Abstract
This paper shows the microstructural differences and phase characterization of pure phases and hydrated products of the cubic and orthorhombic (Na-doped) polymorphs of tricalcium aluminate (C3A), which are commonly found in traditional Portland cements. Pure, anhydrous samples were characterized using scanning transmission X-ray microscopy (STXM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and demonstrated differences in the chemical and mineralogical composition as well as the morphology on a micro/nano-scale. C3A/gypsum blends with mass ratios of 0.2 and 1.9 were hydrated using a water/C3A ratio of 1.2, and the products obtained after three days were assessed using STXM. The hydration process and subsequent formation of calcium sulfate in the C3A/gypsum systems were identified through the changes in the LIII edge fine structure for Calcium. The results also show greater Ca LII binding energies between hydrated samples with different gypsum contents. Conversely, the hydrated samples from the cubic and orthorhombic C3A at the same amount of gypsum exhibited strong morphological differences but similar chemical environments.
Highlights
The most important crystalline phases of clinker Portland are tricalcium silicate (3CaO·SiO2 referred in the chemistry of cement as C3 S), dicalcium silicate (2CaO·SiO2 or C2 S), tricalcium aluminate (3CaO·Al2 O3 or C3 A) and ferrite (4CaO·Al2 O3 ·Fe2 O3 or C4 AF)
The X-ray diffraction (XRD) of the cubic C3 A shows a cubic structure with a d-spacing at 2.69874 Å along the {440}
The orthorhombic C3 A exhibits the expected double refraction line at 2.692 Å and 2.714 Å along the {224} and {400} directions, respectively, which is characteristic of its orthorhombic structure
Summary
The most important crystalline phases of clinker Portland are tricalcium silicate (3CaO·SiO2 referred in the chemistry of cement as C3 S), dicalcium silicate (2CaO·SiO2 or C2 S), tricalcium aluminate (3CaO·Al2 O3 or C3 A) and ferrite (4CaO·Al2 O3 ·Fe2 O3 or C4 AF). The use of STXM along with near-edge X-ray absorption fine structure (NEXAFS) spectra allows the chemical characterization and identifying spatial heterogeneities, providing a specific element quantification of individual species with a high spectral resolution [19]. This technique is well suited for nano-structured or amorphous materials or compounds, such as the main hydrated phases in OPC (C–S–H) as it allows assessing the oxidation states, bond length, coordination numbers and neighboring atoms of the X-ray absorber atom of interest. Unhydrated pure C3 A phases were assessed using X-ray photoelectron spectroscopy (XPS) to validate the chemical state of the elements present in their structures, and X-ray diffraction (XRD) is used for phase identification
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