Elsevier

Journal of Cultural Heritage

Volume 42, March–April 2020, Pages 1-7
Journal of Cultural Heritage

Original article
Experimental measurement of disjoining force at the glass–salt interface: A direct evidence of salt degradation potential caused by crystallization pressure

https://doi.org/10.1016/j.culher.2019.10.003Get rights and content

Abstract

Salt degradation is a well-known but still poorly understood problem. Determination of crystallization pressure that growing crystals exert on the pore walls represents a challenge solved by authors from different points of view. Nevertheless, few papers are aimed at the experimental measurement of the crystallization pressure magnitude. A novel high precision device able to detect repulsive forces generated by a crystal at the crystal/glass interface has been designed. Although some problems with determining the correct contact area of the confined crystal surface, which is most probably not atomically smooth, still exist, the results are comparable to data from other experimental studies. Crystallization experiments were performed with sodium chloride under 30 ± 2% and 60 ± 2% relative humidity (RH) conditions and with sodium sulfate in 30 ± 2% RH. The disjoining pressure values were variable but did not exceed 1 MPa. Special interest was aimed at determination of disjoining pressure of sodium sulfate during phase transition after wetting, since this phenomenon creates most damage during standard crystallization tests. The disjoining pressure values were between 0.957 and 3.159 MPa – sufficiently high to overcome the tensile strength of most of the porous building materials.

Introduction

Crystallization and dissolution cycles of common soluble salts occurring in the pore space of building materials initiate creation and propagation of damage. The most deteriorated region is mostly located close to the material surface where the salt crystals mainly crystallize. The materials most vulnerable to salt degradation are those exposed in the long term to the repeated dissolution – crystallization cycles, i.e. the historic buildings where the salts enter from ground dissolved in the water or from the polluted air in the form of aerosol. The damage usually results in disintegration and scaling of material – namely flaking, sanding, powdering etc. [1]. The damaging action of salt crystals consists in development of crystallization pressure near the contact area with the pore surface. The pressure generates tensile stress in the pore wall, which can overcome the tensile strength of the material under certain conditions [2]. The driving force for the crystal growth as well as for the crystallization pressure magnitude is the supersaturation of a salt solution near the crystal surface close to the pore wall [3]. In highly supersaturated solutions, crystals start to nucleate and grow very quickly yielding corresponding high pressure in case of coming into contact with a barrier [4]. For example, a theoretical upper boundary of NaCl crystallization pressure can reach the yield strength of the crystal – above 200 MPa [5], [6]. However, in real porous building materials containing connected pores with different sizes it is not possible to reach an arbitrary degree of supersaturation [7], [8]. The ability of a crystal to push an obstacle away is fundamentally dependent on the existence of a thin film of a solution between both surfaces through which repulsive forces acting as a barrier prohibiting the true mechanical contact may be transferred [9]. The magnitude of pressure is also highly influenced by the physical-chemical properties of both surfaces close to direct contact [10], [11].

Under real conditions, concentration of ions in the salt solution increases with evaporation of water. Hence, the solution becomes supersaturated and the crystals can nucleate or continue to grow [2]. The rate of evaporation of the solution highly depends on the temperature and relative humidity (RH) conditions, i.e. the lower the RH, the higher the evaporation rate of the solution. The vulnerability of material to salt degradation and corresponding extent of damage are closely related to the materials properties themselves. Nevertheless, besides taking into account the geometric properties of a porous system, the extent of damage that develops in the building material is connected with the properties of the involved type of salt. The phase change between the anhydrous or lower hydrated phase to the high hydrated state of the salt type accompanied by a volume change could be considered as the key damaging factor [12]. Definitely, the most widely known example is sodium sulfate, which has two stable phases – anhydrous thenardite Na2SO4 and decahydrate mirabilite Na2SO4·10H2O. Transition from the anhydrous to hydrated state is accompanied by dissolution of thenardite crystals and creation of a highly supersaturated solution with regards to mirabilite, which starts to crystallize mostly on the surface of dissolving thenardite [13]. This fast transformation is considered in the standard durability test EN 12370 focused on testing building material resistivity against salt crystallization [14]. Unfortunately, the test results do not reflect damage developed under real conditions [15].

Hamilton et al. [16] used an atomic force microscope to measure the interaction force between the surface of a silica particle attached to a cantilever and arcanite (K2SO4) crystal surface. The disjoining pressure transferred through the thin film of a potassium sulfate solution calculated from the repulsive force between silica and arcanite was between 2.4 and 4 MPa. Sekine et al. [17] determined crystallization pressure magnitude by means of the photoelastic effect characterization at the sodium chloride crystal contacts with the polydimethylsiloxane (PDMS) channel wall. They calculated that the crystal developed a maximal pressure of 2 MPa, which was inhomogeneously distributed along the interface. This value represents probably the lower boundary for crystallization pressure, since it wasn’t possible to determine the true contact area, which is possible much more lower, between the crystal and PDMS wall properly. Contrary to the results mentioned above, Desarnaud et al. [11] assessed the crystallization pressure of sodium chloride equal to 150 ± 50 MPa in contact with hydrophilic glass attached to the rheometer.

The lack of studies aimed at the experimental measurement of disjoining pressure developed by growing crystals was one of the reasons for selection of this topic in order to contribute to better understanding of salt crystallization phenomena related problems. Although, real porous building materials contain almost always a mixture of salts whose behavior under the moisture and pore geometry influences is different from a single salt and creates a complex system [18], [19], [20]. The desalination of such a system and consolidation of the material is a quite complicated task and it is necessary to apply a suitable material with corresponding properties to protect the building against a higher content of moisture and accelerated deterioration [21], [22]. Nevertheless, to estimate and model the damage caused by salt action it is necessary to use – among others – proper parameters aimed at a single salt behavior. Therefore, this paper is aimed at the experimental methodology designed according to the Scherer's assumption [5] that a layer of the solution remains trapped between the crystal surface and pore wall. The salt solution evaporates gradually and becomes supersaturated providing a driving force for the subsequent growth of crystal planes against an obstacle, i.e. the pore wall.

Section snippets

Research aims

The study is aimed at the determination of disjoining pressure between growing crystal of single salts and the glass surface. The experimental study was performed using specially designated surface force apparatus. The research aims were the following:

  • to monitor the trend and magnitude of disjoining force acting at the glass–salt interface using inhouse developed device designed according to our novel concept;

  • to calculate the lower boundary of disjoining pressure of sodium chloride and sodium

Direct measurement of the disjoining force between crystal and glass surface

A novel device designed for the direct measurement of the force generated by salt crystal growing was developed (Fig. 1). The device is based on the principle of a glass lever with electrodes, whose position is driven by four electromagnets. The stationary base is composed of a larger glass with two inserted tangs at which the upper narrower glass is placed manually. A drop of the solution is placed carefully into the gap between the glasses and the force developed by electromagnets necessary

Contact area

Roughness of the confined surface of the NaCl crystal was determined along a 26.172 μm profile. The average roughness Ra of such a surface was equal to 0.647 nm and the root mean squared roughness Rq of the profile was equal to 0.761 nm. 3D profilometry data performed in Zygo on a different crystal plane were similar – Ra 0.67 nm and Rq 0.85 nm on a profile of 3.3 μm. Asperities on the surface area equal approximately to 16 μm2 and the 2D profile is shown in Fig. 3. Evidence for the existence of

Acknowledgement

The authors gratefully acknowledge support from the Czech Science Foundation (grant number P105/12/G059) and from the RVO: 68378297.

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