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Characterization of ground cements (as distinguished from clinkers) has been limited, due to its fine particle size and complex microstructure. Characteristic features such as crystal shape, occurrence within the microstructure, and etch color are all utilized for phase identification. The grinding reduces the particle size range to about 1-100 µm with a median size about 12 µm, diminishing the inter-phase relationship clues necessary for identification, and adding an additional phase, gypsum. Analyses using a light microscope generally limit sampling to the larger fragments as some etching techniques often used, may partially or completely dissolve the finer-sized particles. An alternative microscopy method, which avoids many of these difficulties is SEM imaging [13, 14].
The SEM scans a high-energy electron beam across the surface of a specimen and measures one of a number of signals resulting from the interaction between the beam and specimen. Two particularly useful imaging methods are backscattered electron (BE) and X-ray (XR) imaging. Combining the BE and accompanying XR images via image processing allows their segmentation into the constituent phases of the microstructure. Once the image is segmented, it may be analyzed to extract information such as bulk phase abundance, phase surface area, and feature size.
Backscattered electrons are high-energy electrons (>50 eV) that have undergone multiple elastic scattering events within the specimen. The greater energy results in a larger interaction volume and lower spatial resolution compared to the secondary electron image. Contrast is generated by the different phases relative to their average atomic number. This is observed by the differential brightness in the image. The backscatter coefficient 1f is a measure of the backscattered electron fraction and, for a pure element of atomic mass Z, may be estimated from [16] and is shown graphically in Fig. 5.
Fig. 5. Backscattered electron coefficient versus atomic number (Z0, from Goldstein et at. [16].
The back scattered electron coefficient of a multi-element phase is estimated using the mass fractions (Ci) and η values for each constituent:
 |
(1) |
Contrast between constituents may be calculated as:
 |
(2) |
Table 2 is a list of some phases from clinker and cement,
in descending order of their gray level intensity. The
6.8% contrast between alite (
= 15.06) and belite
( = 14.56) is
relatively strong; that between belite and
cubic tricalcium aluminate ( = 14.34), at 1.5%, is
generally too weak to distinguish these constituents. If
the contrast between phases is so weak that it precludes
discrimination, they are usually chemically distinct, and
therefore X-ray imaging can be used to distinguish them
from each other.
| Table 2 Clinker and cement phases, composition, cement chemist notation, densities, average atomic number and backscattered electron coefficient ranked according to relative SEM BE image brightness. |
|||||
|---|---|---|---|---|---|
| Phase | Composition | Notation | Density (Mg/m3) | | η |
| Ferrite | Ca2(Al,Fe)2O5 | C4AF | 3.77 | 16.65 | 0.1860 |
| Free lime | CaO | C | 3.32 | 16.58 | 0.1882 |
| Alite | Ca3SiO5 | C3S | 3.13 − 3.22 | 15.06 | 0.1716 |
| Belite | Ca2SiO4 | C2S | 3.28 to 3.31 | 14.56 | 0.1662 |
| Arcanite | K2SO4 | K |
2.67 | 14.41 | 0.1652 |
| Aluminate-cub. | Ca3Al2O6 | C3A | 3.04 | 14.34 | 0.1639 |
| Aluminate-ort. | NaCaAl3O9 | C3A | 2.56 | 13.87 | 0.1588 |
| Aphthitalite | (Na,K)2SO4 | KN | 2.7 | 13.69 | 0.1577 |
| Syngenite | K2Ca(SO4)2H2O | CK2H | 2.6 | 13.60 | 0.1556 |
| Anhydrite | CaSO4 | CH |
2.98 | 13.41 | 0.1535 |
| Bassanite | 2CaSO4 · H2O | CH
|
2.7 | 13.03 | 0.1489 |
| Gypsum | Ca(SO4)· 2H2O | 2CH |
2.32 | 12.12 | 0.1381 |
| Thenardite | Na2SO4 | N |
2.66 | 10.77 | 0.1249 |
| Periclase | MgO | M | 3.58 | 10.41 | 0.1213 |
Characteristic X-rays are another signal produced as a result of the electron beam-specimen interaction. X-ray microanalysis systems generally employ an energy-dispersive detector. The X-ray signal is used to determine which elements are present and in what concentration, and graphical display of relative concentrations through line scans and X-ray imaging of element spatial distribution and relative concentrations. Mass concentration to a few tenths of a percent can be detected for some elements. The relative accuracy of quantitative analysis (using certified standards, and solid, homogeneous specimens) is about ±20% for concentrations around 1%, and about ±2% for concentrations greater than 50%. More details on X-ray microanalysis may be found in [16].