Travaux Pratiques de Matériaux de Construction

Transcription

1 Travaux Pratiques de Matériaux de Construction Section Matériaux 6 ème semestre 2009 Etude de Matériaux Cimentaire Par Diffraction des Rayons X Responsable: Silke Ruffing silke.ruffing@epfl.ch Tél.: Bureaux: MXG 241 A remettre avant le 19 mai 2009 Avec Date, groupe et vos noms (avec vos s adresses)

2 Introduction Powder X-ray diffraction (XRD) is an important tool for identification and characterisation of cementitious materials. This lecture will show how to prepare fresh and hydrated cement powder for XRD techniques. Furthermore it will give an introduction how to work with analysis programs to determine the phases contained in cement both qualitatively and quantitatively. Measurement Principals Interaction between X-ray and materials (its electrons) is based on different processes. The most important one is coherent scattering which is the precedent condition for X-ray diffraction. The X-ray oscillates electrons and interference occurs. Reinforcement of wave front does occur when the geometry of the experiment is fulfilling so called Bragg reflexion only (see Bragg law). The Bragg law As crystals consists of 3-dimensional and periodic arranged structural units constructive interference takes place only for defined angles. By using the Bragg law these angles are related to distance of specified lattice plane. The Bragg law is described with the mathematical equation: nλ = 2d sinθ n: integer determined by the order given λ: wavelength of X-ray (monochromatic) d: normal to planes θ: Angle between incident beam and scattered planes Interference length is exactly the path difference between two X-rays diffracted on two lattice planes with distance d. This path difference is a whole-number multiple of wavelength in case of constructive interference. It seems macroscopic that X-ray is reflected on sample surface in an angle of 2θ (see Figure 1). This refection does appear for certain angles of lattice plane passels only. These angles are called Bragg angle. Fig. 1: Bragg Law 1

3 Diffraction experiments can be performed on small single crystals and fine homogeneous powder samples showing anisotropic distribution of crystal orientation. Diffraction experiments on powder and mostly single crystal samples employ monochromatic X-rays produced by filters or monochromators. Hence, the major part of the produced X-ray spectrum is blended out and one single line is used for diffraction. Moreover preparation influences quality of diffractogram. So, the grain size should be less than 32 µm for raw cement powder. Furthermore the sample should be prepared with a plain surface to suppress texture effects that affect the position of peak intensities (peak reflexes). Numerous measurements need a special geometric arrangement which is performed with a Bragg-Brentano diffractometer. Bragg-Brentano powder diffractometer Cements are usually investigated with the powder diffraction arrangement of Bragg-Brentano parafocussing geometry. In this geometry the incident beam from the line focus of the X-ray tube diverges in the diffraction plane until it irradiates the sample. The diffracted X-ray beam converges from the sample until it passes through the receiving slit (the natural focussing point on the goniometer circle) before being accepted by the detector. Various configurations are in use. In θ-θ system the sample stage is fixed horizontal whereas the X-ray tube and the detector are moving symmetrically (see Figure 2). If the angle between surface normal of sample and incident rather diffracted beam is θ, the angle between diffracted beam and projection of X-ray beam includes an angle of 2θ. Figure 2: Measurement Configuration The results of XRD measurements are presented in diagrams showing intensity of reflected beam in counts per seconds as a function of reflection angle in 2 /2θ. Since any crystalline phase does have a characteristic set of distances of reflecting lattice planes crystalline phases do show a characteristic set of peak positions in diffractograms. Concentration of phases is indicated by the peak heights, the amorphous content by background hump and peak widths are depending on size and strain of the crystallites. 2

4 Number and position of peaks do emerge from crystallographic properties of phase constituent in sample. To describe their parameters the notation of Miller is used. Miller Indices So called Miller Indices are a vector representation of the orientation of an atomic plane in a crystal lattice. They are defined as the reciprocals of fractional intercepts of the plane with crystallographic axes. Miller Indices are represented by a set of the 3 integer numbers h, k and l. To distinguish these indices from other crystallographic parameters they are written with parenthesis as (h k l). Figure 3 shows the clarification. Figure 3: 4 Examples for Miller Indices Rietveld refinement Application area for powder diffraction is quite large. Besides qualitative information investigations of powders can deliver parameters of lattice and status of crystallinity. Complete determination of crystal structure is quite rare. However, since different programs for structure refinement have been established crystal structure of fine powders is solved more often. Structure refinements are called Rietveld refinements named for one of the pioneers in this field. Base of Rietveld refinement is a full matrix least squares method to compare observed and calculated diffraction profile. During the refinement the difference between both should be minimised. Sometimes it is also called Profile method. In practice the Rietveld refinement needs an accurate powder diffraction intensity data measured in intervals of 2θ a starting model that is reasonably close to the real structure of the investigated material a model accurately describing shapes, widths and any systematic errors in positions of the Bragg peaks in diffractogram 3

5 Workflow of the lecture Introduction Sample preparation X-ray analysis with X`Pert Pro System in Bragg-Brentano Geometry Qualitative Analysis with X`Pert High Score Plus Quantitative Phase Analysis with the Rietveld technique (included in X`Pert High Score Plus) Discussion Samples Slag Fly Ash Cement Hydrated Cement Notes Please see also of Laboratoire de Cristallographie for more information to XRD Literature Wikipedia D.L. Bish & J.E. Post (ed.), Modern Powder Diffraction, Reviews in Mineralogy 20, Mineralogical Society of America, 1989 H. Krischner, Einführung in die Röntgenfeinstrukturanalyse, Vieweg Braunschweig/Wiesbaden,

6 Questions to lecture Etude de Matériaux Cimentaire par Diffraction des Rayons X (if it is more confortable you can do it in French) 1. Entitle the technique of sample preparation for powder-xrd. 2. What are the different steps for qualitative Analysis (catchwords)? 3. Which phases have you found in your qualitative analysis? 4. Describe the principal of Rietveld refinement? 5. What is the quantitative phase content of your found phases? 6. The 4 main clinker phases reacts with water forming hydration products. a) Inscribe their 4 reactions. b) One of the main clinker phases reacts with water fast and forming huge heat release. Which one? What can be added to control its reactivity? c) Which hydration products are difficult to detect with X-ray? Why? 7. As a result of quantitative XRD the hydration degree can be determined. It indicates the advancement of hydration. As a good indicator to control the degree of hydration the amount of Portlandit has to be determined. Which additional technique does allow determining the CH content? 8. Which further techniques based on X-ray do you know? 9. How can you correlate results of those techniques with such of Rietveld? 10. Please outline the hand out journal paper. (5-10 sentences) 5