2014 - stefanidou et al - applying microscopic techniques for the investigation of the behaviour of...

Applying microscopic techniques for the investigation of the behaviour of building materials

M. Stefanidou1, L. Papadopoulou2, E. Pavlidou3 and I. Papayianni1

1 Laboratory of Building Materials, Civil Engineering Department, Aristotle University of Thessaloniki 54124, University campus, Greece

2 School of Geology, Aristotle University of Thessaloniki 54124, University campus, Greece 3 Department of Physics, Aristotle University of Thessaloniki 54124, University campus, Greece

The main challenge in construction field is to build functional, economical and durable structures. In order to accomplish this, an essential precondition is the thorough understanding of the properties of the materials used focusing on their microstructure. For this reason, classical microscopic techniques, like stereoscope, optical microscope and SEM remain the irreplaceable tools for the direct observation and study of the materials. By proper combination of microscopic techniques a deeper penetration into the structure at different scales, precious data as defects of the materials can become evident giving answers to their macroscopic behavior and allowing interpretations for improvements. The aim of this article is to show, after a review of the potential and limits of the microscopic tools, how these may be used to extract concrete remarks about the examined materials.

Keywords: building materials; stereoscope; optical microscope; SEM; AFM

1. Introduction

The term microscopy is referring to the techniques through which the materials can be viewed at an appropriate scale for examination. The image is formed by direct imaging for the optical or light microscopy and electronic processing of the wave nature for electron microscopy. Nowadays, the research fields incorporating microscopes were multiplied and these techniques have evolved rapidly, providing valuable and in many cases unique information. Microscopy could be considered as one non-destructive method of analysis, since a very small sample is required. The sample size and its representativeness, the disturbance of the sample during preparation stages as well as the necessary experience required for gaining and evaluating the results are still some significant drawbacks on the use of these techniques. Building materials despite their homogeneous macroscopic appearance present a rather heterogeneous structure. Moreover, they contain a net of pores and cracks by which their mechanical strength and durability are strongly influenced. A direct view of the pores and cracks in the material is only possible through microscopes which are the basic tools when designing modifications for upgrading the properties of a material. The influence of supplementary additions such as fibers, binary or ternary binding systems, different aggregates, admixtures, consolidates or surface treatments can be efficiently recorded. The monitoring of innovative products can be made by powerful microscopes in order to record any defects, understand their behavior or find the cause and extent of any failure. Microscopic analysis is always used for the study of materials when they do not perform as expected causing problems to the construction. The establishment of regulations in applying microscopic techniques for the examination of building materials contributes to the validation of microscopic analysis in particular in the multi-phase materials [1] [2] [3]. As Sarkar stated “a micro-structural investigation is the only way to identify defects in concrete and is therefore an essential part of any investigation” [4]. Many researchers recognize microscopic techniques as a basic tool for characterizing a material [5] [6] [7] [8] [9].

2. Review of basic microscopes

Stereoscope: This is the first step in order to study of a 3-D bulk material, even large in size, without any processing of the sample and gain the first information about its structure. Actually, the ability to magnify the image up to x100 is an easy and inexpensive way to distinguish the phases of the material and get an idea about its texture, pathology stage, cohesion between inclusions (such as fibers) and binder, the existence of secondary minerals (salts, re-crystallization phases) or distinguish multiple layers for example in the cases of mortars (substrates or renders) [10]. The type of stereoscope used in this study was a Leica Wild M10. Optical microscope: Optical microscopic methods are widely applied in material characterization since they allow the observation of the internal structure of a material. Microstructure features such as grains and grain sizes, precipitates, inclusions, pores and pore sizes, alteration products, textures, shapes and morphology of crystals or aggregates, cracking and interfacial reactions, twin boundaries can be recognized and quantified. Also, crystallization sequence, observe frozen-in reactions, note weathering or alteration, identification of phases and characterize the structure of a material can be determined. For the recognition of minerals and mineral phases, a polarized optical microscope, also called a petrographic microscope, is considered to be the basic tool. Two main types of polarizing optical microscopes

Microscopy: advances in scientific research and education (A. Méndez-Vilas, Ed.)__________________________________________________________________

© FORMATEX 20141064

can be used in material characterization: transmitted light and reflected light [11] [12]. A transmitted light polarizing microscope is used for the observation of transparent minerals while a reflected or incident light microscope is used for the observation of metallic and opaque minerals. Examination under the polarizing microscope can be made with parallel or crossed Nichols. In parallel Nichols only the polarizer is used and optical properties as color, relief, shape, form and cleavage of a crystal can be investigated. In crossed Nichols an analyzer is placed into the light path at right angle to the polarizer and optical properties such as interference colors and distinction can be examined. Examining a thin section (2D, small size material) with transmitted light permits further understanding of the size and the percent of voids and solid particles. The advantages of optical microscopy are the direct imaging and straightforward information. Certain features are better visualized at low magnification, for example directional deformation and twin boundaries, while porosity and phase fractions can be better quantified. The combination of both stereoscope and optical microscope with image analysis systems gives a privilege to quantify the gained information. The disadvantage of this method is the low resolution. Practically, a common optical microscope can distinguish objects that range from 1 to 100μm in size, at a magnification of about 1500X, due to the light diffraction limit. Another drawback can be sample preparation since thin sections are required from all samples studied under the polarizing microscope. In the present study polarized microscope was Leica laborlux 12 pol s, assisted by image analysis system Qwin using a digital camera (ProgRes c5). Scanning electron microscopy (SEM): It has been extensively used for the material characterization, especially in combination with energy dispersive X-ray microanalysis (SEM/EDS) [13]. The SEM technique requires electrically conductive specimens. Insulating specimens need to be covered by a thin (1–10 nm) conducting surface layer, usually applied by sputtering with gold in an argon atmosphere, or with carbon coating in vacuum. The sample size can be maximum 10 cm in diameter and 50 mm in height, but there is a restriction in the specimen’s movement, so the area of interest should be about 2.5 cm in diameter around the center of the goniometer stage. Restrictions also apply regarding the maximum height, tilt and rotation, depending on the specific stage. This particular microscope is very effective in morphology investigation, as easily produced micrographs provide topographical information together with the qualitative and quantitative analysis, evaluating the characteristics of the materials. Micrographs of powder samples may show weather the grains have irregular angular shape, if particle agglomerations composed of spherical aggregates, or smoother surfaces and their size can be measured. The point, either area microanalysis, shows the spectra of the elements composing the sample, with results presented in wt% and at%. SEM micrographs, highlights the diversity of microstructures, as the combination of aggregate with the binder in the case of concrete, or the bond characteristic of various fibrous materials, steel, fiberglass, and carbon-fiber in the reinforced cement. Micrographs of fiber surface, shows weather the fibers are covered densely implying the good bond with cement matrix. If the bond is loose, a space appears between fibers and in many cases they are pulled out forming a smooth surface. This characteristic indicates aging, which causes a possible de-bonding by the decrease of the compressive strength as the years go by. Because of long exposure, lime mortars develop failure in the mode of cracks, crumbling and erosion. By measuring, the area of voids from micrographs taken from historical buildings, qualitative and quantitative results of the damaged area and the materials used can be estimated. To study the ageing of various materials as they degrade with time, it is important to analyze the microstructure changes supplemented with measurement of the area of damages in the form of micro-voids. The micrographs reveal loose microstructures in damaged materials and uniform and dense microstructures in protected and durable materials. The high vacuum SEM observations require specific preparation methods such as removing of water and coating by sputtering of samples. Thus, the images in those cases represent a frozen state. The environmental scanning electron microscope (ESEM) is a direct descendant of the conventional SEM, but also permits wet and insulating samples to be imaged without prior specimen preparation the ESEM. For example it allows the observation of samples with different content of water phase [14] [15]. The SEM used for this study is a JSM 840 A, assisted by EDS analysis OXFORD INCA. Atomic Force Microscopy (AFM): The conventional use of AFM is the study of surface topography both visualize and quantify it. This information has been investigated in relation to other properties such as elastic properties, hydration degree and binder determination [16]. These possibilities as well as their combination with other techniques constituted AFM a popular tool in studying composite materials. This technique is advantageous in relation to other imaging techniques as a reliable 3D image of the sample surface at very high resolution is achieved and it doesn’t require any sample preparation or coating. Its main limitation is the tip convolution which affects tiny dimensions. The AFM used for this study is NT-MDT, Negra.

3. Detection of deterioration

Most building materials are porous and susceptible to deterioration. This process is usually slow and results from the effect of environmental conditions on materials. Different mechanisms can intriguer deterioration and their combination with the materials used can naturally lead to a certain macroscopic pattern. Microscopic techniques can assist on the study of the whole spectrum of deterioration and find the predominant mechanism [17] [18]. Starting from the macroscopic image and going deeper up to the crystal unit means that a researcher can see all the path of the procedure. The cause, the type and the depth of deterioration is possible to be determined by microstructure investigation [19].


1065© FORMATEX 2014

Determining the cause of deterioration is important in order to understand mistakes during designing or applying the materials and by this way it is important to improve the materials’ properties and also it is crucial in order to organize the appropriate repair procedure. For the heterogeneous materials such as concrete and mortars with features ranging from nano to centimeters, microscopy can give information concerning the size, the distribution and the topology in detail [20] [21]. Figure 1 demonstrates the presence of salts as recorded under different microscopes for mortars where salts are in abundance and degradation phenomena were macroscopically observed.

Fig. 1 Mortar from the Medieval walls in Rhodes a)under stereoscope (x8) where pore within crystallized salts is observed, b)under polarized microscope (x150) the presence of angular crystal salts are shown and c)under SEM the morphology and composition of salts can be recorded.

Figure 2 indicates the presence of salts (ettringite) in concrete sample causing a strength reduction as recorded from 25MPa to a healthy sample to 10.5MPa in the sample with salt presence.

element Weight %

O 44.92

Na 0.35

Mg 0.62

Al 6.02

Si 10.97

S 4.80

K 0.54

Ca 31.31

Fe 0.46

Fig. 2 Ettringite fibres inside a concrete structure and the analysis of the fibres by EDS.

Basic minerals in the composition of building materials are often altered causing further degradation problems. For example orthoclase and plagioclase are two minerals of the feldspar group. They both have a hardness of 6, on the Mohs scale. Under chemical weathering, orthoclase is altered to kaolinite, a clay mineral, with a hardness of 2-2.5, while plagioclase is often hydrothermally altered to sericite, a fine-grained mica, with an equal hardness (Figures 3 and 4). As a result, the decrease of mechanical resistance is inevitable. Furthermore, kaolinite can be dissolved from the material causing secondary porosity formations.

a b c



Fig. 3 Alteration of orthoclase to kaolin (parallel Nichols)

Fig. 4 Alteration of plagioclase to sericite (parallel Nichols left and crossed on the right

4. Characterization of porosity and crack pattern

Porosity is a basic characteristic of materials influencing most of their properties such as mechanical strength, deformation, resistance to weathering, insulating properties. There are many, widely applied, indirect techniques for porosity measurement. The major advantage that the microscopic techniques offer is that pore structure has an image and as the observation is direct, the exact size and position of the pore can be identified. In this way pores and cracks occurring inside the matrix or in the transition zones can be distinguished. Microscopic techniques are the only methods that can relate porosity with pathology [22] [23]. Additionally, a wide range of pore sizes can be characterized. Cracks in the materials microstructure pre-exist or are formed during the service life of materials. Their topography recorded by microscopy of different magnification capacity, allows concluding about the main mechanism responsible for them [24] [25]. For example, cracks in the cement matrix of concrete may be related to sulfate attack and ettringite or thaumasite is easily detected from the characteristic crystals. Similarly, cracks originated from aggregate grains may be attributed to alkali-aggregate reaction and the products of this reaction can also be microscopically identified. Cracks in lime-based mortars are mainly localized in the transition zone around aggregates and they are often filled with re-crystallized calcite which contributes to the increase in density and strength of mortars (Figures 5, 6).

Fig. 5 Pore size distribution in mortar using stereoscopic observation and image analysis system

a b



Fig. 6 Re-crystallization of calcite in the contact zone of binder-aggregate filling the crack

5. Detection of interfaces

In composite materials many interfaces exist which are considered as the weak phases of the materials and special care is given to reinforce them. Good compaction, proper additives are often used to avoid weak zones and increase long time service life of materials [26] [27]. The microscopic investigation of the interfaces provides information about the techniques used for building with these materials. Indicatively it can be mentioned that in high quality old mortars taken from monuments, the interfaces between brick-mortars or bedding mortar-renders or even mortar-mortar are quite compact (Figures 7,8,9). In many cases the microscopic examination showed chemical reaction products between the bonding materials (Figure 9). In the case of a roller compacted concrete pavement the cold joint due to premature setting and inadequate compaction of concrete implies for a low performance pavement.

Fig. 7 Interface zones under stereoscope x8 a) stone-mortar b)mortar –render c)brick-mortar

Fig. 8 Interface zones under polarized microscope x150 a)brick-mortar b)mortar-render and c)two mortar layers

a b c

a b c



Fig. 9 SEM micrograph of contact zone of 30μm thickness between brick fragment and binder

6. Study of surface characteristics

Building materials are exposed to environment and the quality of their surface used as the skin of a structure is of great importance for their long service life. Density, roughness porosity, permeability, hydrophobicity are some of them. Since the science of films has been impressively developed the use of AFM in testing surface characteristics has been extended [28] [29]. Recording the effectiveness of a hydrophobic treatment on stone (marble) surfaces using nano-modified coatings, AFM measurements indicate different succeeded roughness justifying different behavior (Fig 10).

Fig. 10 AFM images demonstrating the acute nano-roughness accomplished in nano-modified coating (b) in relation to untreated one (a)

7. Conclusions

Microscopes are valuable tools to the researchers in order to detect the damages, identify phases and develop improved materials used in the construction field. The ability of penetrating and having a direct observation of the structure in the scale of micro or nano allows the detection of the inherent defects as well as ageing or deterioration effects which determine the behavior of the material. The study of building materials has benefited greatly from the microscopy. The evolution and specialization of building materials in order to cover different construction needs was in a significant extent based on them.

References

[1] ASTM C856 - 13 Standard Practice for Petrographic Examination of Hardened Concrete [2] ASTM C1324 Examination and Analysis of Hardened Masonry Mortar [3] ASTM C1721 - 09 Standard Guide for Petrographic Examination of Dimension Stone [4] Sarkar S. H. The importance of microstructure in evaluating concrete Advances in concrete technology ed. V.M. Malhotra,

CANMET Ottawa, Canada, MSL 94-1, 1994 123-202

a b



[5] Middendorf B., Hughes J. J., Callebaut K., Baronio G. and Papayianni I. Investigative methods for the characterization of historic mortars- Part 1: Mineralogical characterization. Materials and Structures, 38, 2005, RILEM Technical Committees, 761-769.

[6] Scrivener K.L. Microscopy methods in cement and concrete science Cement Research and Development 1997, p.92-112 [7] Lindqvist J. E., Sandstrom M. Quantitative analysis of historical mortars using optical microscopy. Materials and Structures,

vol.23 Number 10, 2000 RILEM Technical [8] Elsen J. Microscopy of historic mortars – a review. Cement and Concrete Research 36, 2006, 1416-1424 [9] Charola A.E. Salts in the Deterioration of porous media: An overview JAIC 39, 2000, 327-343 [10] Papayianni I., Stefanidou M. “Microstructural analysis of Old Mortars of Byzantine Period’’ International Conference

STREMAH, Bologna, Italy, Editor C.A. Brebbia, 2001, pp. 45-52 [11] Davidson M.W and Abramowitz M. Optical microscopy. ed. John Wiley & Sons, Published Online: 15 JAN 2002, DOI:

10.1002/0471443395.img074 [12] Yang Leng. Introduction to Microscopic and Spectroscopic Methods Materials Characterization ed. John Wiley & Sons, 2008,

30-34 [13] Pavlidou E., in Conservation science for the cultural heritage, Springer, 2013, Ch 3.2. [14] Donald A.M. The use of environmental scanning electron microscopy for imaging wet and insulating materials Nature

Materials 2, 2003, p.511 - 516 [15] García Santosa A., J.Ma. Rincónb, M. Romerob, R. Taleroc Characterization of a polypropylene fibered cement composite

using ESEM, FESEM and mechanical testing Construction and Building Materials, 19,2005, 5, 396–403 [16] Peleda A., J. Castro, W.J. Weiss Atomic force and lateral force microscopy (AFM and LFM) examinations of cement and

cement hydration products Cement & Concrete Composites 36 (2013) 48–55 [17] Sarkar S.L., S. Chandra, M. Rodhe Microstructural investigation of natural deterioration of building materials in Gothenburg,

Sweden’’ Materials and Structures 1992, vol. 25 p.p 429-435 [18] Riccardi M, Lezzerini M, Caro F, Franzini M, Messoga B (2007) Microtextural and microchemical studies of hydraulic ancient

mortars: Two analytical approaches to understand pre-industrial technology processes. Journal of Cultural Heritage 8: p.350-360

[19] Grattan-Bellew P.E. Microstructural investigation of deteriorated Portland cement concretes Construction and Building Materials 1996, Vol.10, No1, 3-16

[20] Larbi J.A. Microscopy applied to the diagnosis of the deterioration of brick masonry Construction and Building Materials 18 (2004) 299–307

[21] Larbi JA., Visser JHM Diagnosis of chemical attack of concrete structures: the role of microscopy Proc. 7th Euroseminar on Microscopy applied to Building Materials Delft The Netherlands 1999, 55-65

[22] Fitzner B. Porosity analysis- A method for the characterization of building stones in different weathering stages Engineering Geology of Ancient Works Monuments and Historical Sites, 1990, p.2031-2037

[23] Stefanidou M. “Methods for measuring porosity of lime-based mortars” Construction and Building Materials 24, 2010, 2572-2578

[24] Robinson G.C. 1984 The relationship between pore structure and durability of brick Ceramic Bulletin vol.63 No2 295-300 [25] Daniel Q. et.al. ‘’Microstructure and transport properties of porous building materials’’ Materials and Structures Vol.31 Issue

209 June 1998 pp.317-324 [26] Lawrence S.J., Cao H.T. 1987 Microstructure of the Interface between brick and Mortar Proc. 8th International Brick/Block

masonry conference Dublin Ed. J.W. DeCourcy vol.1 194-204 [27] Barnes B.D. Diamond S. Dolch W.L. Micromorphology of the interfacial zone around aggregates in Portland cement mortar

The American Ceramic Society 62 1979, 21-24 [28] Manoudis P.N., A. Tsakalof, I. Karapanagiotis, I. Zuburtikudis, C. Panayiotou Fabrication of super-hydrophobic surfaces for

enhanced stone protection Surface & Coatings Technology 203, 2009, 1322–1328 [29] Zielecka M., Bujnowska E., Silicone-containing polymer matrices as protective coatings: Properties and applications Progress

in Organic Coatings, Volume 55, Issue 2, 2006, p. 160–167



2014 - stefanidou et al - applying microscopic techniques for the investigation of the behaviour of...

Documents