Investigation on Building Materials with the SEM in the ESEM Mode to Demonstrate Their Capillarity Using the Contact Angle Method

Abstract

The chapter describes the use of the Scanning Electron Microscope (SEM) in the Environmental Scanning Electron Microscope (ESEM) mode on building materials, whose capillarity is to be examined. The abbreviation SEM means Scanning Electron Microscope. The abbreviation ESEM means Environmental Scanning Electron Microscope. On the basis of condensation in the ESEM, the hydrophobicity of capillary building materials is demonstrated with the help of the contact angle method. In the chapter, the investigation in the ESEM is shown using capillary building materials that have been given subsequent injections. Due to the problem of rising masonry moisture on capillary masonry in the absence of a cross-section sealing, injection agents, which have a hydrophobic and pore-filling effect, subsequently are used in the borehole method. Such a subsequent masonry sealing must be checked for effectiveness. In addition to already existing macroscopic methods, a new microscopic detection method is presented. This detection method uses ESEM technology in the SEM to generate and detect in situ dew processes at samples taken from the injection level of the examined masonry. The output of the results is done by image or film. By means of the condensation with the medium of water, the contact angle measurement method on the dew drops can be used to make accurate statements about the water-repellent capabilities of the examined sample and thus about the sealing success. There are detectable correlations to the macroscopic detection methods. The contact angles measured in the ESEM during condensation are connected to the conventional macroscopic measurement methods. The method presented in this chapter offers the advantage to have very small samples and to be investigated in a short time with very precise results. The new detection method is suitable for practical use.

1. Introduction

In the refurbishment of old buildings, especially in the area of monuments [1, 2], capillary building materials, for example, consisting of brick masonry are found very often. Since the building materials of old buildings are often capillary-active materials [3, 4, 5], there is distinctive water absorption and release as well as water transport behaviour on this material. This process is called capillarity. Investigating the capillarity of old building materials can be of considerable relevance. The water absorption and release behaviour is, for example, crucially important for moisture transport and the moisture penetration of components.

The possibilities that result from examining the capillarity of a material in the Environmental Scanning Electron Microscope (ESEM) are explained in this abstract, using a subsequent sealing with injection agents on capillary masonry.

In many cases when renovating old buildings [6], sealing has to be carried out later because no sealing was installed when the object was built or the sealing is no longer adequately functional now. The proof of capillarity and the description of the moisture behaviour of the building materials play a major role here.

A good example is the construction of a subsequent cross-section sealing with injection agents. Although already known from antiquity, the regular use of functioning building waterproofing at capillary building materials began around 1890. Nevertheless, there were no uniform rules for the execution of structural waterproofing at that time. Only in the 1930s, structural waterproofing was normatively regulated. Although the cross-section sealing in massive walls had already a higher priority than other seals on buildings at the end of the nineteenth century, cross-section seals were regularly installed in masonry walls since 1930 onwards. In the old building area, there are very often buildings to find that have a cellar, even if the space requirements made this cellar unnecessary. This is related to the previous construction use, in which the basement was due to lack of sealing technology while permanently moist, but served as a ‘buffer’ to the upper floors, which thereby could be kept sufficiently dry. In this way, the buildings were built without a cross-sectional sealing. For these reasons, solid brick walls in cellars are often encountered in old building renovation and monument preservation, in which there is rising masonry moisture due to non-existent cross-sectional sealing. However, due to usage or conversion requirements of a value retention, there is a great need to permanently seal capillary masonry walls against increasing moisture in the wall cross section in the refurbishment in the renovation of historical monuments. A main group in the retrofitting of masonry cross-section seals is the masonry injection methods, which do not require static interventions [7, 8].

There are currently about 150 different injection agents [7] available for the subsequent cross-sectional sealing of capillary masonry. All injection agents have in common that they are applied by the production of borehole chains in the masonry. The injection agents react chemically. The sealant layer in the masonry is physically formed. There are pressurized and non-pressurized processes to apply the injection medium into the masonry.

However, all injection agents work in the same way: they change the capillarity of the building material and thus also the water transport properties of the material [9]. Therefore, the capillarity of a capillary-active substance that is changed by the injection agent can be used as a reference for the investigations in the Scanning Electron Microscope (SEM) in the ESEM mode. The investigations with the SEM in the ESEM mode can provide information about the efficiency of such a subsequent sealing by means of injection. In order to be able to monitor the injection procedure and to be able to demonstrate the effectiveness of the injection medium, it is therefore necessary to monitor the quality and quantity of the actually changed capillarity on the object [10].

The detection method presented here is based on the contact angle method in the Environmental Scanning Electron Microscope, ESEM. The ESEM is a modified version of a Scanning Electron Microscope, SEM [11, 12]. In contrast to the SEM, the ESEM can be used in low-vacuum mode. This circumstance allows the supply of a medium (here water steam) during the investigation. A cooling table in the chamber allows the sample to be refrigerated while the air in the chamber is at 100% relative humidity. Changing the chamber pressure causes condensation in the ESEM chamber [13]. During the investigation in the ESEM, condensation water droplets are formed on the sample. The contact angles can be determined on the formed drops of water [14]. When measuring the contact angle, one makes use of the interfacial tension of the water. Both static contact angles and dynamic contact angles can be measured in the ESEM. The contact angles provide information about the changed capillarity of the sample material on which the drops were formed. The contact angle can be measured directly in the ESEM or afterwards. The data obtained from this show a geometrically differentiated picture of the changed capillarity of the examined material. The method can provide information about the quality of the injection as well as about the geometric penetration with the injection agent.

2. The SEM in the ESEM mode

The Environmental Scanning Electron Microscope (ESEM) is a special variant of the Scanning Electron Microscope (SEM). The main difference to a conventional Scanning Electron Microscope lies in the lower vacuum in the measuring chamber [15]. Moreover, a special detector is installed for the operation of the ESEM. Due to the lower vacuum (low vac mode), a medium can be supplied to the chamber in ESEM mode. For the examination of building materials, the supplied medium is usually water/water vapour. The gas pressure in the chamber of the ESEM is usually 130–1.300 pascals. In the same way as when using the SEM, the sample is scanned by a focused electron beam in the ESEM. The signal resulting from an interaction with the sample is picked up by the detector and used to generate the image. The ESEM uses the generation of low-energy secondary electrons (0–50 eV), which are emitted from the sample surface as slow electrons. For signal amplification, the ESEM uses the gas in the sample chamber, which generates an amplification cascade through ionization. This system also neutralizes charges on the samples. The most important difference between the ESEM and operation in high vacuum (= SEM) is that in low vacuum the water is not ‘expelled’ from the sample and condensation processes (droplet formation) and can thus be made visible. For this purpose, the detector of the ESEM is not sensitive to light or temperature. In order to be able to visualize the water wetting and drying processes in the ESEM, different aggregate states of the medium, here water, which are pressure- and temperature-dependent, are used.

For this purpose, a cooling table connected to a recirculating cooler is arranged in the chamber of the ESEM. The sample is glued to this cooling table with carbon or conductive silver to ensure optimal temperature conductivity. As part of the investigations in the ESEM, the temperature conditions of the cooling table are fixed while the chamber pressure is changed. This causes a change in the state of aggregation of the medium in the chamber (from gaseous to liquid). If the dew point is reached on the sample, the water condenses out on the sample surface. This process is recorded with the help of pictures. In the ESEM, the forming contact angle of a drop can be measured in situ using the contact angle measurement method. Progressive, receding or static contact angles can be measured on ripe droplets. If a drying process is to be shown, the condensation water that has formed can be evaporated by reducing the chamber pressure, and the drying process is made visible. Technically, the ESEM is very well suited to show dynamic condensation in situ [16, 17, 18, 19, 20] (Figure 1).

Figure 1.

Phase diagram of water, P. Körber.

The figure above shows the limit curves of the three phases: gaseous (water vapour), liquid (water) and solid (water-ice). These phase areas meet at the triple point. At this point, the three phases are in thermodynamic equilibrium. The formation of condensation is related to the dependence of the state of aggregation of the phases on temperature and pressure.

Due to the change in the pressure conditions in the ESEM, the dew point inevitably occurs during the investigation in the ESEM, and the water that is gaseous in the medium becomes liquid in the form of droplets on the sample surface (Figures 2 and 3).

Figure 2.

Cooling table with circulation cooler, P. Körber.

Figure 3.

ESEM chamber: Cooling table with circulation cooler, P. Körber.

3. Contact angle method

The contact angle θ is the angle between the liquid surface and the outline of the contact surface at an interface between a liquid and a solid. The external stress of a liquid is defined by the imbalance of molecules within the liquid and at the liquid boundary (interface between liquid and gas). This intermolecular force that contacts the surface is called surface tension. A drop is formed due to the surface tension of a liquid. In addition, external influences such as gravity play a role shaping the drop. The contact angle of a drop occurs at the contact surface of the drop on a solid and also depends on the shape of the drop. The contact angle can provide information about the wettability of a solid with a liquid. The contact angle of a drop of water placed on a component surface can be measured macroscopically or microscopically. Water is well suited for carrying out a contact angle measurement, as it is characterized by a relatively high surface tension (=0.072 n/m). The principle of the contact angle measurement is illustrated in the Figure 4. The static contact angle is measured by applying a tangent to the point where the water droplet touches the solid surface and the ambient phase (here it is air). The contact angle decreases with increasing wettability of the solid (building material surface). The contact angle θ is defined as an angle at the phase boundary of the gaseous, liquid and solid phases of liquids on a solid surface surrounded by gas [21, 22]. This relationship was already defined in 1805 by Thomas Young.

Figure 4.

Contact angle measurement with the tangent method, P. Körber.

Eq. (1): Interfacial tension between solid and gaseous:

$${\gamma }_{SG}={\gamma }_{SL}+{\gamma }_{LG}cos\theta$$E1

In Young’s equation, the solid-gas interfacial tension is calculated by adding the solid-liquid interfacial tension to the liquid-gas interfacial tension and multiplying it by the contact angle. The equation below is used to calculate the Young’s contact angle.

Eq. (2): Young’s equation for calculating the contact angle:

$$cos\theta =\frac{{\sigma }_S-{\sigma }_{LS}}{{\sigma }_L}$$E2

Surface and interfacial tension defines the ‘work’ required to increase the interface area. Within the liquid, the molecules interact in all directions (cohesion), while at the interface there is no interaction of the liquid molecules with the outside (adhesion). Young’s equation describes the balance of these forces. This is viewed at the three-phase contact line and exists when the contact line is balanced and at rest. Then the horizontal forces acting on the contact line exactly cancel each other out. The interfacial tension is temperature-dependent, so the contact angles also depend on the temperature and, for most substances, decrease with increasing temperature. However, because there are other forces acting on the contact line in addition to surface tension, the Young’s contact angle cannot be measured per se. If there are movements of the contact line, one speaks of ‘dynamic contact angles’. When the drop volume increases, one speaks of ‘advancing contact angles’, while when liquids evaporate, one speaks of ‘receding contact angles’. In this context, it can be assumed that the advancing contact angle is always greater than the receding contact angle. The difference between these two contact angles is called ‘contact angle hysteresis’.

The hydrophilic or hydrophobic properties of substances can be precisely determined using the ‘Drop Shape Analysis System’. In this method, a droplet illuminated from behind is observed with a camera and displayed on a monitor. With this method a static contact angle is measured by assuming, for the sake of simplicity, that static conditions are present for the contact angle measurement. In fact, this is not the case, because contact angles determined in this way are also subject to certain, very small, changes during the measurement. However, this inaccuracy is included in the tolerance to be estimated and can therefore remain irrelevant for the purposes considered here.

The static contact angle can be measured using the tangent method, as shown above. The results of the contact angle measurements on building materials are differentiated using the 90° limit in A) hydrophobic > 90° and B) hydrophilic < 90°. In addition to this 90° angle definition, the angle measurements also provide information about the gradual water absorption capacity of the substance being examined. If a dynamic condensation/evaporation process is present, static contact angles can only be measured when the dynamic equilibrium between condensation and evaporation is reached. The contact angle method described here for determining the capillarity of a building material can also be applied macroscopically. In the present case, however, this method is applied microscopically in the ESEM. A reliable optical method for drop shape measurement can be carried out on the drops measured in the ESEM: The Drop Shape Analysis (DSA).

In the Drop Shape Analysis (DSA), images are taken of the droplets that are formed and then examined by using computer software. The contact angle is determined by the use of an image. The software can sharpen the captured image and recognize the contour of the drop. The measurement is computer-aided utilizing a geometric model. In the next step, the surface tension can be calculated applying the ‘Young-Laplace-Fit’ if the density difference and thus the imaging scale between the droplet phase and the surrounding phase are known. Accuracies of ±0.2° can be achieved here.

The optically measured drop contour can be calculated using a conic section equation =>Conic Section Method. The conic section method is based on the assumption that the contour of the drop to be measured describes an arc of an ellipse.

4. Hydrophilic or hydrophobic

The term ‘hydrophobe’ (hydrophobic) is borrowed from ancient Greek (hẏdor = water and phob = repellent). Hydrophobic describes a water-repellent property of a substance. This means that the substance does not dissolve in water and cannot be wetted by water. The term ‘hydrophil’ (hydrophilic) is also borrowed from the ancient Greek (hẏdor = water and phílos = loving). Hydrophilic means that a substance is water-friendly, water-loving. Hydrophilic describes a water-accepting property of a substance. This means that the substance can be dissolved in water and wetted with water (Figures 5–7).

Figure 5.

Contact angle measurement: superhydrophobic, P. Körber.

Figure 6.

Contact angle measurement: hydrophobic, P. Körber.

Figure 7.

Contact angle measurement: hydrophilic, P. Körber.

5. Investigations in the ESEM on building materials

The examination method described here for building materials in the ESEM can be used in particular for the question in the hydrophobicity of the substance to be examined. During the investigation in the ESEM, condensation processes are carried out, which provide information on how water-absorbent the examined substance is [23].

The procedure for examining building materials in the ESEM is explained below on the basis of a subsequent waterproofing of capillary building materials using injection agents. Such an injection method is used in particular in historic buildings made of solid building materials and in renovations [9].

Investigations in the ESEM are particularly useful when the question arises to what extent a building material is capillary-active or to what extent the capillarity of the building material has changed. Such changes are conceivable, for example, through the use of injection agents in building waterproofing. In the case of injection agent seals, for example, on brick masonry, the penetration of the injection agent changes the capillarity of the building material in such a way that the transfer of water in the building material is impeded or prevented. In this way, it is possible to subsequently create a cross-section seal in masonry walls. For such a procedure, it is necessary to prove the changed capillarity of the building material. This proof documents the sealing success. In this respect, the condensation in the ESEM is predestined to provide evidence for the use of injection agents for the subsequent sealing. The advantage of the examination method in the ESEM is that very small sample quantities can be evaluated in a very short time. The ESEM investigations on building materials are therefore qualitative, microscopic (imaging) detection methods. In addition to these microscopic methods, there are also macroscopic detection methods to investigate the changed capillarity in building materials [24, 25, 26, 27, 28, 29].

The qualitative microscopic (imaging) detection method in the ESEM as described here can also be used by verifying the macroscopic, also qualitative, detection methods. It can provide comprehensive information about the changes in the capillary building material under the influence of the injection agent used.

Furthermore, the extent to which an injection material was used at all and to which the building material now exhibits hydrophobic properties after use can be verified. With the exact measurements, the hydrophobicity of the building material can be gradually verified. A comparative examination of the building material before and after injection is possible. With the qualitative microscopic (imaging) detection method, as with the macroscopic investigations, local samples are taken from the injection level using mini core drillings and are analysed in the laboratory in the ESEM.

Essentially, significantly less material has to be removed from the structure for the verification method described here.

To examine the building material in the ESEM, samples are to be taken from the injection level. In addition, reference samples of the masonry without adding the injection agent are required.

The reference samples and the samples from the injection level are subjected to condensation in the ESEM. In the ESEM mode, condensation on a microscopic scale can be brought about within the pores of the building material (bricks and mortar). The condensation process is recorded using pictures and, if necessary, using film. During the condensation process, there is a time window in which the contact angle of the forming water droplets can be measured. The measurement of the contact angle can be carried out directly in the ESEM.

In addition, the measurement can also be carried out retrospectively on the images generated in the ESEM. The measured contact angles can be used to determine whether the substance is hydrophobic or hydrophilic. The examination results consist of an imaging procedure that can be evaluated afterwards. In particular, comparisons with the reference samples are possible.

The qualitative microscopic, imaging detection method described here can be flanked and verified by macroscopic examinations. The macroscopic investigations can serve as a calibration function for the ESEM measurements. In this way, serial tests can be carried out, in which differentiated proof of the sealing success of the subsequent building sealing with injection agents can be provided [29].

In Figure 8a–f, shown below, it becomes clear how the condensation water droplets form in the ESEM. After the time window for the formation of the drops has expired, these attract each other and then merge into one another, so that the time window for the measurement is over. The maximum achievable contact angle is relevant for the measurement.

Figure 8.

Condensation process on a brick sample in the ESEM, P. Körber.

The following figures show examples of the formation of condensation water droplets in the ESEM on brick and mortar samples (Figures 9–11). Figure 12 shows the contact angle measurement, which takes place directly in the ESEM.

Figure 9.

Condensation process on a brick sample in the ESEM, P. Körber.

Figure 10.

Condensation process on a mortar sample in the ESEM, P. Körber.

Figure 11.

Condensation process on a brick sample in the ESEM, P. Körber.

Figure 12.

Measurement of the contact angle in the ESEM, P. Körber.