Easy Illustration of Salt Damage in Stone - Journal of Chemical

Demonstration Cite This: J. Chem. Educ. XXXX, XXX, XXX−XXX

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Easy Illustration of Salt Damage in Stone Francesco Caruso,*,†,‡ Timothy Wangler,† and Robert J. Flatt† †

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Physical Chemistry of Building Materials (PCBM), Institute for Building Materials (IfB), ETH Zürich, Stefano-Franscini-Platz 3, 8093 Zurich, Switzerland ‡ Conservation Studies, Department of Archaeology, Conservation and History (IAKH), University of Oslo, P.O. Box 1008, Blindern, 0315 Oslo, Norway ABSTRACT: Salt-induced stone weathering is often erroneously listed among the physical causes of damage when it is in fact physicochemical. We introduce here a succinct version of the theory behind crystallization pressure along with a simple yet dramatic demonstration of the damage induced in sandstone by sodium sulfate. The contents here presented might be implemented in chemistry for cultural heritage, conservation science, geosciences, and materials science undergraduate courses.

KEYWORDS: Upper-Division Undergraduate, Demonstrations, Physical Chemistry, Misconceptions/Discrepant Events, Thermodynamics

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contributed substantially to the topic since the beginning of the 20th century, and there are several works that have led to a more comprehensive understanding of the salt weathering phenomena, especially in the Earth system.12−23 Salt crystallization is considered to be one of the most important degradation mechanisms affecting natural stone as well as most inorganic building materials.24−28 Herodotus described it, nearly two and half millennia ago:29,30 I have observed that Egypt runs out into the sea further than the adjoining land, and that shells are found upon the mountains of it, and an efflorescence of salt forms upon the surface, so that even the pyramids are being eaten away by it. Salt contamination is a serious hazard for the artistic value and the structure of historical structures (e.g., Figure 1), outdoor and indoor monuments,16 and wall paintings. UNESCO world heritage sites such as the Nubian monuments in Egypt, the Nabataean caravan-city of Petra in Jordan,31 the Historic Center of Bukhara in Uzbekistan,32 Old Havana and its Fortification System, and many others,24 are all threatened by salt crystallization. Salt weathering has an important role in shaping Earth’s33 (coastal, urban, and dry34) environment and Mars’ landscape.35,36 Salt crystallization also has a large impact on geotechnical engineering, as illustrated by the high profile case of the historical town center of Staufen in Germany, where

norganic and organic building materials (concrete, stone, wood, metals, etc.) have had a huge impact on our history and present everyday life. Romans produced concrete for their infrastructural works,1,2 using mixtures of lime and natural pozzolans as cementitious binders. Nowadays, cement is manufactured much more efficiently and at a very large scale, so that concrete has become ubiquitous in modern construction and it is practically impossible imagining a future without using cementitious materials for building purposes.3 Concrete is sometimes referred to as “artificial stone”, a material that has an even greater significance in built cultural heritage worldwide. While properties of stone vary greatly from one type to another,4,5 we can nevertheless state rather generally that its broad accessibility, good mechanical properties in compression, and perceived long-term durability led it to be used to erect temples and monuments of utmost importance.6,7 Many of these are still standing, but suffer various forms of weathering. One of such forms is presented in this paper with the hope of illustrating its working mechanism through a simple experiment that can be performed in a classroom. Educating tomorrow’s professionals on building materials requires a balanced mix of physics, chemistry, geology, and materials science. Such a mix is so important that selfsustaining and developing disciplines have emerged and been taught at the undergraduate level in engineering and architecture in the past 30 years.8−10 A substantial section of the teaching about building materials, and, more in general, about materials of wide interest, is devoted to the study of the weathering phenomena (and the chemistry behind them) that can seriously alter their durability.11 Geosciences have © XXXX American Chemical Society and Division of Chemical Education, Inc.

Received: October 24, 2017 Revised: May 30, 2018

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DOI: 10.1021/acs.jchemed.7b00815 J. Chem. Educ. XXXX, XXX, XXX−XXX

Journal of Chemical Education

Demonstration

On the other hand, porosity is the void volume fraction of a material.42,43 Both permeability and porosity contribute to the diffusivity of a ionic solution in a porous medium. To explain how supersaturation develops in a porous system, let us first consider a crystal in a solution surrounded by a semipermeable elastic membrane that could be used to apply hydrostatic pressure to the crystal. Such a membrane would allow the exchange of ions and water between the crystal and the solution. Let us start with a saturated solution and a large crystal at equilibrium with it. The pressure exerted by the membrane at equilibrium with it is zero. If pressure is applied on the crystal (but not on the solution), then the chemical potential of the crystal increases and equilibrium only gets restored if the crystal dissolves enough. In other words, the solution concentration must increase to a degree of saturation defining a crystal under pressure at equilibrium with a solution at ambient pressure. Another way of looking at the problem is that if a crystal is placed in a supersaturated solution, then a specific pressure must be applied to it through the semipermeable membrane to prevent the crystal from growing. In both cases the pressure is the same, but some communities refer to it as “pressure solution” and others as “crystallization pressure”. The magnitude of this pressure is given by44,45 Figure 1. Salt damage in the columns of a building on the Malecón, in the historical center of Havana (Cuba).

pA =

geothermal drilling has led to ground upheaval and significant damage due to anhydrite to gypsum conversion.37 Salts by themselves are, however, not damaging. They require the presence of water for their aggressiveness to become evident.25,28 In fact, the source of the damage is crystallization pressure, which originates from the growth of salt crystals in a supersaturated solution. Furthermore, salt weathering is often erroneously described as a physical process38 when it is in fact physicochemical. The following is a short introduction to salt weathering and the thermodynamic basis of the damage due to crystallization pressure from supersaturated solutions.

RT Q ln − γCLκC vC K

(1)

where pA is the pressure exerted by the pore wall, R is the ideal gas constant, T is the absolute (thermodynamic) temperature, vC is the molar volume of the crystal, Q is the reaction quotient, K is the equilibrium constant, κC is the curvature of the crystal, and γCL is the interfacial energy between the crystal and the liquid phases. The first term of the right-hand side of eq 1 is the thermodynamic driving force (the composition of the liquid phase in the system), whereas the second term corresponds to an effect of consumption due to the curvature of the crystal (geometric effect). As explained above, pA defines the pressure that must be exerted to prevent a crystal placed in a supersaturated solution from growing. If this pressure were not exerted, the natural tendency of the system would be to consume ions from the solution to form more solid. This would take place until the solution concentration reaches the saturation concentration. So, in order for a crystal to exert pressure, there must be a special condition allowing for the supersaturation to be sustained without crystals freely growing in unrestrained zones.44 One such case is to consider that the porous network is composed of long pores with a small radius. These may be represented by cylindrical geometry and with cylindrical crystals having hemispherical ends (Figure 2). The importance of such situations is that the crystals can exert pressure while not completely filling the pore space. It therefore represents an equilibrium condition that can develop more or less independently of kinetic concerns.46 Another situation is to consider a porous network with a combination of small (≤50 nm) and large (micrometer range) pores. The small pores would not allow crystal growth because of curvature effects, but could supply the larger pores with materials to sustain crystal growth in them. Under these conditions, crystallization pressure would only develop once the large pore is filled, and in this case the full chemical driving force would be available:

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SALT CRYSTALLIZATION PRESSURE According to IUPAC, a solution is said to be saturated when its solute has the same concentration as one that is in equilibrium with undissolved solute at a defined temperature and pressure.39 The concentration of the solute in such a solution is called “solubility”. Therefore, a solution is said to be supersaturated when the concentration of the solute is higher than its solubility. This solution is consequently in a metastable state, and if it is in contact with a crystal, this will tend to grow. As an example, supersaturation of a solution containing a solute whose solubility increases with temperature can be achieved by rapidly cooling warm saturated solutions so that the cooling rate is faster than crystallization.40 The magnitude of the crystallization pressure that can develop in a porous system depends not only on supersaturation, but also on pore geometry (CT images that accurately explain the nature of the porous network in a stone can, for instance, be found in the Web site of the HighResolution X-ray Computed Tomography Facility at the University of Texas at Austin).41 Permeability and porosity are important (and related) properties of a porous medium. The first one represents the ability of the porous system to let a fluid flow through it without damaging the internal structure. B



Demonstration

Figure 2. (a) Cylindrical salt crystal with hemispherical ends immersed in a cylindrical pore filled with a supersaturated solution of the same salt. (b) A section of the same system. pC = pressure exerted by the crystal, pA = pressure exerted by the pore wall, pL = pressure exerted by the liquid surrounding the crystal, κC = curvature of the crystal, f = surface stress. The arrows indicate the normal forces of the pressures exerted by the liquid surrounding the crystal and by the pore wall.

pA =

RT Q ln vC K

Figure 3. Experimental data showing the temperature dependence of the solubility of thenardite (anhydrous sodium sulfate) and mirabilite (decahydrate sodium sulfate) at standard pressure conditions.53 At values higher than 32.38 °C, thenardite is the only thermodynamically stable form.

(2)

These two examples highlight the fact that, depending on the pore size distribution, part or all of this pressure may be transferred to the solid. Additionally, kinetic factors of transport and crystal growth will also affect the local supersaturation and the resulting disjoining pressure from the pore walls. These conditions will however not be discussed here in detail. We will just mention that in the worst-case scenario, the entire pressure ends up being uniformly distributed within the material and would be defined by eq 2. The Special Case of Sodium Sulfate

Sodium sulfate is known as the most damaging salt for stones and other porous building materials.24,47,48 Because of this, sodium sulfate is employed in salt crystallization durability standard tests.49−52 Typically, such tests consist of cycles of impregnation in a sodium sulfate solution of a porous material, followed by its drying. As the number of cycles increases, samples progressively lose mass, which is taken as a quantitative measure of damage. In this test, damage occurs during the wetting. The reason comes from the existence of different phases of this salt with different solubilities.53 Sodium sulfate exists in eight different phases: two thermodynamically stable ones, three metastable ones, and three phases that are stable under high temperature and pressure conditions.44,54−57 Thenardite (Na2SO4) and mirabilite (Na2SO4·10H2O) are the two stable phases.44,54,55 The metastable phases are the heptahydrate (Na2SO4· 7H2O),57,58 and two anhydrous forms called sodium sulfate (III) and sodium sulfate (V).54 Sodium sulfate (I) and (II) exist only at temperatures higher than 270 and 225 °C, respectively.55 The octahydrate (Na2SO4·8H2O) is formed when the pressure is higher than 1.54 GPa.56 The temperature dependence of solubility of thenardite and mirabilite, which are relevant for the demonstration discussed in this paper, is shown in Figure 3. The phase diagram (Figure 4) describes the zones of stability for the solution, thenardite and mirabilite, as a function of temperature and relative humidity.44,59 The discontinuous line extends the solubility line of thenardite below 32.38 °C, which is the temperature value at which mirabilite becomes thermodynamically stable. Any point

Figure 4. Phase diagram of sodium sulfate. The three regions indicate the values of temperature and relative humidity at which sodium sulfate solutions, mirabilite and thenardite are stable. The discontinuous line describes solutions in metastable equilibrium with respect to thenardite and supersaturated with respect to mirabilite. Adapted with permission from ref 44. Copyright 2002 Elsevier.

on this line would correspond to a situation in which a solution would be in equilibrium with thenardite and supersaturated with respect to mirabilite. If mirabilite grew from such a solution, it would develop crystallization pressure if restrained during its growth. More specifically, this is the condition that the test presented here creates in order to enhance damage by crystallization pressure.

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DEMONSTRATION

Preparation

A small cube (we used 3 × 3 × 3 cm3 cubes) of porous building stone (we usually employ Bollingen sandstone,60 but a porous limestone, like the one from Portland (UK),61 may be used as well) is impregnated with a 20% (w/w) solution of C



Demonstration

anhydrous sodium sulfate at 35 °C for 12 h. This stage of the preparation can be carried out in a small crystallizing dish, filled with the solution and covered with a Petri dish, previously equilibrated at 35 °C in an oven (see Figure 5).

It is worth noting that the salt degradation pattern changes from stone to stone and that, for the sake of the demonstration, a sandstone with relatively large grains is preferred. Explanation

On the basis of what was explained in the paragraphs on salt crystallization pressure and sodium sulfate, the impregnation cycles (preparation) occur at a temperature at which only thenardite could form. During the wetting phase (execution), the stone is again saturated with a sodium sulfate solution but at room temperature. The solution encounters the thenardite formed during the impregnation cycles. This thenardite, present in the stone, dissolves in the entering solution and increases the ion concentration well above the mirabilite saturation. The pore solution is now supersaturated with respect to mirabilite, and this starts forming in the pore network. It has been shown that mirabilite spontaneously nucleates under such conditions, but if the solution is cooled, the heptahydrate forms first with a subsequent lower pressure.57,62,63 The mirabilite continues growing in the pores, and the pressure exerted by the crystal is transmitted to the solution and consequently to the pore walls, leading to damage. For damage to occur a sufficient volume fraction of salts in the pores is needed,24 which explains that a minimum number of impregnation cycles are needed.

Figure 5. Sketch of the experimental apparatus needed for carrying out the impregnation cycles in a stone cube. The oven has been previously set at 35 °C. The cube is placed in a crystallizing dish with 20% (w/w) sodium sulfate solution equilibrated at 35 °C. The crystallizing dish is covered with a Petri dish to avoid evaporation and concentration of the solution.

Guided inquiry and Discovery

The experiment here presented can have higher pedagogical value if delivered in the form of a guided inquiry laboratory.64,65 We provide here some suggestions and questions to turn this expository demonstration into a guided inquiry experiment. The explanation is kept in the hand of the instructor, and the students are provided with the phase diagram of sodium sulfate (Figure 4) and the following systems: sandstone saturated with sodium sulfate, granite saturated with sodium sulfate, hardened cement paste (suggested water to ordinary Portland cement, e.g. type I 52.5, ratio: 0.5) saturated with sodium sulfate, and sandstone not previously impregnated. The instructor then guides the students throughout the different systems with the following set of questions:

It makes sense to have a crystallizing dish slightly taller than the stone sample and use the smallest possible volume of solution. This would avoid a potential loss of the salt in the stone by outward diffusion. Eventually, the stone cube is taken out of the solution and kept in the oven at 105 °C for 12 h. The stone cube is then moved out of the oven and put in a desiccator to cool down to room temperature. The entire cycle is repeated for around 4−5 times or until constant mass is reached (this depends on the porosity of the stone). Execution

1. How does the system look at room temperature? What is the state of sodium sulfate at this temperature? 2. What happens to the system when it starts being impregnated with the sodium sulfate solution (i.e., increase of the relative humidity)? 3. What happens to the system, when not completely damaged? Is it taken out of the solution and dried? 4. Can you predict the behavior of the system if the temperature is increased to 35 °C? 5. If pure water is used instead in place of sodium sulfate, what would happen to the system? With these kinds of questions, the students are supposed to sketch a (graphical) representation of what is occurring at the pore and macroscopic scales, highlighting the role of the different components in the system. On the basis of the level of the class, selected sections from the open access paper by Flatt et al.24 could be provided to the students. In this way, an additional semiquantitative analysis of the experiment that would involve calculations of the crystallization pressure, volume fraction of the pore space

At room temperature (between 20 and 25 °C), the stone cube is placed at the center of an empty small crystallizing dish. Some of the previously used sodium sulfate solution is poured around the stone cube. The stone cube is then naturally impregnated by capillarity. The effect of the salt in the stone against time is shown in Figure 6.

Figure 6. Dramatic effect over time of a 20% (w/w) solution of sodium sulfate entering into a sandstone cube previously impregnated with sodium sulfate. D



Demonstration

filled with crystals, and macroscopic tensile stress could be carried out.

(9) ETH Zürich. Concrete Material Science. http://www.ifb.ethz. ch/education/msc-courses/msc-concrete-science.html (accessed Oct 7, 2017). (10) KU Leuven. Advanced Building Materials Science. https:// onderwijsaanbod.kuleuven.be/syllabi/v/e/H03V5CE.htm#activetab= doelstellingen_idp2868784 (accessed Oct 7, 2017). (11) Charola, A. E. Acid Rain Effects on Stone Monuments. J. Chem. Educ. 1987, 64 (5), 436. (12) Becker, G. F.; Day, A. L. The Linear Force of Growing Crystals. Proc. Wash. Acad. Sci. 1905, VII, 283−288. (13) Taber, S. The Growth of Crystals under External Pressure. Am. J. Sci. 1916, s4−41 (246), 532−556. (14) Correns, C. W.; Steinborn, W. Experimente Zur Messung Und Erklärung Der Sogenannten Kristallisationskraft. Z. Kristallogr. - Cryst. Mater. 1939, 101 (1), 117−133. (15) Correns, C. W. Growth and Dissolution of Crystals under Linear Pressure. Discuss. Faraday Soc. 1949, 5 (0), 267−271. (16) Arnold, A.; Zehnder, K. Salt Weathering on Monuments. In Proceedings of the 1st International Symposium “The conservation of monuments in the Mediterranean Basin”; Zezza, F., Ed.; Grafo: Bari, 1990; pp 31−58. (17) Brehler, B. Ü ber das Verhalten gepreßter Kristalle in ihrer Lösung. Neues Jahrb. Für Mineral. - Monatshefte 1951, 110−131. (18) Weyl, P. K. Pressure Solution and the Force of Crystallization: A Phenomenological Theory. J. Geophys. Res. 1959, 64 (11), 2001− 2025. (19) Khaimov-Mal’kov, V. Y. Experimental Measurement of Crystallization Pressure. In Growth of Crystals; Shubnikov, A. A. V., Sheftal’, D. G.-M. S. N. N., Eds.; Springer: US, 1995; pp 14−19. (20) Flatt, R. J.; Steiger, M.; Scherer, G. W. A Commented Translation of the Paper by C.W. Correns and W. Steinborn on Crystallization Pressure. Environ. Geol. 2007, 52 (2), 187−203. (21) Caruso, F.; Flatt, R. J. Further Steps towards the Solution of Correns’ Dilemma. In Proceedings of SWBSS 2014; De Clercq, H., Ed.; Royal Institute for Cultural Heritage (KIK-IRPA): Brussels, 2014; pp 199−209. (22) Espinosa Marzal, R. M.; Scherer, G. W. Crystallization of Sodium Sulfate Salts in Limestone. Environ. Geol. 2008, 56 (3−4), 605−621. (23) Espinosa-Marzal, R. M.; Hamilton, A.; McNall, M.; Whitaker, K.; Scherer, G. W. The Chemomechanics of Crystallization during Rewetting of Limestone Impregnated with Sodium Sulfate. J. Mater. Res. 2011, 26 (12), 1472−1481. (24) Flatt, R. J.; Caruso, F.; Aguilar Sanchez, A. M.; Scherer, G. W. Chemo-Mechanics of Salt Damage in Stone. Nat. Commun. 2014, 5, 4823 DOI: 10.1038/ncomms5823. (25) Scherer, G. W. Stress from Crystallization of Salt. Cem. Concr. Res. 2004, 34 (9), 1613−1624. (26) Scherer, G. W. Crystallization in Pores. Cem. Concr. Res. 1999, 29 (8), 1347−1358. (27) Espinosa-Marzal, R. M.; Scherer, G. W. Mechanisms of Damage by Salt. Geol. Soc. Spec. Publ. 2010, 331 (1), 61−77. (28) Espinosa-Marzal, R. M.; Scherer, G. W. Advances in Understanding Damage by Salt Crystallization. Acc. Chem. Res. 2010, 43 (6), 897−905. (29) Herodotus: Histories; Rawlinson, G., Translator; 440AD; Vol. Book II, http://classics.mit.edu/Herodotus/history.2.ii.html. (30) Charola, A. E. Salts in the Deterioration of Porous Materials: An Overview. J. Am. Inst. Conserv. 2000, 39 (3), 327−343. (31) Bala’awi, F. Salt Damage at Petra, Jordan: A Study of the Effects of Wind on Salt Distribution and Crystallisation. Ph.D. Thesis, University of London, 2006. (32) Akiner, S.; Cooke, R. U.; French, R. A. Salt Damage to Islamic Monuments in Uzbekistan. Geogr. J. 1992, 158 (3), 257−272. (33) Goudie, A. S.; Viles, H. A. Salt Weathering Hazards; John Wiley & Sons, Ltd: Chichester, 1997. (34) Goudie, A. The Salt Weathering Hazard in Deserts. In Geomorphological Hazards in High Mountain Areas; Kalvoda, J.,

In-Class Use

To fully appreciate the destruction of the stone cube, the expository demonstration should be carried out at the beginning of a 2−2.5 h class. The demonstration was successfully implemented during the “Stone as a Building Material” laboratory session (lasting around 3.5 h) of the Werkstoffe III (Materials III) course of Civil Engineering Bachelor’s degree course at ETH Zurich.

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HAZARDS AND SAFETY Anhydrous sodium sulfate is an irritant in the case of eye contact, slightly irritating in the case of skin contact, and slightly hazardous in the case of ingestion or inhalation. Its LD50 for mice is 5989 mg/kg.66 A laboratory coat, goggles, and safety gloves should be worn during the course of the experiment. Heat resistant gloves and tongs should be used when taking the stone out of the oven at 105 °C.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Francesco Caruso: 0000-0002-0369-3194 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are very thankful to Yurena Seguı ́ Femenias (IfB−ETH Zürich) and Fulvio Caruso (ABB Semiconductors Switzerland Ltd.) for their assistance in using the software to help create Figure 2 and Figure 5, respectively. We are also thankful to Giulia Gelardi (IfB−ETH Zürich) for reading and commenting on the manuscript and to Asel Maria Aguilar Sanchez (IfB− ETH Zürich) for her technical assistance. Finally, we wish to thank the four anonymous reviewers who read our manuscript and provided us with useful comments for its improvement.

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REFERENCES

(1) Blezard, R. G. 1-The History of Calcareous Cements. In Lea’s Chemistry of Cement and Concrete, 4th ed.; Hewlett, P. C., Ed.; Butterworth-Heinemann: Oxford, 1998; pp 1−23. (2) MacLaren, D. C.; White, M. A. Cement: Its Chemistry and Properties. J. Chem. Educ. 2003, 80 (6), 623. (3) Flatt, R. J.; Roussel, N.; Cheeseman, C. R. Concrete: An Eco Material That Needs to Be Improved. J. Eur. Ceram. Soc. 2012, 32 (11), 2787−2798. (4) Siegesmund, S.; Török, Á . Building Stones. In Stone in Architecture; Siegesmund, S., Snethlage, R., Eds.; Springer: Berlin, 2014; pp 11−95. (5) Siegesmund, S.; Dürrast, H. Physical and Mechanical Properties of Rocks. In Stone in Architecture; Siegesmund, S., Snethlage, R., Eds.; Springer: Berlin, 2014; pp 97−224. (6) Hurcombe, L. M. Archaeological Artefacts as Material Culture; Routledge: Milton, 2007. (7) Snethlage, R. Natural Stones in Architecture: Introduction. In Stone in Architecture; Siegesmund, S., Snethlage, R., Eds.; Springer: Berlin, 2014; pp 1−9. (8) Materials Science of Concrete: Cement and ConcreteTrends and Challenges, Special Volume; Boyd, A. J., Mindess, S., Skalny, J. P., Eds.; Wiley, 2006. E



Demonstration

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Easy Illustration of Salt Damage in Stone - Journal of Chemical

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