CaL_2,3-edge near edge X-ray absorption fine structure of tricalcium aluminate, gypsum, and calcium (sulfo)aluminate hydrates

Abstract

Tricalcium aluminate (cement clinker phase), gypsum, katoite, ettringite, and calcium monosulfoaluminate hydrate (abbreviated as kuzelite) are the major minerals in the hydration reaction of tricalcium aluminate in the presence of gypsum and have critical impacts on the kinetics and thermodynamics of early-age cement hydration mechanisms. Here, spectroscopic analysis of these minerals is conducted using scanning transmission X-ray microscopy (STXM). Their Ca L 2,3-edge near edge X-ray absorption fine structure (NEXAFS) spectra are measured and correlated to the known Ca coordination environments. The results indicate that these minerals have unique Ca environments that can be differentiated from one another based on the intensities and positions of the absorption peaks at 346.5-348.5 and 350.5-351.5 eV. It is concluded that Ca in tricalcium aluminate (cubic and orthorhombic polymorphs) and katoite is in cubic-like coordination with negative 10Dq, whereas Ca is in an octahedral-like coordination with positive 10Dq in ettringite, gypsum, and kuzelite. For tricalcium aluminate, the Ca atoms in both polymorphs are in similar chemical environments with slightly more distortion in the orthorhombic polymorph. As a common issue in STXM experiments, absorption saturation of NEXAFS spectra is also investigated. It is demonstrated that the optical density difference between pre-and post-edge absorption levels provides a reliable indication of the sample thickness in the systems studied. The present work provides a reference for the STXM study of the calcium (sulfo)aluminate reactions in cement hydration and natural aqueous environments, and in the former case, provides a more complete understanding of a system that may serve as a low-C alternative to Portland cement.

Quantum Nature of Stability Mechanisms of Calcium Apatite Structure

“…Analysis of the literature data and our results allows suggesting that the atomic effects in the studied substances play a significant role in the formation of the form of calcium L  -spectra. This is confirmed by the spectra of the quantum yield of photoemission, which show a significant similarity of the spectra for completely different compounds-carbonate and fluoroapatite of calcium, as well as the spectra given in publications [11][12][13][14][15][16][17][18][19][20][21].…”

Nanometer-Resolved Spectroscopic Study Reveals the Conversion Mechanism of CaO·Al2O3·10H2O to 2CaO·Al2O3·8H2O and 3CaO·Al2O3·6H2O at an Elevated Temperature

“…22,25 The Ca−O coordination of C 3 AH 6 is cubic-like as the peak 2 of the Ca L 2,3 -edge NEXAFS spectrum is located between peak a 1 and a 2 ; see Figure 1a. 22 In contrast, the coordination of Ca− O in CAH 10 and C 2 AH 8 are octahedral-like, similar in shape to ettringite and kuzelite (also named monosulfate-AFm). As shown in Figure 1b, the atomic configuration of Ca−O polyhedron in CAH 10 28 is very similar to that of ettringite: 22,29 a rectangle (four O atoms) and a parallelogram (four O atoms) approach the Ca from opposite directions and are aligned along each other's diagonals.…”

“…water suspension, contained in 2 mL polypropylene vials, was rapidly heated to 60 ± 2°C in a water bath. After being heated for the desired time durations (30 min, 1 h, and 3 h), a 0.1 μL drop of the suspension was transferred onto a 100 nm-thick-Si 3 N 4 window, followed by quick removal of the liquid with Kimwipes (Kimtech Science TR ), as described in ref 22. The top of the sample was covered with another Si 3 N 4 window.…”

Role of Adsorption Phenomena in Cubic Tricalcium Aluminate Dissolution

“…The positions, number and size of the model units shown are approximate: readers are referred to the crystallographic literature for accurate structures of the solid phases depicted. 4,5,52 C3A dissolution is occurring from right to left and is shown to rearrange near-surface structural units in the Al-rich leached layer. The C3A surface is depicted to bear a negative charge, which is expected at pH > 12, from preferential dissolution of Ca structural units, 17 and from outer-sphere Ca adsorption in this pH range.…”

“…2 The structure of C3A contains tetrahedral Al in Al6O18 18rings and Ca atoms in distorted octahedral or irregular bonding environments. 3,4,5 Dissolution of C3A in portlandite (Ca(OH)2)-saturated aqueous CaSO4 solutions leads mainly to the precipitation of ettringite (Ca6Al2S3O18•32H2O) and 'alumino-ferrite-mono' (AFm) phases in CaO-Al2O3-SO3-H2O systems with mechanistically comparable yet slightly lower S/Al molar ratios (≈ 0.1 6 ) than that present in PC systems (≈ 0.5 7 ). AFm phases are layered double hydroxides based on substituted Ca(OH)2-type structures.…”