous solution containing 0.1 M DAP + 0.1 mM CaCl2 (no ethanol was added). CaCl2 was added to prevent dissolution of the substrate [32]. Ethanol was added only during the first treatment, because its addition to the second treatment had proven to make the layer too thick and to cause no further benefit [24]. This procedure was the most effective among those investigated in preliminary tests on powders [24]. Samples “3M” were analyzed for comparison's sake, using another formulation of the DAP treatment Table 1 Labeling and description of the treatments. In the labels, “0.1M” and “3M” indicate the concentration of DAP in the treating solution, “E” indicates the addition of ethanol, “D” indicates the double application of a treatment, “AmOx” stands for treatment by ammonium oxalate. Specimen
Treatment description
Application method
UT 0.1MED
Untreated First treatment: 0.1 M DAP + 0.1 mM CaCl2 + 0.5 wt.% EtOH Second treatment 0.1 M DAP + 0.1 mM CaCl2 First treatment: 3 M DAP Second treatment: 1.7 g/l Ca(OH)2 5 wt.% AmOx First treatment: 5 wt.% AmOx Second treatment: 0.1 M DAP + 0.1 mM CaCl2
– Immersion
3M AmOx AmOx + 0.1 M
169
Immersion Brushing Poultice Brushing Immersion Immersion
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previously proposed by the Authors [10,27], involving a higher DAP concentration (3M, close to saturation of DAP in water at room temperature) and no ethanol addition. Samples were first treated with a 3M DAP aqueous solution applied by brushing up to apparent refusal (corresponding to 8 brush strokes). Then, after reaction for 48 h (samples being wrapped in a plastic film), rinsing with water and drying, samples were further treated by application of a limewater poultice for 24 h (dry poultice:limewater ratio 1:6). According to this methodology, calcium ions necessary for HAP formation come from millimolar dissolution of the substrate during the first step, then they are provided externally by the limewater poultice during the second step, aimed at converting any unreacted DAP to HAP [10,27]. During the final phase of drying in contact with the samples, the limewater poultice also removes unreacted DAP that might remain in the stone, given the high concentration of precursor used. Samples “AmOx” were treated according to the method proposed in [36]; samples were treated for 24 h with a 5 wt% ammonium oxalate aqueous solution, applied by brushing up to apparent refusal (15 brush strokes). Finally, samples “AmOx + 0.1M” were treated twice, the first time by immersion for 24 h in a 5 wt% ammonium oxalate aqueous solution and the second time (after drying) by immersion for 24 h in an aqueous solution containing 0.1 M DAP + 0.1 mM CaCl2. The idea was to create an insoluble HAP layer on top of a more soluble, but continuous, calcium oxalate layer. This procedure, although less effective than the double treatment with DAP and ethanol, had given encouraging results in preliminary tests on powders [24].
Fig. 1. Schematic representation of the dripping apparatus.
performed, so drying between the 1st and the 2nd dripping period of one day lasted 3 h, while after the 2nd dripping period samples were left to dry overnight. Preliminary tests were carried out to verify that 3 h were enough to complete the drying of the samples. Twenty-four wetting/drying cycles were performed in total, thus exposing each sample to an average volume of simulated rain of about 29 liters. Considering the annual average rain in Bologna (800 mm) and the size of the specimens (30 × 30 mm2), this volume of solution corresponds to about 40 years of rain in Bologna. Deionized water was used for the tests because there is a plateau in dissolution rate above about pH 6, so there is no advantage in raising pH higher than that; moreover, dissolution in clear rain is more relevant for European climates than dissolution in acidic rain. By doing so, the regime chosen is the same as for on-site degradation; hence, no substantial changes in the dissolution behavior of marble are expected with respect to those that would be obtained in the real environment [11]. A different behavior, not necessarily representative of that on site, would be obtained by adopting lower pH values (e.g., pH 4, sometimes used for accelerating marble dissolution and hence reducing the test duration [7]). Moreover, regarding the behavior of the coatings, solubility of calcium phosphates depends on the pH, hence misleading results, unrepresentative or even opposite to the behavior at higher pH, might be obtained by carrying out the tests at too low a pH (for example, brushite, CaHPO4·2H2O, is less soluble than HAP for pH < 4 but it is much more soluble than HAP for pH > 4, which is the relevant case in the field) [44].
2.3. Assessment of color change The possible effects of the treatments on the aesthetic appearance of marble were evaluated to verify that no formulation had to be discarded because of excessive alterations in the appearance of the stone. Color change assessment was carried out by comparing the color parameters of the same samples before and after the application of each treatment. The color parameters used refer to the CIELAB color space (1976), established by the “Commission Internationale de L’Eclairage” (CIE). Briefly, each color can be represented by the three coordinates (or color parameters) L*, a* and b*, which are related to the perception of color by the human eye in the 3 directions of the tridimensional color space defined by the three axes black-white, green-red and yellow-blue [42,43]. L* is the lightness variable, ranging from 0 (for black) to 100 (for white), while a* and b* are chromaticity coordinates, a* referring to the red-green components (positive for red and negative for green) and b* referring to the yellow-blue components (positive for yellow and negative for blue) [42,43]. The difference between two colors can hence be determined by comparing the values of each coordinate, according to the formula ΔE = (ΔL*2 + Δa*2 + Δb*2)1/2, [42,43] here used to evaluate color change before and after treatment. ΔL* = L*1-L*2, Δa* = a*1-a*2 and Δb* = b*1-b*2 are here obtained as the difference between a*, b* and L* before (condition“1”) and after (condition“2”) treatment. Color parameters were determined by spectrophotometer (Mercury 2000 Datacolor spectrophotometer). Each value is average of 6 measurements for each sample.
2.4.2. Characterization after the dripping test Dissolution of treated and untreated specimens was evaluated by measuring Ca2+ ion content in the runoff solution. For HAP-based treatments, PO43− ion content was also determined, as this is indicative of coating dissolution. Because HAP is basically insoluble in the given pH range, the presence of PO43− ions in the runoff solution indicates that pieces of the coating were removed and captured in the analyte or that soluble calcium phosphate (CaP) phases formed alongside HAP. In fact, formation of metastable CaP phases instead of or together with HAP can occur, depending on the treatment conditions and precursors [10,24,26]. This is a possible issue with the treatment, because some of these phases have high solubility in water [45–48] and can have a negative impact on the resistance of the coating. Calcium and phosphate ion content was measured after cycles 1, 2, 8 and 24. Ca2+ concentration was determined by high pressure liquid chromatography (HPLC, instrument equipped with a Universal cation 70 Column, Alltech). From data obtained by HPLC the cumulative loss of Ca2+ ions was calculated for each sample, assuming that the loss rate varies linearly between the points measured after cycles 1, 2, 8 and 24 and summing the relevant results. PO43− concentration was measured by first adding a phosphate marker to the samples of runoff solution that experiences a color shift towards blue, the intensity of the color change being a function of the PO43− concentration; then, the PO43−
2.4. Assessment of resistance to simulated rain 2.4.1. Dripping test Before the dripping test, all the faces of the sample were sealed with an impermeable paint, except for the face to be treated by brushing, and the upper face in samples treated by total immersion, to ensure that these faces were the only ones actually exposed to the simulated rain. Resistance to dissolution was determined by a dripping apparatus consisting of two lines of nozzles, each with a separate water supply. One sample for each condition was tested as sketched in Fig. 1. Deionized water (at initial pH 6.8) was dripped onto the samples, alternating periods of dripping (2.5 h) and drying. Two cycles per day were 170
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PO43− ions are released in very low concentrations (slightly above the detectability limit) in the first two cycles. This suggests that, at the beginning, either the detachment of fragments of the coating occurred, or some soluble CaP phases were present in very low concentrations, that were washed away during the first hours of dripping. This explains the slightly higher amount of Ca2+ ions leached in the first cycle compared to the other cycles: in the first cycle, a contribution from both the marble substrate and the phosphate coating is present. Nevertheless, the Ca2+ concentration after the 24th cycle is still lower than that of the untreated reference (Fig. 2), which indicates that some protection is maintained even after 40 years of simulated rain. Notably, samples 0.1MED exhibit the lowest variability between the two replicates (scatter bars in Fig. 2); in particular, the scatter is essentially negligible for the first cycles and starts being more significant as the test proceeds, exposing some parts of the marble substrate. SEM observations confirmed that the coating is still present after 24 cycles of dripping, although it looks patchy and diffusely detached, so that the underlying etched marble is visible in some areas (Fig. 3c–e). Observation at high magnification reveals that the coating seems quite porous (Fig. 3g), which raises some concern as porosity may allow the solution to corrode the underlying marble. Further reduction of the coating porosity, by optimization of ethanol addition, seems opportune. This work is currently in progress [31]. The evaluation of the behavior of the 3M samples is made difficult by the high variability exhibited by the two samples (Fig. 2). While for one sample the Ca2+ release was lower than that of the untreated reference, for the other sample a very high value was registered during the first cycle (Fig. 2). This high Ca2+ release, that also makes the cumulative loss higher than that of the untreated samples (Table 3), might indicate a significant formation of soluble CaP phases in one of the samples. This seems to be confirmed by the high release of PO43− ions registered during the first cycle for one of the samples (Fig. 2). In general, there seems to be a fair agreement between Ca2+ and PO43− ions, which suggests formation of some soluble phases or the detachment of relevant parts of the coating. This, in turn, explains the high scatter in Fig. 2c for both calcium and phosphate ions, deriving from a different behavior of the two samples examined. SEM observation revealed the presence of some uncoated areas (Fig. 3i), as well as areas of new CaP phases with needle-like morphology (Fig. 3k), both possibly indicating formation of soluble phases. Where it is still present, the coating appears to be intact, but largely affected by cracking, (Fig. 3l), which is consistent with the high concentration of the DAP solution and hence with the high thickness expected for the resulting coating. As a result, the treatment based on application of a 3M DAP solution, despite having been found to provide very good results for marble consolidation, appears not ideal for protection of marble against dissolution. The AmOx treatment does not significantly improve resistance to corrosion, as the amount of Ca2+ ions dissolved in each cycle is comparable to that of the untreated reference and so is the cumulative loss (Fig. 2 and Table 3). After the first cycles, the registered values are slightly higher than those of the untreated samples, possibly because of the coating contribution. This behavior is consistent with the relatively high solubility of the coating, which has basically the same solubility as calcite [52]. Some variability is also found between the behavior of different samples and between the cycles (scatter bars in Fig. 2), possibly deriving from different contributions to dissolution of the coating and of the substrate at different cycles. SEM observations showed that AmOx samples are severely etched after dripping and the coating is no longer visible (Fig. 2l,m), which confirms that the protection given by the treatment is not satisfactory. Ammonium oxalate-hydroxyapatite mixed coatings (AmOx + 0.1M samples) exhibit the worst behavior, as Ca2+ concentration is even higher than that of the untreated reference at each cycle, so that the cumulative Ca2+ ions loss is the highest among the examined samples (Fig. 3 and Table 3). Accordingly, high concentrations of PO43− are detected as well, especially during the first cycle. This suggests that
Table 2 Color change after treatment (values are averages for 6 measurements on two different samples for each condition; errors are expressed as the difference between the average and the maximum/minimum values). Sample
ΔE*
UT 0.1MED 3M AmOx AmOx + 0.1M
– 0.45 0.63 0.89 0.71
± ± ± ±
0.25 0.21 0.29 0.08
concentration was determined quantitatively by a photometer (instrument HI 83200, Hanna Instruments). To verify whether the marker would react with detached fragments of the coating, the very same tests (i.e. staining by a phosphate marker and measuring color shift towards blue by photometer) were carried out using pure hydroxyapatite powder, that was reacted with the marker in 10 ml of deionized water, exactly as the samples of runoff solution. HAP powder caused a strong shift towards blue, hence indicating that the marker is capable of reacting with solids. To investigate whether the coatings are preserved after dripping and the possible etching of the underlying marble, indicating corrosion of the substrate, sample observation by SEM/EDS was performed after the 24th dripping cycle. 3. Results Values of color change after treatment are reported in Table 2. All values are well below the ΔE = 3 threshold of human eye detectability [49], so none of the treatments was discarded as incompatible. The concentrations of Ca2+ and PO43− ions measured in the runoff solution for each sample at different cycles, indicative of the dissolution of the substrate and of the coatings, are illustrated in Fig. 2. The extent of dissolution of the samples was evaluated by calculating the cumulative loss of calcium ions, reported in Table 3: the dissolution is greatest for samples AmOx + 0.1M and 3M, and least for sample 0.1MED, where dissolution is remarkably reduced. No significant differences are found between untreated samples and samples treated by ammonium oxalate, for which dissolution is slightly higher. The morphology of the treated surface after dripping is illustrated in Fig. 3. Significant etching is found for untreated samples, indicating that significant corrosion was induced by the test, consistently with the simulation of 40 years of rain. 4. Discussion SEM/EDS analysis detected traces of magnesium in the untreated specimens, which is not uncommon as dolomite is often a minor component in marbles. However, because Mg is a hydroxyapatite-growth inhibitor and can also reduce HAP crystal size [50,51], its presence might negatively influence the growth of HAP coatings, resulting in more patchy phosphate layers. As for the effects of the simulated rain tests on the samples, measured in terms of release of Ca2+ and PO43− ions, untreated samples (UT) exhibit a noticeable release of calcium ions (Fig. 2), which indicates progressive dissolution. Accordingly, after dripping severe etching of the marble surface can be observed by SEM (Fig. 3a–b). In the case of the 0.1MED samples, Ca2+ ions released after each cycle are lower than those released by the untreated references, indicating that the coating is able to offer some protection, even if corrosion of the substrate is not completely prevented. Noticeably, the ratio between the cumulative loss of sample 0.1MED and that of the untreated reference is about 0.6 (Table 3), thus indicating that sample 0.1MED releases about 40% less calcium ions compared to untreated marble. Consistently, 171
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Fig. 2. Concentration of Ca2+ ions (light grey bars) and PO43− ions (dark grey bars) in the runoff solution in (a) untreated samples; (b) samples 0.1MED; (c) samples 3M; (d) samples AmOx and (e) samples AmOx + 0.1M. For untreated reference samples, a PO43− concentration of 0.01 mg/l was registered, which corresponds to the instrument detectability threshold and has to be attributed to the error in the measurements. Values are averages for 2 samples; error bars indicate the difference between the average and the maximum/minimum values.
treatment is not promising, because significant dissolution occurs both in and under the coating.
Table 3 Cumulative losses for untreated and treated samples. The Ca2+ concentration is normalized by the area of the sample. Sample
Cumulative Ca2+ loss [mg/l cm2]
UT 0.1MED 3M AmOx AmOx + 0.1M
5.78 3.51 7.02 6.43 9.00
5. Conclusions The resistance to dissolution in simulated rain was evaluated for Carrara marble treated with different HAP-based formulations, in terms of concentration of the phosphate precursors and possible ethanol addition, and were compared to a treatment based on ammonium oxalate. The following conclusions can be derived.
significant dissolution of both the coating and the substrate occurred: essentially, most of the coating dissolves during the first hours of dripping, leaving large areas of the substrate bare and thus causing relevant dissolution also in the following cycles. Consistently, SEM observation (Fig. 3n-p) shows that the surface of the samples is essentially bare and etched, with the HAP and underlying oxalate layer visible in only a few spots and no traces of the calcium oxalate layer alone. The dissolution of the calcium oxalate layer, initiated where some defect in the HAP layer was present, likely caused the detachment of the HAP-layer. As in the case of the 3M treatment, the presence of soluble phases leads to detachment of portions of the coating and causes a very high variability between different samples and between different cycles, dissolution generally being maximal in the first cycle, when most of the soluble phases dissolve. In general, comparing the behavior of the different samples, it was found that whenever there is a significant dissolution of the coating (which contributes to the amount of calcium ions detected in the solution), quite high variability is found between the different samples and different cycles. These results clearly indicate that the ammonium oxalate-hydroxyapatite sequential
1. Double treatments involving ethanol addition to the 0.1 M DAP solution during the first step provide fairly good protection, as dissolution of the substrate is significantly reduced with respect to the untreated reference. After exposure to simulated rain, the coating appears to be detached in several areas, but still the total amount of calcium ions released in the runoff solution is about 40% lower than for the untreated marble, hence significant protection is offered against the equivalent of 40 years of simulated rain; 2. The ammonium oxalate treatment alone does not offer satisfactory protection, as the layer is too soluble and dissolves during the test. This is particularly relevant as it indicates the advantage of the HAPbased treatment. 3. In the light of results obtained in this study, the treatment based on hydroxyapatite formation by ethanol addition to the DAP solution appears to be the most promising method, as ethanol allows increasing surface coverage without causing significant cracks and soluble phases formation, thus creating an insoluble phosphate layer that reduces corrosion of the underlying substrate.
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Fig. 3. SEM images of treated and untreated samples after dripping: (a,b) UT samples; (c–h) 0.1MED samples; (i–k) 3 M samples; (l,m) AmOx samples, AmOx+0.1M samples (n,p). Etching of the marble substrate is visible for both untreated (a,b) and AmOx-treated samples (l,m). In samples 0.1MED, some coating detachment can be observed (c,d), leading to exposure of the underlying marble (e). In samples 3M, depending on the area, it is possible to see uncoated and etched marble (i), complete but cracked layer (j), or a needle-like morphology, suggesting the presence of phases other than HAP (k). In AmOx + 0.1 M the coating has been almost completely corroded, so that only traces of the HAP coating can be detected (o,p), while the calcium oxalate layer was not observed anywhere in the sample.
References
Acknowledgements We are grateful to M.Eng. Matteo Glorioso for collaboration on dripping tests and to Dr. Lorella Guadagnini for assistance in HPLC measurements.
[1] E.M. Winkler, Stone in Architecture, 3rd ed., Springer, Berlin, 1997. [2] K. Lal Gauri, J.K. Bandyopadhyay, Carbonate Stone Chemical Behavior, Durability and Conservation, Wiley, New York, 1999. [3] T.T.N. Lan, R. Nishimura, Y. Tsujino, Y. Satoh, N.T.P. Thoa, M. Yokoi, Y. Maeda, The effects of air pollution and climatic factors on atmospheric corrosion of marble
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G. Graziani et al. under field exposure, Corros. Sci. 47 (2005) 1023–1038. [4] J.B. Johnson, M. Montgomery, G.E. Thompson, G.C. Wood, P.W. Sage, M.J. Cooke, The influence of combustion-derived pollutants on limestone deterioration: 1. The dry deposition of pollutants gases, Corros. Sci. 38 (1996) 105–131. [5] J.B. Johnson, M. Montgomery, G.E. Thompson, G.C. Wood, P.W. Sage, M.J. Cooke, The influence of combustion-derived pollutants on limestone deterioration: 2. The wet deposition of pollutants species, Corros. Sci. 38 (1996) 267–278. [6] A. Bonazza, P. Messina, C. Sabbioni, C.M. Grossi, P. Brimblecombe, Mapping the impact of climate change on surface recession of carbonate buildings in Europe, Sci. Total Environ. 407 (2009) 2039–2050. [7] E. Franzoni, E. Sassoni, Correlation between microstructural characteristics and weight loss of natural stones exposed to simulated acid rain, Sci. Total Environ. 412–413 (2011) 278–285. [8] S. Siegesmund, K. Ullemeyer, T. Weiss, E.K. Tschegg, Physical weathering of marbles caused by anisotropic thermal expansion, Int. J. Earth Sci. 89 (2000) 170–182. [9] A. Bonazza, C. Sabbioni, C. Guaraldi, P. De Nuntiis, Climate change impact: mapping thermal stress on Carrara marble in Europe, Sci. Total Environ. 407 (2009) 4506–4512. [10] E. Sassoni, G. Graziani, E. Franzoni, Repair of sugaring marble by ammonium phosphate: comparison with ethyl silicate and ammonium oxalate and pilot application to historic artifact, Mater. Des. 88 (2015) 1145–1157. [11] E.L. Sjcoberg, D.T. Rickard, The effect of added calcium on calcite dissolution kinetics in aqueous solutions at 25 °C, Chem. Geol. 49 (1985) 405–413. [12] T.T.N. Lan, N.T.P. Thoa, R. Nishimura, Y. Tsujino, M. Yokoi, Y. Maeda, New model for the sulfation of marble by dry deposition Sheltered marble—the indicator of air pollution by sulfur dioxide, Atmos. Environ. 39 (2005) 913–920. [13] S.J. Haneef, J.B. Johnson, G.E. Thompson, G.C. Wood, Effects of dry deposition of pollutant gases on the degradation of pentelic marble, Corros. Sci. 35 (1993) 743–750. [14] D.A. Dolske, Deposition of atmospheric pollutants to monuments, statues, and buildings, Sci. Total Environ. 167 (1995) 15–31. [15] F.W. Lipfert, Atmospheric damage to calcareous stones comparison and reconciliation of experimental findings, Atmos. Environ. 23 (1989) 415–429. [16] M. Matteini, Inorganic treatments for the consolidation and protection of stone artifacts, Conserv. Sci. Cult. Herit. 8 (2008) 13–27. [17] Q. Liu, B. Zhang, Z. Shen, H. Lu, A crude protective film on historic stone and its artificial preparation through biomimetic synthesis, Appl. Surf. Sci. 253 (2006) 2625–2632. [18] R.M. Ion, D. Turcanu-Caruţiu, R.C. Fierăscu, I. Fierăscu, I.R. Bunghez, M.L. Ion, S. Teodorescu, G. Vasilievici, V. Rădiţoiu, Caoxite-hydroxyapatite composition as consolidating material for the chalk stone for basarabi-murfatlar churches ensemble, Appl. Surf. Sci. 358 (2015) 612–618. [19] C. Esposito Corcione, R. Striani, M. Frigione, UV-cured siloxane-modified methacrylic system containing hydroxyapatite as potential protective coating for carbonate stones, Prog. Org. Coat. 76 (2013) 1236–1242. [20] P. Cardiano, S. Sergi, M. Lazzari, P. Piraino, Epoxy-silica polymers as restoration materials, Polymer 43 (2002) 6635–6640. [21] C. Esposito Corcione, M. Frigione, Influence of stone particles on the rheological behavior of a novel photopolymerizable siloxane-modified acrylic resin, J. Appl. Polym. Sci. 122 (2011) 942–947. [22] D. Pinna, B. Salvadori, S. Porcinai, Evaluation of the application conditions of artificial protection treatments on salt-laden limestone and marble, Constr. Build. Mater. 25 (2011) 2723–2732. [23] S. Naidu, E. Sassoni, G.W. Scherer, New treatment for corrosion-resistant coatings for marble and consolidation of limestone, Jardins de Pierres – Conservation of stone in Parks, Gardens and Cemeteries, XL Print, PARIS, 2011, pp. 289–294. [24] G. Graziani, E. Sassoni, E. Franzoni, G.W. Scherer, Hydroxyapatite coatings for marble protection: optimization of calcite covering and acid resistance, Appl. Surf. Sci. 368 (2016) 241–257. [25] S. Naidu, J. Blair, G.W. Scherer, Acid-resistant coatings on marble, J. Am. Ceram. Soc. 99 (2016) 3421–3428. [26] E. Sassoni, S. Naidu, G.W. Scherer, The use of hydroxyapatite as a new inorganic consolidant for damaged carbonate stones, J. Cult. Herit. 12 (2011) 346–355.
[27] E. Franzoni, E. Sassoni, G. Graziani, Brushing: poultice or immersion? Role of the application technique on the performance of a novel hydroxyapatite-based consolidating treatment for limestone, J. Cult. Herit. 16 (2015) 173–184. [28] E. Sassoni, G. Graziani, E. Franzoni, An innovative phosphate-based consolidant for limestone. Part 1: effectiveness and compatibility in comparison with ethyl silicate, Constr. Build. Mater. 102 (2016) 918–930. [29] E. Sassoni, G. Graziani, E. Franzoni, An innovative phosphate-based consolidant for limestone. Part 2. Durability in comparison with ethyl silicate, Constr. Build. Mater. 102 (2016) 931–942. [30] F. Yang, Y. Liu, G. Zuo, X. Wang, P. Hua, Q. Ma, G. Dong, Y. Yue, B. Zhang, Hydroxyapatite conversion layer for the preservation of surface gypsification marble relics, Corros. Sci. 88 (2014) 6–9. [31] E. Sassoni, G. Graziani, G.W. Scherer, E. Franzoni, Some recent findings on marble conservation by aqueous solutions of diammonium hydrogen phosphate, MRS Adv. 2 (2017) 2021–2026. [32] S. Naidu, G.W. Scherer, Nucleation, growth and evolution of calcium phosphate films on calcite, J. Colloid Interface Sci. 435 (2014) 128–137. [33] M. Matteini, A. Moles, S. Giovannoni, Calcium oxalate as a protective mineral system for wall paintings: methodology and analyses, in: V. Fassina, H. Ott, F. Zezza (Eds.), III Int. Symp. Conservation of Monuments in the Mediterranean Basin (1994). [34] C. Conti, C. Colombo, D. Dellasega, M. Matteini, M. Realini, G. Zerbi, Ammonium oxalate treatment: evaluation by μ-Raman mapping of the penetration depth in different plasters, J. Cult. Herit. 12 (2011) 372–379. [35] P. Meloni, F. Manca, G. Carangiu, Marble protection: an inorganic electrokinetic approach, Appl. Surf. Sci. 273 (2013) 377–385. [36] B. Doherty, M. Pamplona, R. Selvaggi, C. Miliani, M. Matteini, A. Sgamellotti, B. Brunetti, Efficiency and resistance of the artificial oxalate protection treatment on marble against chemical weathering, Appl. Surf. Sci. 253 (2007) 4477–4484. [37] G. Kaufmann, W. Dreybrodt, Calcite dissolution in the system CaCO3-H2O-CO2 at high undersaturation, Geochim. Cosmochim. Acta 71 (2007) 1398–1410. [38] C. Chiavari, E. Bernardi, A. Balbo, C. Monticelli, S. Raffo, M.C. Bignozzi, C. Martini, Atmospheric corrosion of fire-gilded bronze: corrosion and corrosion protection during accelerated ageing tests, Corros. Sci. 100 (2015) 435–447. [39] C. Chiavari, E. Bernardi, C. Martini, F. Passarini, A. Motori, M.C. Bignozzi, Atmospheric corrosion of Cor-Ten steel with different surface finish: accelerated ageing and metal release, Mater. Chem. Phys. 136 (2012) 477–486. [42] C. Gómez-Polo, M. Portillo Muñoz, M. Cruz Lorenzo Luengo, P. Vicente, P. Galindo, A.M. Martín Casado, Comparison of the CIELab and CIEDE2000 color difference formulas, J. Prosthet. Dent. 115 (2016) 65–70. [43] E. Ksinopoulou, A. Bakolas, I. Kartsonakis, C. Charitidis, A. Moropoulou, Particle modified consolidants in the consolidation of porous stone, Proceedings of the 12th International Congress on the Deterioration and Conservation of Stone Columbia University New York (2012). [44] S. Mats, A. Johnsson, G.H. Nancollas, The role of brushite and octacalcium phosphate in apatite formation, Crit. Rev. Oral Biol. Med. 3 (1992) 61–82. [45] S.V. Dorozhkin, Amorphous calcium orthophosphates: nature, chemistry and biomedical applications, Int. J. Mater. Chem. 2 (2012) 19–46. [46] S.V. Dorozhkin, Biphasic, triphasic and multiphasic calcium orthophosphates, Acta Biomater. 8 (2012) 963–977. [47] S.V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its biodegradable alloys, Acta Biomater. 10 (2014) 2919–2934. [48] S.V. Dorozhkin, Calcium orthophosphates, Biomatter 1 (2) (2011) 121–164. [49] C. Miliani, M.L. Velo-Simpson, G.W. Scherer, Particle-modified consolidants: a study on the effect of particles on sol–gel properties and consolidation effectiveness, J. Cult. Herit. 8 (2007) 1–6. [50] A. Bigi, G. Falini, E. Foresti, M. Gazzano, A. Ripamonti, N. Roveri, Magnesium influence on hydroxyapatite crystallization, J. Inorg. Biochem. 49 (1993) 69–78. [51] G. Graziani, M. Bianchi, E. Sassoni, A. Russo, M. Marcacci, Ion-substituted calcium phosphate coatings deposited by plasma-assisted techniques: a review, Mater. Sci. Eng. C 74 (2017) 219–229. [52] D.J. White, G.H. Nancollas, The kinetics of dissolution of calcium oxalate monohydrate: a constant composition study, J. Cryst. Growth 57 (1982) 267–272.
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Resistance to simulated rain of hydroxyapatite- and calcium oxalate-based coatings for protection of marble against corrosion
MARK
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Gabriela Graziania, Enrico Sassonia, , George W. Schererb, Elisa Franzonia a b
Department of Civil, Chemical, Environmental and Materials Engineering (DICAM), University of Bologna, Via Terracini 28, 40131 Bologna, Italy Department of Civil and Environmental Engineering (CEE), Princeton University, Eng. Quad E-319, Princeton, NJ 08544, USA
A R T I C L E I N F O
A B S T R A C T
Keywords: A. Marble B. Calcium phosphates C. Acid corrosion C. Acid inhibition C. Atmospheric corrosion
Dissolution in pure and acid rain is among the main environmental degradation processes of marble. Coatings based on hydroxyapatite (HAP) have been proposed to prevent marble corrosion, providing encouraging results. In this paper, the resistance to dissolution was investigated on marble treated with different HAP-treatment formulations and compared to commercial ammonium oxalate. The ability of the coating to prevent corrosion was evaluated by performing wetting/drying cycles, by a custom-designed apparatus simulating rain. When ethanol is added to the solution used to form HAP, a significant reduction in calcium ions leached from marble is found, which is indicative of reduced corrosion.
1. Introduction Marble is among the most used materials in historic and modern architecture, but when exposed to the outdoor environment, it experiences severe decay, mainly because of the characteristics of calcite, its main mineral constituent [1,2]. Calcite has a relatively high solubility in water (which leads to the surface recession of marble elements) [1–7] and an unusual anisotropic thermal behavior (which leads to marble sugaring at the micro-scale and bowing of marble slabs at the macro-scale) [8–10]. Reaction products that can form as a result of calcite interactions with the environment, such as gypsum and calcium nitrate, also have very high solubility, thus further causing surface recession and threatening the conservation of marble artifacts. Dissolution and consequent surface recession of marble can occur in both acidic and neutral environment [11]. In particular, there are three main processes leading to dissolution of marble on site. i) Dissolution in pure rain in equilibrium with atmospheric CO2: pH in these conditions is around 5.6, but might span from 5 to 7 in European environments [6]; ii) Dissolution in an acidic environment, where the acidic pH results from the presence of atmospheric pollutants, such as SO2 and NOx, that can push the pH down to 4; iii) Dry deposition of gaseous atmospheric pollutants, such NOx and, mostly, SO2, that cause conversion of marble into more soluble products, such as gypsum and calcium nitrate, which are easily washed away by rain [12–14].
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According to current data and to predictions for this phenomenon in European climates in the near future, dissolution in pure rain (the socalled Karst effect [6]) is the main mechanism. It accounts for about 50–90% of the total surface recession of carbonate stones [6]. The rate and mechanism of calcite dissolution fall into different regimes, depending on the rain pH: i) acid solutions, ii) neutral and alkaline solutions, and iii) transitional regime. The boundaries between the regimes are at about pH 4 and 5.5 for KC1 solutions at 25 °C, but they might vary slightly. What changes substantially in each regime is the dependence of dissolution rate upon H+ concentration and presence of Ca2+ in solution [11]. No significant differences are expected for a pH change within the ambient range, while dissolution rate rises significantly as pH drops in the acid range and dissolution kinetics may vary significantly when switching from one regime to another. Dissolution has caused the corrosion of several millimeters from historical surfaces in the past centuries, resulting in the damage of architectural and sculptural details, and further recession is to be expected in the future, with the consequent loss of precious material. Surface recession depends on several parameters and can be modeled by different equations (for example, according to the Lipfert equation [15], surface dissolution is dependent on the solubility of calcium carbonate in water in equilibrium with CO2, the amount of precipitation per year, the H+ concentration, the deposition velocity of SO2 and HNO3, and the concentrations of SO2 and HNO3). For this reason, current data and estimates for the near future vary to a very high extent, depending on the geographical location and on the rain pH: values between 4 and 130 μm/year surface recession are predicted by Lipfert
Corresponding author. E-mail address: [email protected] (E. Sassoni).
http://dx.doi.org/10.1016/j.corsci.2017.08.020 Received 13 March 2017; Received in revised form 15 August 2017; Accepted 18 August 2017 Available online 26 August 2017 0010-938X/ © 2017 Elsevier Ltd. All rights reserved.
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[15] for carbonate stones, while values of about 14 μm/year are indicated in [6]. In the latter case, 96–99% of recession is because of the Karst effect. For this reason, the search for a suitable protective treatment that might retard or prevent dissolution is one of the main goals in cultural heritage preservation. However, none of the currently used protectives can be considered satisfactory, because they either lack efficacy (typically, in the case of traditional inorganic products [16–18]) or efficacy and/or durability (typically, in the case of organic products [19–22]). To overcome these limitations, treatments based on hydroxyapatite (Ca10(PO4)6(OH)2, HAP) have been proposed for marble protection [23–25], in view of the very low solubility and dissolution rate of the mineral HAP [23,26]. Together with low solubility and slow dissolution rate, HAP has shown an excellent compatibility with calcite substrates and is non-hazardous for the operators and the environment [10,27–30], which also pushed towards its use for stone conservation. HAP can be obtained by reacting a solution of diammonium hydrogen phosphate ((NH4)2HPO4, DAP) with calcium ions, coming from the substrate and/or externally supplied by adding CaCl2 to the DAP solution [23–26]. Notably, no unreacted DAP, chlorides (when CaCl2 is added to the DAP solution) or reaction by-products (such as ammonium carbonate [2]), all potentially harmful to the stone, remain after rinsing with water at the end of the treatment, as verified in previous studies by several techniques, i.e. SEM-EDS [10,26–29,32], FT-IR [24,27–29], Raman [10], and GI-XRD [32]. The advantage of the low solubility of HAP was first suggested in [23] and later evaluated in [24] and [25]: tests carried out so far on both marble powders and massive samples confirmed that HAP treatments can increase the resistance of marble to dissolution. In particular, tests performed on marble powders [24] indicated that, by adding ethanol (EtOH) to the DAP solution, it is possible to significantly enhance surface coverage while reducing cracking and porosity, because ethanol influences the hydration shell of ions in the solution, thus favoring HAP formation [24,31]. This, in turn, boosts the resistance of the samples against corrosion [31]. Moreover, superimposed layers formed at low DAP concentration were found to have a higher efficacy than one layer formed at high DAP concentration, because in this latter case the HAP layer is cracked [24]. The results of acid resistance tests carried out so far on marble powders [24] still need to be validated on massive specimens, i.e. samples of at least a few square centimeters surface, so as to get closer to the conditions of marble buildings and statues on site. In fact, the formation of HAP coatings and their resistance to dissolution in acid may differ between powders and massive specimens: in addition to the different specific surface area, in powders the coverage of each particle and its resistance to dissolution is basically independent of the behavior of the other particles; on the contrary, in massive samples each calcite grain, having a different orientation, may positively or negatively influence the surrounding grains. Indeed, on the one hand, grains with the most favorable crystallographic orientation may promote HAP nucleation and spread to the adjacent grains; on the other hand, grains with the least favorable orientation and hence poor coverage by HAP may allow acid to reach the marble substrate and trigger dissolution at the calcite-HAP interface also in adjacent grains, in spite of their good initial coverage with HAP. Therefore, in this paper the resistance to dissolution was investigated on massive samples of Carrara marble, treated with different concentrations of the DAP solution, with and without ethanol addition, and by performing single and double applications. The efficacy of the DAP-based treatments was compared to that of ammonium oxalate (AmOx), which is currently one of the most used inorganic protectives for marble [16,17,33–36]. To better reproduce the real conditions experienced by marble on site, resistance to dissolution was evaluated with a custom-designed apparatus able to simulate rain instead of using an apparatus with a finite volume of acid, as in references [24,25]. Unlike dissolution tests carried out by exposing samples to acid solutions in a beaker, which are
very useful for screening the most promising treatments, this apparatus prevents accumulation of calcium ions (that originate from marble dissolution) near the marble surface, which would reduce the dissolution rate and hence possibly influence the reliability of the results [11,24,37]. Moreover, by means of this rain simulating apparatus, it is also possible to prevent any influence deriving from the stirring rate of the solution, which might also have an impact on the dissolution rate [11], and also to reproduce the slight mechanical action of rain drops hitting the surface of the stone [38,39]. Finally, differently from references [24,25], calcium and phosphate ion concentrations were determined to measure the dissolution of the substrate and the coating, instead of pH shift over time. 2. Materials and methods 2.1. Materials Specimens (30 × 30 × 20 mm3) were sawn from a single slab of Carrara marble (supplier: Imbellone Michelangelo s.a.s.). DAP (> 99%, Sigma Aldrich), calcium chloride (assay > 99.0%, Sigma Aldrich), ethanol (Fisher-Scientific) and ammonium oxalate (≥99.99%, Sigma Aldrich) were used for the treatments. Prior to treatment and characterization, prisms were rinsed with water and ethanol to remove possible surface impurities and dried in an oven at 40 °C until constant weight. 2.2. Treatments For each treatment, duplicate samples were used. The number of samples for each treatment was considered sufficient based on preliminary tests carried out on marble powders and 1 × 1 × 1 cm3 cubes, for which no significant differences in surface coverage or in coating morphology, composition and acid resistance were found among different samples treated with the same procedure [24]. The conditions used are summarized in Table 1 and described in the following. Two reference samples were left untreated (Samples “UT”). Samples “0.1MED” were treated twice, the first time by total immersion for 24 h in an aqueous solution containing 0.1 M DAP + 0.1 mM CaCl2 + 0.5 wt% EtOH, and the second time (after drying) by total immersion for 24 h in an aqueous solution containing 0.1 M DAP + 0.1 mM CaCl2 (no ethanol was added). CaCl2 was added to prevent dissolution of the substrate [32]. Ethanol was added only during the first treatment, because its addition to the second treatment had proven to make the layer too thick and to cause no further benefit [24]. This procedure was the most effective among those investigated in preliminary tests on powders [24]. Samples “3M” were analyzed for comparison's sake, using another formulation of the DAP treatment Table 1 Labeling and description of the treatments. In the labels, “0.1M” and “3M” indicate the concentration of DAP in the treating solution, “E” indicates the addition of ethanol, “D” indicates the double application of a treatment, “AmOx” stands for treatment by ammonium oxalate. Specimen
Treatment description
Application method
UT 0.1MED
Untreated First treatment: 0.1 M DAP + 0.1 mM CaCl2 + 0.5 wt.% EtOH Second treatment 0.1 M DAP + 0.1 mM CaCl2 First treatment: 3 M DAP Second treatment: 1.7 g/l Ca(OH)2 5 wt.% AmOx First treatment: 5 wt.% AmOx Second treatment: 0.1 M DAP + 0.1 mM CaCl2
– Immersion
3M AmOx AmOx + 0.1 M
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Immersion Brushing Poultice Brushing Immersion Immersion
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previously proposed by the Authors [10,27], involving a higher DAP concentration (3M, close to saturation of DAP in water at room temperature) and no ethanol addition. Samples were first treated with a 3M DAP aqueous solution applied by brushing up to apparent refusal (corresponding to 8 brush strokes). Then, after reaction for 48 h (samples being wrapped in a plastic film), rinsing with water and drying, samples were further treated by application of a limewater poultice for 24 h (dry poultice:limewater ratio 1:6). According to this methodology, calcium ions necessary for HAP formation come from millimolar dissolution of the substrate during the first step, then they are provided externally by the limewater poultice during the second step, aimed at converting any unreacted DAP to HAP [10,27]. During the final phase of drying in contact with the samples, the limewater poultice also removes unreacted DAP that might remain in the stone, given the high concentration of precursor used. Samples “AmOx” were treated according to the method proposed in [36]; samples were treated for 24 h with a 5 wt% ammonium oxalate aqueous solution, applied by brushing up to apparent refusal (15 brush strokes). Finally, samples “AmOx + 0.1M” were treated twice, the first time by immersion for 24 h in a 5 wt% ammonium oxalate aqueous solution and the second time (after drying) by immersion for 24 h in an aqueous solution containing 0.1 M DAP + 0.1 mM CaCl2. The idea was to create an insoluble HAP layer on top of a more soluble, but continuous, calcium oxalate layer. This procedure, although less effective than the double treatment with DAP and ethanol, had given encouraging results in preliminary tests on powders [24].
Fig. 1. Schematic representation of the dripping apparatus.
performed, so drying between the 1st and the 2nd dripping period of one day lasted 3 h, while after the 2nd dripping period samples were left to dry overnight. Preliminary tests were carried out to verify that 3 h were enough to complete the drying of the samples. Twenty-four wetting/drying cycles were performed in total, thus exposing each sample to an average volume of simulated rain of about 29 liters. Considering the annual average rain in Bologna (800 mm) and the size of the specimens (30 × 30 mm2), this volume of solution corresponds to about 40 years of rain in Bologna. Deionized water was used for the tests because there is a plateau in dissolution rate above about pH 6, so there is no advantage in raising pH higher than that; moreover, dissolution in clear rain is more relevant for European climates than dissolution in acidic rain. By doing so, the regime chosen is the same as for on-site degradation; hence, no substantial changes in the dissolution behavior of marble are expected with respect to those that would be obtained in the real environment [11]. A different behavior, not necessarily representative of that on site, would be obtained by adopting lower pH values (e.g., pH 4, sometimes used for accelerating marble dissolution and hence reducing the test duration [7]). Moreover, regarding the behavior of the coatings, solubility of calcium phosphates depends on the pH, hence misleading results, unrepresentative or even opposite to the behavior at higher pH, might be obtained by carrying out the tests at too low a pH (for example, brushite, CaHPO4·2H2O, is less soluble than HAP for pH < 4 but it is much more soluble than HAP for pH > 4, which is the relevant case in the field) [44].
2.3. Assessment of color change The possible effects of the treatments on the aesthetic appearance of marble were evaluated to verify that no formulation had to be discarded because of excessive alterations in the appearance of the stone. Color change assessment was carried out by comparing the color parameters of the same samples before and after the application of each treatment. The color parameters used refer to the CIELAB color space (1976), established by the “Commission Internationale de L’Eclairage” (CIE). Briefly, each color can be represented by the three coordinates (or color parameters) L*, a* and b*, which are related to the perception of color by the human eye in the 3 directions of the tridimensional color space defined by the three axes black-white, green-red and yellow-blue [42,43]. L* is the lightness variable, ranging from 0 (for black) to 100 (for white), while a* and b* are chromaticity coordinates, a* referring to the red-green components (positive for red and negative for green) and b* referring to the yellow-blue components (positive for yellow and negative for blue) [42,43]. The difference between two colors can hence be determined by comparing the values of each coordinate, according to the formula ΔE = (ΔL*2 + Δa*2 + Δb*2)1/2, [42,43] here used to evaluate color change before and after treatment. ΔL* = L*1-L*2, Δa* = a*1-a*2 and Δb* = b*1-b*2 are here obtained as the difference between a*, b* and L* before (condition“1”) and after (condition“2”) treatment. Color parameters were determined by spectrophotometer (Mercury 2000 Datacolor spectrophotometer). Each value is average of 6 measurements for each sample.
2.4.2. Characterization after the dripping test Dissolution of treated and untreated specimens was evaluated by measuring Ca2+ ion content in the runoff solution. For HAP-based treatments, PO43− ion content was also determined, as this is indicative of coating dissolution. Because HAP is basically insoluble in the given pH range, the presence of PO43− ions in the runoff solution indicates that pieces of the coating were removed and captured in the analyte or that soluble calcium phosphate (CaP) phases formed alongside HAP. In fact, formation of metastable CaP phases instead of or together with HAP can occur, depending on the treatment conditions and precursors [10,24,26]. This is a possible issue with the treatment, because some of these phases have high solubility in water [45–48] and can have a negative impact on the resistance of the coating. Calcium and phosphate ion content was measured after cycles 1, 2, 8 and 24. Ca2+ concentration was determined by high pressure liquid chromatography (HPLC, instrument equipped with a Universal cation 70 Column, Alltech). From data obtained by HPLC the cumulative loss of Ca2+ ions was calculated for each sample, assuming that the loss rate varies linearly between the points measured after cycles 1, 2, 8 and 24 and summing the relevant results. PO43− concentration was measured by first adding a phosphate marker to the samples of runoff solution that experiences a color shift towards blue, the intensity of the color change being a function of the PO43− concentration; then, the PO43−
2.4. Assessment of resistance to simulated rain 2.4.1. Dripping test Before the dripping test, all the faces of the sample were sealed with an impermeable paint, except for the face to be treated by brushing, and the upper face in samples treated by total immersion, to ensure that these faces were the only ones actually exposed to the simulated rain. Resistance to dissolution was determined by a dripping apparatus consisting of two lines of nozzles, each with a separate water supply. One sample for each condition was tested as sketched in Fig. 1. Deionized water (at initial pH 6.8) was dripped onto the samples, alternating periods of dripping (2.5 h) and drying. Two cycles per day were 170
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PO43− ions are released in very low concentrations (slightly above the detectability limit) in the first two cycles. This suggests that, at the beginning, either the detachment of fragments of the coating occurred, or some soluble CaP phases were present in very low concentrations, that were washed away during the first hours of dripping. This explains the slightly higher amount of Ca2+ ions leached in the first cycle compared to the other cycles: in the first cycle, a contribution from both the marble substrate and the phosphate coating is present. Nevertheless, the Ca2+ concentration after the 24th cycle is still lower than that of the untreated reference (Fig. 2), which indicates that some protection is maintained even after 40 years of simulated rain. Notably, samples 0.1MED exhibit the lowest variability between the two replicates (scatter bars in Fig. 2); in particular, the scatter is essentially negligible for the first cycles and starts being more significant as the test proceeds, exposing some parts of the marble substrate. SEM observations confirmed that the coating is still present after 24 cycles of dripping, although it looks patchy and diffusely detached, so that the underlying etched marble is visible in some areas (Fig. 3c–e). Observation at high magnification reveals that the coating seems quite porous (Fig. 3g), which raises some concern as porosity may allow the solution to corrode the underlying marble. Further reduction of the coating porosity, by optimization of ethanol addition, seems opportune. This work is currently in progress [31]. The evaluation of the behavior of the 3M samples is made difficult by the high variability exhibited by the two samples (Fig. 2). While for one sample the Ca2+ release was lower than that of the untreated reference, for the other sample a very high value was registered during the first cycle (Fig. 2). This high Ca2+ release, that also makes the cumulative loss higher than that of the untreated samples (Table 3), might indicate a significant formation of soluble CaP phases in one of the samples. This seems to be confirmed by the high release of PO43− ions registered during the first cycle for one of the samples (Fig. 2). In general, there seems to be a fair agreement between Ca2+ and PO43− ions, which suggests formation of some soluble phases or the detachment of relevant parts of the coating. This, in turn, explains the high scatter in Fig. 2c for both calcium and phosphate ions, deriving from a different behavior of the two samples examined. SEM observation revealed the presence of some uncoated areas (Fig. 3i), as well as areas of new CaP phases with needle-like morphology (Fig. 3k), both possibly indicating formation of soluble phases. Where it is still present, the coating appears to be intact, but largely affected by cracking, (Fig. 3l), which is consistent with the high concentration of the DAP solution and hence with the high thickness expected for the resulting coating. As a result, the treatment based on application of a 3M DAP solution, despite having been found to provide very good results for marble consolidation, appears not ideal for protection of marble against dissolution. The AmOx treatment does not significantly improve resistance to corrosion, as the amount of Ca2+ ions dissolved in each cycle is comparable to that of the untreated reference and so is the cumulative loss (Fig. 2 and Table 3). After the first cycles, the registered values are slightly higher than those of the untreated samples, possibly because of the coating contribution. This behavior is consistent with the relatively high solubility of the coating, which has basically the same solubility as calcite [52]. Some variability is also found between the behavior of different samples and between the cycles (scatter bars in Fig. 2), possibly deriving from different contributions to dissolution of the coating and of the substrate at different cycles. SEM observations showed that AmOx samples are severely etched after dripping and the coating is no longer visible (Fig. 2l,m), which confirms that the protection given by the treatment is not satisfactory. Ammonium oxalate-hydroxyapatite mixed coatings (AmOx + 0.1M samples) exhibit the worst behavior, as Ca2+ concentration is even higher than that of the untreated reference at each cycle, so that the cumulative Ca2+ ions loss is the highest among the examined samples (Fig. 3 and Table 3). Accordingly, high concentrations of PO43− are detected as well, especially during the first cycle. This suggests that
Table 2 Color change after treatment (values are averages for 6 measurements on two different samples for each condition; errors are expressed as the difference between the average and the maximum/minimum values). Sample
ΔE*
UT 0.1MED 3M AmOx AmOx + 0.1M
– 0.45 0.63 0.89 0.71
± ± ± ±
0.25 0.21 0.29 0.08
concentration was determined quantitatively by a photometer (instrument HI 83200, Hanna Instruments). To verify whether the marker would react with detached fragments of the coating, the very same tests (i.e. staining by a phosphate marker and measuring color shift towards blue by photometer) were carried out using pure hydroxyapatite powder, that was reacted with the marker in 10 ml of deionized water, exactly as the samples of runoff solution. HAP powder caused a strong shift towards blue, hence indicating that the marker is capable of reacting with solids. To investigate whether the coatings are preserved after dripping and the possible etching of the underlying marble, indicating corrosion of the substrate, sample observation by SEM/EDS was performed after the 24th dripping cycle. 3. Results Values of color change after treatment are reported in Table 2. All values are well below the ΔE = 3 threshold of human eye detectability [49], so none of the treatments was discarded as incompatible. The concentrations of Ca2+ and PO43− ions measured in the runoff solution for each sample at different cycles, indicative of the dissolution of the substrate and of the coatings, are illustrated in Fig. 2. The extent of dissolution of the samples was evaluated by calculating the cumulative loss of calcium ions, reported in Table 3: the dissolution is greatest for samples AmOx + 0.1M and 3M, and least for sample 0.1MED, where dissolution is remarkably reduced. No significant differences are found between untreated samples and samples treated by ammonium oxalate, for which dissolution is slightly higher. The morphology of the treated surface after dripping is illustrated in Fig. 3. Significant etching is found for untreated samples, indicating that significant corrosion was induced by the test, consistently with the simulation of 40 years of rain. 4. Discussion SEM/EDS analysis detected traces of magnesium in the untreated specimens, which is not uncommon as dolomite is often a minor component in marbles. However, because Mg is a hydroxyapatite-growth inhibitor and can also reduce HAP crystal size [50,51], its presence might negatively influence the growth of HAP coatings, resulting in more patchy phosphate layers. As for the effects of the simulated rain tests on the samples, measured in terms of release of Ca2+ and PO43− ions, untreated samples (UT) exhibit a noticeable release of calcium ions (Fig. 2), which indicates progressive dissolution. Accordingly, after dripping severe etching of the marble surface can be observed by SEM (Fig. 3a–b). In the case of the 0.1MED samples, Ca2+ ions released after each cycle are lower than those released by the untreated references, indicating that the coating is able to offer some protection, even if corrosion of the substrate is not completely prevented. Noticeably, the ratio between the cumulative loss of sample 0.1MED and that of the untreated reference is about 0.6 (Table 3), thus indicating that sample 0.1MED releases about 40% less calcium ions compared to untreated marble. Consistently, 171
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Fig. 2. Concentration of Ca2+ ions (light grey bars) and PO43− ions (dark grey bars) in the runoff solution in (a) untreated samples; (b) samples 0.1MED; (c) samples 3M; (d) samples AmOx and (e) samples AmOx + 0.1M. For untreated reference samples, a PO43− concentration of 0.01 mg/l was registered, which corresponds to the instrument detectability threshold and has to be attributed to the error in the measurements. Values are averages for 2 samples; error bars indicate the difference between the average and the maximum/minimum values.
treatment is not promising, because significant dissolution occurs both in and under the coating.
Table 3 Cumulative losses for untreated and treated samples. The Ca2+ concentration is normalized by the area of the sample. Sample
Cumulative Ca2+ loss [mg/l cm2]
UT 0.1MED 3M AmOx AmOx + 0.1M
5.78 3.51 7.02 6.43 9.00
5. Conclusions The resistance to dissolution in simulated rain was evaluated for Carrara marble treated with different HAP-based formulations, in terms of concentration of the phosphate precursors and possible ethanol addition, and were compared to a treatment based on ammonium oxalate. The following conclusions can be derived.
significant dissolution of both the coating and the substrate occurred: essentially, most of the coating dissolves during the first hours of dripping, leaving large areas of the substrate bare and thus causing relevant dissolution also in the following cycles. Consistently, SEM observation (Fig. 3n-p) shows that the surface of the samples is essentially bare and etched, with the HAP and underlying oxalate layer visible in only a few spots and no traces of the calcium oxalate layer alone. The dissolution of the calcium oxalate layer, initiated where some defect in the HAP layer was present, likely caused the detachment of the HAP-layer. As in the case of the 3M treatment, the presence of soluble phases leads to detachment of portions of the coating and causes a very high variability between different samples and between different cycles, dissolution generally being maximal in the first cycle, when most of the soluble phases dissolve. In general, comparing the behavior of the different samples, it was found that whenever there is a significant dissolution of the coating (which contributes to the amount of calcium ions detected in the solution), quite high variability is found between the different samples and different cycles. These results clearly indicate that the ammonium oxalate-hydroxyapatite sequential
1. Double treatments involving ethanol addition to the 0.1 M DAP solution during the first step provide fairly good protection, as dissolution of the substrate is significantly reduced with respect to the untreated reference. After exposure to simulated rain, the coating appears to be detached in several areas, but still the total amount of calcium ions released in the runoff solution is about 40% lower than for the untreated marble, hence significant protection is offered against the equivalent of 40 years of simulated rain; 2. The ammonium oxalate treatment alone does not offer satisfactory protection, as the layer is too soluble and dissolves during the test. This is particularly relevant as it indicates the advantage of the HAPbased treatment. 3. In the light of results obtained in this study, the treatment based on hydroxyapatite formation by ethanol addition to the DAP solution appears to be the most promising method, as ethanol allows increasing surface coverage without causing significant cracks and soluble phases formation, thus creating an insoluble phosphate layer that reduces corrosion of the underlying substrate.
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Fig. 3. SEM images of treated and untreated samples after dripping: (a,b) UT samples; (c–h) 0.1MED samples; (i–k) 3 M samples; (l,m) AmOx samples, AmOx+0.1M samples (n,p). Etching of the marble substrate is visible for both untreated (a,b) and AmOx-treated samples (l,m). In samples 0.1MED, some coating detachment can be observed (c,d), leading to exposure of the underlying marble (e). In samples 3M, depending on the area, it is possible to see uncoated and etched marble (i), complete but cracked layer (j), or a needle-like morphology, suggesting the presence of phases other than HAP (k). In AmOx + 0.1 M the coating has been almost completely corroded, so that only traces of the HAP coating can be detected (o,p), while the calcium oxalate layer was not observed anywhere in the sample.
References
Acknowledgements We are grateful to M.Eng. Matteo Glorioso for collaboration on dripping tests and to Dr. Lorella Guadagnini for assistance in HPLC measurements.
[1] E.M. Winkler, Stone in Architecture, 3rd ed., Springer, Berlin, 1997. [2] K. Lal Gauri, J.K. Bandyopadhyay, Carbonate Stone Chemical Behavior, Durability and Conservation, Wiley, New York, 1999. [3] T.T.N. Lan, R. Nishimura, Y. Tsujino, Y. Satoh, N.T.P. Thoa, M. Yokoi, Y. Maeda, The effects of air pollution and climatic factors on atmospheric corrosion of marble
173
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G. Graziani et al. under field exposure, Corros. Sci. 47 (2005) 1023–1038. [4] J.B. Johnson, M. Montgomery, G.E. Thompson, G.C. Wood, P.W. Sage, M.J. Cooke, The influence of combustion-derived pollutants on limestone deterioration: 1. The dry deposition of pollutants gases, Corros. Sci. 38 (1996) 105–131. [5] J.B. Johnson, M. Montgomery, G.E. Thompson, G.C. Wood, P.W. Sage, M.J. Cooke, The influence of combustion-derived pollutants on limestone deterioration: 2. The wet deposition of pollutants species, Corros. Sci. 38 (1996) 267–278. [6] A. Bonazza, P. Messina, C. Sabbioni, C.M. Grossi, P. Brimblecombe, Mapping the impact of climate change on surface recession of carbonate buildings in Europe, Sci. Total Environ. 407 (2009) 2039–2050. [7] E. Franzoni, E. Sassoni, Correlation between microstructural characteristics and weight loss of natural stones exposed to simulated acid rain, Sci. Total Environ. 412–413 (2011) 278–285. [8] S. Siegesmund, K. Ullemeyer, T. Weiss, E.K. Tschegg, Physical weathering of marbles caused by anisotropic thermal expansion, Int. J. Earth Sci. 89 (2000) 170–182. [9] A. Bonazza, C. Sabbioni, C. Guaraldi, P. De Nuntiis, Climate change impact: mapping thermal stress on Carrara marble in Europe, Sci. Total Environ. 407 (2009) 4506–4512. [10] E. Sassoni, G. Graziani, E. Franzoni, Repair of sugaring marble by ammonium phosphate: comparison with ethyl silicate and ammonium oxalate and pilot application to historic artifact, Mater. Des. 88 (2015) 1145–1157. [11] E.L. Sjcoberg, D.T. Rickard, The effect of added calcium on calcite dissolution kinetics in aqueous solutions at 25 °C, Chem. Geol. 49 (1985) 405–413. [12] T.T.N. Lan, N.T.P. Thoa, R. Nishimura, Y. Tsujino, M. Yokoi, Y. Maeda, New model for the sulfation of marble by dry deposition Sheltered marble—the indicator of air pollution by sulfur dioxide, Atmos. Environ. 39 (2005) 913–920. [13] S.J. Haneef, J.B. Johnson, G.E. Thompson, G.C. Wood, Effects of dry deposition of pollutant gases on the degradation of pentelic marble, Corros. Sci. 35 (1993) 743–750. [14] D.A. Dolske, Deposition of atmospheric pollutants to monuments, statues, and buildings, Sci. Total Environ. 167 (1995) 15–31. [15] F.W. Lipfert, Atmospheric damage to calcareous stones comparison and reconciliation of experimental findings, Atmos. Environ. 23 (1989) 415–429. [16] M. Matteini, Inorganic treatments for the consolidation and protection of stone artifacts, Conserv. Sci. Cult. Herit. 8 (2008) 13–27. [17] Q. Liu, B. Zhang, Z. Shen, H. Lu, A crude protective film on historic stone and its artificial preparation through biomimetic synthesis, Appl. Surf. Sci. 253 (2006) 2625–2632. [18] R.M. Ion, D. Turcanu-Caruţiu, R.C. Fierăscu, I. Fierăscu, I.R. Bunghez, M.L. Ion, S. Teodorescu, G. Vasilievici, V. Rădiţoiu, Caoxite-hydroxyapatite composition as consolidating material for the chalk stone for basarabi-murfatlar churches ensemble, Appl. Surf. Sci. 358 (2015) 612–618. [19] C. Esposito Corcione, R. Striani, M. Frigione, UV-cured siloxane-modified methacrylic system containing hydroxyapatite as potential protective coating for carbonate stones, Prog. Org. Coat. 76 (2013) 1236–1242. [20] P. Cardiano, S. Sergi, M. Lazzari, P. Piraino, Epoxy-silica polymers as restoration materials, Polymer 43 (2002) 6635–6640. [21] C. Esposito Corcione, M. Frigione, Influence of stone particles on the rheological behavior of a novel photopolymerizable siloxane-modified acrylic resin, J. Appl. Polym. Sci. 122 (2011) 942–947. [22] D. Pinna, B. Salvadori, S. Porcinai, Evaluation of the application conditions of artificial protection treatments on salt-laden limestone and marble, Constr. Build. Mater. 25 (2011) 2723–2732. [23] S. Naidu, E. Sassoni, G.W. Scherer, New treatment for corrosion-resistant coatings for marble and consolidation of limestone, Jardins de Pierres – Conservation of stone in Parks, Gardens and Cemeteries, XL Print, PARIS, 2011, pp. 289–294. [24] G. Graziani, E. Sassoni, E. Franzoni, G.W. Scherer, Hydroxyapatite coatings for marble protection: optimization of calcite covering and acid resistance, Appl. Surf. Sci. 368 (2016) 241–257. [25] S. Naidu, J. Blair, G.W. Scherer, Acid-resistant coatings on marble, J. Am. Ceram. Soc. 99 (2016) 3421–3428. [26] E. Sassoni, S. Naidu, G.W. Scherer, The use of hydroxyapatite as a new inorganic consolidant for damaged carbonate stones, J. Cult. Herit. 12 (2011) 346–355.
[27] E. Franzoni, E. Sassoni, G. Graziani, Brushing: poultice or immersion? Role of the application technique on the performance of a novel hydroxyapatite-based consolidating treatment for limestone, J. Cult. Herit. 16 (2015) 173–184. [28] E. Sassoni, G. Graziani, E. Franzoni, An innovative phosphate-based consolidant for limestone. Part 1: effectiveness and compatibility in comparison with ethyl silicate, Constr. Build. Mater. 102 (2016) 918–930. [29] E. Sassoni, G. Graziani, E. Franzoni, An innovative phosphate-based consolidant for limestone. Part 2. Durability in comparison with ethyl silicate, Constr. Build. Mater. 102 (2016) 931–942. [30] F. Yang, Y. Liu, G. Zuo, X. Wang, P. Hua, Q. Ma, G. Dong, Y. Yue, B. Zhang, Hydroxyapatite conversion layer for the preservation of surface gypsification marble relics, Corros. Sci. 88 (2014) 6–9. [31] E. Sassoni, G. Graziani, G.W. Scherer, E. Franzoni, Some recent findings on marble conservation by aqueous solutions of diammonium hydrogen phosphate, MRS Adv. 2 (2017) 2021–2026. [32] S. Naidu, G.W. Scherer, Nucleation, growth and evolution of calcium phosphate films on calcite, J. Colloid Interface Sci. 435 (2014) 128–137. [33] M. Matteini, A. Moles, S. Giovannoni, Calcium oxalate as a protective mineral system for wall paintings: methodology and analyses, in: V. Fassina, H. Ott, F. Zezza (Eds.), III Int. Symp. Conservation of Monuments in the Mediterranean Basin (1994). [34] C. Conti, C. Colombo, D. Dellasega, M. Matteini, M. Realini, G. Zerbi, Ammonium oxalate treatment: evaluation by μ-Raman mapping of the penetration depth in different plasters, J. Cult. Herit. 12 (2011) 372–379. [35] P. Meloni, F. Manca, G. Carangiu, Marble protection: an inorganic electrokinetic approach, Appl. Surf. Sci. 273 (2013) 377–385. [36] B. Doherty, M. Pamplona, R. Selvaggi, C. Miliani, M. Matteini, A. Sgamellotti, B. Brunetti, Efficiency and resistance of the artificial oxalate protection treatment on marble against chemical weathering, Appl. Surf. Sci. 253 (2007) 4477–4484. [37] G. Kaufmann, W. Dreybrodt, Calcite dissolution in the system CaCO3-H2O-CO2 at high undersaturation, Geochim. Cosmochim. Acta 71 (2007) 1398–1410. [38] C. Chiavari, E. Bernardi, A. Balbo, C. Monticelli, S. Raffo, M.C. Bignozzi, C. Martini, Atmospheric corrosion of fire-gilded bronze: corrosion and corrosion protection during accelerated ageing tests, Corros. Sci. 100 (2015) 435–447. [39] C. Chiavari, E. Bernardi, C. Martini, F. Passarini, A. Motori, M.C. Bignozzi, Atmospheric corrosion of Cor-Ten steel with different surface finish: accelerated ageing and metal release, Mater. Chem. Phys. 136 (2012) 477–486. [42] C. Gómez-Polo, M. Portillo Muñoz, M. Cruz Lorenzo Luengo, P. Vicente, P. Galindo, A.M. Martín Casado, Comparison of the CIELab and CIEDE2000 color difference formulas, J. Prosthet. Dent. 115 (2016) 65–70. [43] E. Ksinopoulou, A. Bakolas, I. Kartsonakis, C. Charitidis, A. Moropoulou, Particle modified consolidants in the consolidation of porous stone, Proceedings of the 12th International Congress on the Deterioration and Conservation of Stone Columbia University New York (2012). [44] S. Mats, A. Johnsson, G.H. Nancollas, The role of brushite and octacalcium phosphate in apatite formation, Crit. Rev. Oral Biol. Med. 3 (1992) 61–82. [45] S.V. Dorozhkin, Amorphous calcium orthophosphates: nature, chemistry and biomedical applications, Int. J. Mater. Chem. 2 (2012) 19–46. [46] S.V. Dorozhkin, Biphasic, triphasic and multiphasic calcium orthophosphates, Acta Biomater. 8 (2012) 963–977. [47] S.V. Dorozhkin, Calcium orthophosphate coatings on magnesium and its biodegradable alloys, Acta Biomater. 10 (2014) 2919–2934. [48] S.V. Dorozhkin, Calcium orthophosphates, Biomatter 1 (2) (2011) 121–164. [49] C. Miliani, M.L. Velo-Simpson, G.W. Scherer, Particle-modified consolidants: a study on the effect of particles on sol–gel properties and consolidation effectiveness, J. Cult. Herit. 8 (2007) 1–6. [50] A. Bigi, G. Falini, E. Foresti, M. Gazzano, A. Ripamonti, N. Roveri, Magnesium influence on hydroxyapatite crystallization, J. Inorg. Biochem. 49 (1993) 69–78. [51] G. Graziani, M. Bianchi, E. Sassoni, A. Russo, M. Marcacci, Ion-substituted calcium phosphate coatings deposited by plasma-assisted techniques: a review, Mater. Sci. Eng. C 74 (2017) 219–229. [52] D.J. White, G.H. Nancollas, The kinetics of dissolution of calcium oxalate monohydrate: a constant composition study, J. Cryst. Growth 57 (1982) 267–272.
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