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WEATHERING AND PRESERVATION OF CHALK STONES FROM BASARABI CHURCHES (ROMANIA) Rodica-Mariana Ion (ICECHIM, Romania and Valahia University, Targoviste, Romania) ( e-mail: [email protected](corresponding author) Radu-Claudiu Fierascu , Irina Fierascu (ICECHIM, Romania) Mihaela-Lucia Ion (“Atelierul” NGO, Bucharest, Romania) Daniela Turcanu-Carutiu (Ovidius University, Constanta, Romania) Maria Mihaly, Adina Rogozea, Aurelia Meghea (Polytechnica University, Bucharest, Romania)

Keywords: weathering, preservation, Basarabi chalk Church

Abstract Environmental and human impacts endanger historic monuments. The preservation and the condition assessment are essential for the selection of proper materials. The Basarabi whole cave has a special place in terms of chronological data, being recognized as the first religious monument from medieval Dobrogea. Discovered on 1957, the Basarabi Murfatlar Ensemble is one of the most impressive archaeological sites of Europe, consisting of churches dated from 9th - 11th century. Situated in the cliff of a chalk stone hill, this ensemble is built from amorphous calcium carbonate and very sensitive to humidity, frost, salts etc. Atmospheric pollution and acid deposition materials are recognized as the most important and common causes of decay the heritage monuments. Studies on chemical-mineralogical-physical changes in monument caused by weathering will also make a part of the paper. Techniques to measure and evaluate environmental damage and degradation processes on different materials, measurements of the extent of damage caused by atmospheric pollution, will be presented in this paper, as follows: petrographical and physical-chemical aspects (Energy-dispersed X-ray fluorescence (EDXRF), X-ray diffraction (XRD)) for chalk stone wall prelevated from Basarabi whole cave. The methods of restoration and analyses of materials that are used in monument preservation, especially consolidants, will be also discussed, pointing out the effectiveness of protection, preservation and conservation procedures. Some materials (nanomaterials) as calcium hydroxide Ca(OH)2, magnesium hydroxide Mg(OH)2, hydroxyapatite (HAp), are prepared and analytical investigated (Dynamic Light Scattering (DLS), Scanning electron microscopy (SEM), Atomic force microscopy (AFM)), and used for consolidating the wall chalk stone. The pores diameters and the specific surface determinations will complete the experimental tests. Mechanical properties as compressive strength and capillary water uptake test, have been evaluated and discussed, comparing the model-samples with chalk samples, treated as shown above.

Introduction With a mineral-inorganic nature, the main causes for stone deterioration are acid attack (caused by rains and humidity condense in polluted urban atmospheres) and soluble salts cyclic crystallization [1]. The first mechanism (acids) induces corrosion to carbonatic materials while silicatic stones are only poorly affected. The second mechanism (salts) is active towards porous stones. Some studies on environmental factors on degradation stone monuments, such as temperature (average, minimum, maximum), frost and sunny days, humidity annual average rainfall amount and chemical composition, currents air, are evaluated, too [2].

Causes of stone deterioration, mechanisms, characterization of building materials, in terms chemical and mineralogical composition, the physical, chemical and mechanical properties, have been studied in this paper. Also, the composition will be decelated by petrographic analysis, and analytical techniques: X-ray diffraction (XRD), thermal analysis, scaning electron microscopy (SEM), Fourier transformed infrared spectroscopy (FTIR) and energy-dispersive X-ray fluorescence (EDXRF). Mechanical properties as compressive strength and capillary water uptake test, have been evaluated and discussed, comparing the model-smaples with chalk samples, treated as shown above. For restoration/conservation of historical buildings and monuments is necessary to develop materials (micro- and nano) compatible with natural and artificial stone [3]. The new recently developed consolidants based on hydroxyapatite, calcium hydroxide and magnesium hydroxyde nanoparticles dispersed in alcohols, are treated in this paper, for some synthetic-model samples very similar with Basarabi Chuck Church, ensamble of Churches located in the south-east part of Romania, very close of the Danube-Black Sea Channel, dated from 9th - 11th century and discovered on 1957 [4]. Now this historical building is highly degraded, fast and efficient solutions for restoration being required.

Materials Specimen samples preparation The substrates were Specimens imitating highly corroded lime mortar (4x4x4 cm3 ) formed from mixture of crushed limestone (size max. 2 mm), sand (size max. 2 mm) and demineralised water (2:4:1 by volume). These samples were ground in an agate mortar and pestle before analyses, in order to reduce the particle size and to secure homogeneity. It was graduated siliceous sand obtained from a mixture of three different calibrated sands with mean particle sizes < 2 mm. Different samples were prepared, such as mortar prisms (40 mm × 40 mm × 40 mm). It was choosed this receipt in order to achieve a good simulation of corroded lime mortar and to distinguish the difference between the lime binder contained in prepared samples and the influence of consolidant used during the treatment. The samples prelevated from Basarabi Church (samples collected from the exterior area of the monument, without any value for this church.

Consolidants Nanosuspension of Mg(OH)2 and Ca(OH)2 dispersed in ethanol, are synthesized in the lab. They have been dispersed in isopropanol and suspended into isopropanol as solvent. By comparison, has been used CaLoSiL E25 composed of lime nanoparticles suspended in alcohol [5-7]. It is supplied as CaLoSiL E25 consisting of 25g particles per litre of ethanol. In this study, hydroxyapatite was obtained by chemical precipitation method from calcium nitrate tetrahydrate Ca(NO3)2.4H2O and dibasic ammonium phosphate (NH4)2HPO4, at room temperature. The powder was dry mortar in a mortar and pestle and then calcined in alumina crucible at 1200 0C for 1h. Each type of mortar substrates was treated with the above –mentioned three consolidants. The application of all types of consolidant was carried out by immersion until saturation. After each cycle the all samples were covered for one day by a slightly opened cover to avoid quick evaporation of solvent. Next day the cover was removed and the specimens were exposed to laboratory conditions to get dry. The next application cycle was done when the specimens became completely dry. For the chalk samples, the consolidants have been applied by spraying.

Characterization techniques The samples were analyzed by X-ray diffraction, carried out with a DRON UM1 diffractometer using an iron filter for the CoKα radiation (1.79021Å) and also, with a XRD, Philips Diffractometer PW 1840, 40kV/20mA, Cu Kα radiation, X-ray fluorescence analysis, performed with an energy dispersive instrument, EDXRF PW4025, type MinipalPanalytical, with a Si(Li)-detector of 150 eV resolution at 5.89 keV (Mn-K-line). The particles size and theirs size distribution have been measured by Dynamic Light Scattering (DLS) technique. For microscopic analysis, scanning electron microscopy (SEM) with Quanta 200 Scanning Electron Microscope (SEM) with magnifications of over 100000x, and atomic force microscopy (AFM), carried out with an Agilent 5500 SPM system, for morphology and 3D topographical images and section analysis. The conservation efficiency of the consolidant was estimated by compressive strength, with Silver Schmidt Hammer L, with a compressive range 5-30 N/mm2 and 0.735 Nmm impact energy (EN 12 504-2) and by capillary water uptake tests, determined according to the method according to EN ISO 15148.

Results and discussion From petrographic point of view, the prelevated chalk sample is a clay bioclastic limestone (chalk), a variety of precipitation limestone, porous, finely granular and relatively friable (loose cohesive powder, white). It has an equigranular texture and a microcrystalline binder. The chalk sample has both an organogenic chemical structure, with calcite (and/or vaterite) and minerals clay as constituents, and an autigens chemical structure with iron oxides and hydroxides, and a bioclastic foraminiphera, diagenesis local recrystallization of calcite. It has a microcrystalline texture, a diagenetic structure, with autigens constituents: silica, calcite, iron oxides and hydroxides. As bioclast, it contains radiolarian and foraminiphera, and the specific diagenesis, local recrystallization of silica and a silicification of foraminiphera.

Figure 1. Petrographic analysis of chalk stone sample

The petrographic microscopy investigations of the samples confirm the same conclusion from the literature: vaterite, which is bery unstable (is stable only under 10 OC) has the tendency to form framboidal structures, in the presence of CO 2. These framboidal structures are aggregations of smaller, mostly spherical, particles, with an average size of these elementary spheres comprised between 36 and 150 nm. Broken vaterite framboids as shown in Figure 1a clearly show that the center of the framboidal aggregates is built up by the same small particles as the outer part [8].

The concentrations of trace metals, e.g., Mg, Sr or Ba, in biogenic calcite are used as palaeo-proxies for the reconstruction of past environmental conditions (e.g. [9]), while sorption to calcite could be an alternative pathway for the immobilization of hazardous metals, e.g., Cd and other radionuclides [10]. The presence of Sr is favouring the calcite stability, is able to interact primarily with the sterically open sites on the surface of calcite during dissolution and that, competition between the precipitation/adsorption of SrCO 3 and the dissolution of CaCO3 is occurring at these sites. On the other hand, Sr 2+ causes a significant reduction in the growth and dissolution rates of aragonite. This was attributed to reversible adsorption of Sr2+ ions at growth sites (kinks) [11]. Cu2+ and Zn2+ could form soluble oxides and carbonated over the calcite surface, while Sc3+ could inhibite the calcite solubilization [12].

Figure 2. XRD of samples prelevated from inside and outside Vaterite is highly unstable when exposed to water; it can recrystallize to calcite within 20 to 25 h at room temperature [13]. Vaterite being unstable outside, due to a higher humidity, its concentration is decreasing constantly. As the humidity is higher, the vaterite concentration decreases, figure 2.

Figure 3. EDXRF spectrum of the chalk sample Hydroxyapatite, Ca10(PO4)6(OH)2, is the main inorganic constituent of tooth enamel and bone [14]. HAp has a much lower solubility and dissolution rate having the ability to

confer protection in acidic environments, with a dense coating of only about 10 μm. The performance of HAp has already been proven in restoring the strength of weathered limestone [15]. Other workers have also investigated the use of HAp as a consolidant [1618]. In this paper, we discuss its use as a surface-protective layer for chalk in order to obtain a noticeable retardation of the weathering. Special attention was paid to the effectiveness of consolidation treatment observable namely in the pores and cracks. It is mostly the size of pore openings and the chemical/ mineralogical nature of the pore walls which are of relevance to the treatment by consolidant.

Figure 4. Size distribution for Ca(OH)2 nanoparticles dispersed in 2-propanol Cumulative distribution often provides a more accurate representation of the data. For hydroxyapatite powder was made a global analysis where reproducibility is better between sets of measurements, but measured particles seem more independent particles and not aggregates.

Figure 5. HAp size distribution of nanoparticles by number In the case of Ca(OH)2, the light part of the image can be the consequence of the presence of a thick part of sizing material, possibly to an aggregate form [19,20]. For HAp, less white deposits of revealed a rough surface architecture for HAp, the predominant size of grains being in the range of 90-100nm. For Ca(OH)2 the consolidation film is characterized by the presence of plate-like nanoparticles that aggregate into micro-sized clusters, which are compact and polydispersed. According to previous studies [21] the carbonation of nanolime particles originate in oriented crystal grains, which promote the agglomeration of the particles [22]. On the other hand, specimens treated with HAp present a more uniform distribution of the consolidation product and homogeneous infilling of the matrix voids.

Figure 6a. AFM image of Ca(OH)2 on surface obtained in contact mode (a), 3D AFM image of Ca(OH)2 on surface (b)

Figure 6b. HAp sprayed on the surface (tapping mode) (a), 3D AFM morphology for HAp nanocrystals on surface (b)

Figure 6c. AFM image of Mg(OH)2 on surface obtained in contact mode (a), 3D AFM image of Mg(OH)2 on surface (b) Three specimens of cube shape (4x4x4 cm3) of each type of substrate were used for measurement. Before testing, all samples were dried up to constant weight at 80 °C in a drying chamber for 24 hours. After the drying process the substrates were left to get cold for two hours in a dessicator and their aspect was measured, Figure 6a,b,c. HAp is easier to dry and induce more grip than the other components, first of all due to the contribution of hydroxyapatite which binds weathered stone blocks together providing a substantial reinforcement, and has the possibility to penetrate deep into damaged zones without limitations due to the particle size. Despite of its relatively low stability, HAp is uniform layer, and don’t induce a significant whiter colour of the treated surface.

Figure 7. The aspect of one cube specimen before (up) and after treatment (down) with nanomaterial HAp : Ca(OH)2 For the model-samples, the compressive strength determined with Silver-Schmidt Hammer, indicated that the most effective treated sample has a compressive strength of 20.33 MPa and is that treated with HAp. Undoubtedly, this is caused by the network of hydroxyapatite, which can bind weathered stone blocks together providing a substantial reinforcement. In the case of Ca(OH) 2, we have to take into acount the non-uniform thickness of the consolidant, due to the aggregation tendency of Ca(OH)2 [22]. For chalk samples, HAp indicates a certain reconsolidation, too, while the treatment with Ca(OH)2 do not cause any increase in cohesion, rather a decrease. Even both methods are measuring the same parameter – compressive strength – there is a difference between their values, versus the depth. At Silver Schmidt hammer, the depth is 5 mm. This depth has been is imposed by the stone resistance and for feasibility of the measurements. Suplimentary studies are necessary to correlate the external factors influence on these values.

Conclusions In this paper has been treated the structural, morphological and compositional aspects of chalk stone sample prelevated from Basarabi Chalk Church (Romania) (outside of monument area), for which a new method based on nanoparticles has been tested. The petrographic microscopy investigations of the samples coupled with XRD concluded the presence of vaterite (stable only under 10 OC and with a tendency to form framboidal structures, in the presence of CO2), possibly responsible for further damages of the stone (open framboidal structure). The mechanical parameter compressive strength either determined with Silver Schmidt hammer, indicated us highest value for HAp. This is caused by the network of hydroxyapatite, which can bind weathered stone blocks together providing a substantial reinforcement.

References 1. P.P.Ferreira and R.J.Delgado “Stone consolidation: The role of treatment procedures”, J. Cult. Herit. Vol.9, 2008, pp.38-53. 2. C.A.Price, Stone Conservation: An Overview of Current Research., Getty conservation Institute, 1996. 3. B.L. Clarke and J.Ashurst, Stone Preservation Experiments. Building Research Establishment, 1972.

4. E.Sassoni, S.Naidu and G.W.Scherer, “The use of hydroxyapatite as a new inorganic consolidant for damaged carbonate stones”, J Cult Herit.,Vol. 12, 2011, pp.346-355. 5. G.Ziegenbalg “Colloidal calcium hydroxide: A New Material for Consolidation and Conservation of Carbonatic Stones”, 11th Int. Congr. Deterioration and Conservation of Stone, Torun, Poland, 2008, pp. 1109-1115. 6. G. Ziegenbalg, K.Brümmer and J. Pianski Nano-Lime - a New Material for the Consolidation and Conservation of Historic Mortars, 2nd Historic Mortars Conference, 2010, pp. 1301-1309. 7. M.Drdácký, Z. Slížková and G. Ziegenbalg, “Α Nano Approach to Consolidation of Degraded Historic Lime Mortars”, J. Nano Res., Vol.8, 2009, pp. 13-22. 8. G. Nehrke, “Calcite precipitation from aqueous solution: transformation from vaterite and role of solution stoichiometry, Doctoral thesis Utrecht University, 2007, pp.67-96. 9. E.A.Boyle, “Cd,Zn,Cu and Ba in foraminifera tests”, Earth Planet Sci Lett. Vol.53, 2008, pp.11-35. 10. E.Curti, Coprecipitation of radionuclides, PSI Bericht, 1981. 11. A.Gutjahr, H.Dabringhaus and R.Lacmann “Studies of the growth and dissolution kinetics of the CaCO3 polymorphs calcite and aragonite II. The influence of divalent cation additives on the growth and dissolution rates”, J. Cryst. Growth. Vol.158, 1996, pp.310–315. 12. I.Nestaas I. and S.G.Terjesen, “The inhibiting effect of scandium ions upon the dissolution of calcium carbonate”, Acta Chem. Scand., Vol.23, 1969, pp.2519-2531. 13. S.T.Silk, Factors governing polymorph formation in calcium carbonate. Ph.D. thesis. New York University, 1970. 14. W.Johannes and D.Puhan, “The calcite-aragonite transition, reinvestigated”, Contrib Mineral Petrol. 31, 1971, pp.28–38. 15. J.H. Yang, K.J. Oh, DS Pandher. “Hydroxyapatite crystal deposition causing rapidly destructive arthropathy of the hip joint”, Indian J Orthop. Vol.45, 2011, pp. 569-572. 16. D.Liu, K. Savino and M.Z.Yates, “Coating of hydroxyapatite films on metal substrates by seeded hydrothermal deposition”. Surface & Coatings Technology. Vol.205, 2011, pp.3975–3986. 17. M.Matteini, S.Rescic, F.Fratini and G. Botticelli, “Ammonium Phosphates as Consolidating Agents for Carbonatic Stone Materials Used in Architecture and Cultural Heritage: Preliminary Research, Int. J. Arch. Heritage, Vol.5, 2011, pp.717736. 18. R.M. Ion, IR Bunghez, SF Pop, RC Fierascu, ML Ion and M. Leahu, “Chemical Weathering of chalk stone materials from Basarbi Churches”, Metalurgia International. Vol.2, 2013, pp.89-93. 19. E.Caner, E.N.Caner-Saltık, G.Orial and J.D. Mertz, Deterioration Mechanisms of Historic Limestone and Development of Its Conservation Treatments with Nanodispersive Ca(OH)2 Solutions, 8th Int. Symp. Conservation of Monuments in the Mediterranean Basin, Ed. M.Koui, F. Zezza, 2010. 20. P.López-Arce, A.Zornoza-Indart, L.Gomez-Villalba, and R.Fort, "Short and Longer Term Consolidation Effects of Portlandite (CaOH) 2 Nanoparticles in Carbonate Stones." J. Mater. Civ. Eng., Vol. 25, 2012, pp.1655-1665. 21. E.Franzoni, F.Sandrolini and S. Bandini, Metodologia per il monitoraggio congiunto dei sali e dell’umidità nelle murature storiche: sperimentazione su modelli di laboratorio, IV Convegno Monitoraggio e conservazione preventiva dei beni culturali, AVELLINO, Poligrafica Ruggiero. 2010. Acknowledgements: This work was supported by a grant of the Romanian National Authority for Scientific Research, CNDI-UEFISCDI, project number 222/2012.