Experimental program
3. MICROSTRUCTURAL CHARACTERIZATION OF AGGREGATES AND CONCRETE
3.1. X-RAY POWDER DIFFRACTION
X-ray powder diffraction (XRD) is a non-destructive technique generally employed for phase identification of polycrystalline samples. Currently, it is one of the most applied methods for mineralogical material characterization.
Diffraction methods are based on the phenomenon of wave interferences, and the prerequisite that X-ray wavelengths are of the same order of magnitude as interplanar spacing in crystal lattices (Klockenkämper and Bohlen, 2014). Monochromatic X-ray beams incident on a crystalline solid will be diffracted according to Bragg’s law (3.5) as illustrated in Figure 3.9.
nr∙λx=2∙ds∙sin θ (3.5)
with nr a positive integer, also called order of reflection, which is conventionally established as the unity (Fultz and Howe, 2013) [-], λx the wavelength of the incident X-ray beam [nm], ds the interplanar spacing in the ionic lattice [nm], and θ the angle of incidence of the X-ray on the crystal [º].
By continuously changing the incident angle of the X-ray beam, information on the spacing between atomic planes is gathered and a spectrum of diffraction is recorded. Various intensity peaks located at different angles 2θ provide the “fingerprint” for a crystalline solid which enables the mineral phase identification among known patterns from a standard sample or from a calculation (Leng, 2013a).
The analysis was performed on the aggregates and conducted through the Debye–Scherrer method, as is the most appropriate for polycrystalline samples. Sample preparation consisted of a particle size reduction to grain sizes smaller than 88 µm by means of an agate mortar. Then, the homogeneous powder was placed on an aluminium sample holder and pressed at 0.60 MPa in order to ensure a flat surface.
79 Figure 3.9: Schematic explanation of Bragg’s Law (Callister, 2007)
XRD tests were performed on a Bruker D8 Advanced (Theta/Theta) non-monochromator diffractometer (Figure 3.10). The operating parameters used for X-ray generation were tungsten cathode, 2.2 kW copper anode (CuKα1: 1.5406 Å; CuKα2: 1.5444 Å), 30 mA intensity, 40 kV voltage and a 6 mm slit. Radiation data were gathered over the angular 2θ range of 5–60° with a step size of 0.019° and a count time per step of 0.5 s.
Finally, the mineralogical phase identification was determined by the comparison of the positions of the diffraction peak maxima and the relative peak intensities of the powder diffraction patterns obtained and the ones indexed on the Powder Diffraction File (PDF) of the International Centre for Diffraction Data(ICDD, 2015)via the search-match DiffracPlus Evaluation software.
Figure 3.10: X-ray powder diffraction spectrometer
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3.2. X-RAY FLUORESCENCE
An X-ray fluorescence (XRF) analysis, a non-destructive technique employed for chemical material characterization, was carried out for both the aggregates and the concrete mixtures pertaining to the first research phase.
Fluorescence occurs when a sample is irradiated by X-rays. The high energy photons emitted induce an ionization of inner shell electrons (K, L, M) of the atoms in the specimen creating electron vacancies leading to the emission of secondary radiation. The spectrum is then dispersed by a single crystal of known interplanar spacing (d) causing that each wavelength detected will diffract at a specific angle given by Bragg’s law (3.5). And, as stated by Moseley’s law (3.6), there is a relationship between the energy - or reciprocal wavelength - of the secondary X-ray emitted and the atomic number of the element present in the sample. Therefore, the measurement of wavelength or energy and intensity of the characteristic photons emitted from the sample allows the identification of the elements present and the determination of their mass or concentration (Jenkins, 2001; Janssens, 2003; Klockenkämper and Bohlen, 2014)
E= 1241 λ=k × Z-σ 2 (3.6)
with E the energy of the photons emitted by the sample [eV], λ the wavelength of the radiation released by the sample [nm], k a constant that takes different values for each spectral series [eV]
(values of different transition energies can be found in Deslattes et al. (2003)), Z the atomic number of the absorbing element [-] and σ a shielding constant [-] (different values can be found in Klockenkämper and von Bohlen (2014)).
Figure 3.11: X-ray fluorescence spectrometer
81 XRF tests were performed on a Bruker S8 Tiger wavelength dispersive X-ray fluorescent spectrometer (Figure 3.11) with tungsten tube, lithium fluoride (LiF) crystal analyser and generator of 4 kW. As specimen preparation is an all-important factor in the ultimate accuracy of any X-ray determination, to obtain quantitatively correct results, the sample was crushed and the powder was pressed into tablets by fusion with lithium tetraborate (Li2B4O7) in a platinum/gold crucible to form a stable glass bead. The analysis was carried out using Quant Express standardless calibration software (SpectraPlus package) and taking into account the resulting data from a loss on ignition (LOI) analysis determined in conjunction with the XRF.
3.3. SCANNING ELECTRON MICROSCOPY
The scanning electron microscopy (SEM) is positively one of the most used non-destructive methods as it allows simultaneous morphological and chemical characterization by raster-scanning the surface of the specimen. The polyvalence of this technique is based on the different manners in which the focused beam of electrons interacts with the matter in the sample, producing diverse electromagnetic responses. The most frequently detected signals are high-energy backscattered electrons, low-high-energy secondary electrons and X-rays, while less common detections include Auger electrons, cathodoluminescence, and measurements of beam-induced current (Scheu and Kaplan, 2012).
The information gathered from the most usual detected signals allows a profound knowledge (imaging, qualitative and quantitative analysis) of the microstructure related to the relative position in the specimen surface. On one hand, signals of secondary electrons (SE) - due to inelastic electron scattering and with low energies (<50 eV) - are used to obtain SEM images that provide a topographical representation of the sample surface. On the other hand, the detection of backscattered electrons (BSE) - due to elastic electron scattering and with high energies (>50 eV) - is associated with the atomic number of the elements present in the sample. As such, a BSE image displays a compositional contrast - on a greyscale ranging from 0 (black) to 255 (white) - where brighter areas correspond to heavier elements. Consequently, the different phases can be distinguished by their differential brightness. Finally, an energy dispersive spectroscopy (EDS) could be performed based on the measurements of the characteristic intensity of the X-rays leaving the sample which, based on Mosely’s law equation (3.6), allows obtaining the chemical composition in a point analysis or over a much broader raster area to create an element map (Kristiansen, 1997; Wang and Petrova, 2012; Leng, 2013b).
The microstructural studies were conducted on the aggregates and the concrete mixture of phase I using a Hitachi S-4800 scanning electron microscope (Figure 3.12) with tungsten as X-ray source, a Si/Li detector and a Brucker XFlash 5030 EDS analyser. In order to preserve the features of the specimen surface, samples used for SEM were subjected to minimal preparation, which involved their placement in a metallic holder by means of a bi-adhesive graphite film and a subsequent carbon coating to ensure conductivity and avoid signal masking.
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Figure 3.12: Scanning electron microscope
3.4. MERCURY INTRUSION POROSIMETRY
Mercury intrusion porosimetry (MIP) is an extremely valuable characterization procedure for porous materials. The method is based on the behaviour of mercury with respect to most solids.
As a non-wetting liquid - one having a contact angle greater than 90º, the application of an external pressure is required to force the mercury into the specimen pores. Washburn’s equation (3.7) states that the pressure needed for the intrusion of a non-wetting liquid into a circular cross-section pore is inversely proportional to its diameter and directly proportional to the surface tension of the liquid and the angle of contact with the solid surface (Washburn, 1921).
=-4∙γ∙ cos θ
P (3.7)
with di the equivalent diameter of the intruded pores [nm], γ the surface tension of mercury [N/nm], θi the contact angle between mercury and the pore walls [º] and P the pressure at which a given increment of mercury intrudes into de pore system [N/nm2]. The values established for the surface tension of mercury and the contact angle - both, advancing and receding angle - were here set at 485·10-12 N/nm and 141.3° respectively.
In a MIP analysis, the test specimen is placed in a glass cell of a sealed penetrometer that is filled with mercury which is forced to successively intrude in smaller pores of the sample by gradually increasing the setup pressure or to sequentially extrude of pores by progressively decreasing the pressure at the end of the test (Giesche, 2002).
The results obtained with this technique provide a wide range of information, such as the average pore diameter, the porosity, the skeletal and apparent density, the threshold pressure, the tortuosity, and the specific surface area of a sample among others. Although the determination of the pore size distribution is also possible, the reliability of this method for cement-based materials has been profoundly discussed (Abell et al., 1999; Diamond, 2000, 2003; Gallé, 2001, 2003; Kumar and Bhattacharjee, 2003) to finally be accepted as a reasonable estimate (Bermejo et al., 2010).
83 The test was conducted to ASTM D4404-10 (2010) on a Micromeritics Autopore IV 9500 mercury intrusion porosimeter (Figure 3.13) capable of operating at up to 33,000 psia (227.5 MPa). The operating parameters were mercury filling pressure of 0.45 psia (~3.10 Pa), maximum intrusion volume of 0.50 ml/g and equilibration time of 10 seconds.
The effect of sample preconditioning has been thoroughly investigated (Winslow and Diamond, 1970; Konecny and Naqvi, 1993; Gallé, 2001, 2003; Rübner et al., 2002; Diamond, 2003; Kumar and Bhattacharjee, 2003). However, there is no agreement in the method specimens should be prepared before undergoing MIP (Bermejo et al., 2010). In the framework of this PhD, samples were dried to constant weight at 40 ºC and degassed with a vacuum pump for 30 minutes in order to ensure moisture removal. Moreover, for recycled concrete characterizations, samples were selected guaranteeing the presence of at least one ceramic coarse aggregate with enough quantity of mortar adhered.
Figure 3.13: Mercury intrusion porosimeter
3.5. AIR VOID ASSESSMENT
The assessment of the air void system of the concretes mixtures from phase II was performed through the linear traverse method described in ASTM C457 (2012) Procedure A. By means of a RapidAir 457 apparatus (Figure 3.14), which allows an automatic image analysis of the samples, the air content, void frequency, spacing factor, specific surface, chord length and paste-air ratio were determined for hardened concrete specimens at 28 days.
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Figure 3.14: RapidAir 457 apparatus
For each concrete mixture, four slices were cut from the same 100x100 mm cube specimen. A concrete saw with diamond edge was employed to cut four slices of approximately 2.5 mm in thickness in the perpendicular direction of the trowelled surface of the cube.
Since scratches must be avoided for this determination, the sawn surface of the sample was mechanically flattened and polished with a sequence of wet diamond polishing pads (grit 50, 100, 200, 400, 800, 1500, 3000 and 8000). The polishing pad was switched when the raster chalk applied to control the smoothing process was completely erased from the surface. After the polishing procedure, each slice was further cut to remove the unavoidable unpolished part due to the holding configuration of the test machine, and specimens of 65x100 mm were finally used for the air void examinations.
In order to ensure a proper detection with the air void analyser, a contrast enhancement process was also followed. The samples were coloured black with a broad tipped black marker, taking care of overlapping two consecutive ink lines. After air-drying for a few seconds, the same colouring process was repeated in the perpendicular direction to guarantee a perfect coating and the sample was allowed to dry. Next, a barium sulphate (BaSO4) white powder, with average grain size of 2 μm, was poured in the black-coated surface. To fill the air voids, the powder was tamped with a rubber stopper until all the voids were completely occupied and the excess powder was removed by pressing and dragging a steel blade over the sample surface. Then, the fine dust was cleaned in a circular motion with soft paper. The final step consisted of the colouring of the voids or cracks present in the aggregates, in which the white powder also penetrated, by means of a fine tipped black marker. Finally, the quality of the sample preparation was verified under microscope, and the enhancement process was repeated.
Once the treatment was deemed satisfactory, the sample was mounted and levelled in the sample holder of the RapidAir apparatus. After adjusting the light and focus, the sample was automatically analysed using 5 traverse lines per frame and a total traverse length of 2413.1 mm.
Two analyses were performed per sample, by turning the sample over 90° and an average of the two readings of the sample was reported. As the examination was based on a white and black image analysis, a threshold value was implemented in order to distinguish between colours.
85 However, this value varies with light/contrast settings of the system and general room lighting among others (Jacobsen and Arntsen, 2007), and a proper threshold value needed to be chosen for each sample.
During the test, the total chord length traversed, the chord length traversed through air voids, the chord length traversed through paste and the number of air voids intersected by the traverse line were automatically gathered by the RapidAir software. To avoid any lack of representativeness between the sample and the concrete batch, the paste content in the original mix was manually determined and input in the software to be factored in the automatic analysis. Lately, all this parameters were used to fully characterize the air void system parameters, i.e. the air content (3.8), the specific surface (3.9), the spacing factor (3.10) and the average chord length (3.11).
Ac=Ta
Ttot∙100 (3.8)
αs=4N
Ta (3.9)
Ls=Tp 4N
Ls= 3
αs 1.4∙ 1+ p Ac
13
-1
for p/Ac≤4.33
for p/Ac>4.33
(3.10)
Lm=Ta
N (3.11)
with Ac the air content [%], Ta the total chord length of air voids [mm], Ttot the total surface distance traversed [mm], αs the specific surface [mm-1], N the total number of air void chord lengths [-], Ls the spacing factor [µm], Tp the total chord length of paste [mm], p the paste content [%], Lm the average chord length [mm].