Abstract
The impact of low-pressure treatment on the crystal structure, morphology, and chemical composition of ettringite, due to their major importance with respect to processability (i.a., drying conditions) and to the analysis of ettringite-containing samples, is examined utilizing X-ray diffraction, thermogravimetric analysis, Raman spectroscopy, and environmental scanning electron microscopy. Synthetic ettringite was treated for various durations (5 min up to 72 h) and at two different levels of low-pressure (4.0 mbar and 60 µbar). Evaluation showed a correlation between the procedural parameters (time and pressure), the chemical composition, and the morphology of ettringite. The experiments reveal that, when exposed to 4 mbar pressure, nearly no changes occur in the ettringite’s morphology, whereas the crystals undergo swelling and slight deformations at very low pressures (60 µbar and 35.3 nbar), which is attributed to the loss of bound water and the partial transformation from ettringite to quicklime, anhydrite, and calcium aluminate. Furthermore, the strongly dehydrated ettringite shows the same morphology.
Highlights
Ettringite (Ca6 Al2 (SO4 )3 (OH)12 ·26H2 O) is formed as small needle-shaped crystals on the surface of cement particles during the early stages of hydration [1,2,3,4,5,6] and is of significant technical importance, as it influences the rheological properties of fresh cement suspensions
In the angle range of 7.5◦ ≤ 2θ ≤ 51◦ [23], which align with the data by Hartman et al [23]. Both discrepancies in the intensity of the reflections at 2θ = 32.03◦ and 2θ = 34.84◦ compared with the literature are most likely induced by the faceting of the crystals
Synthetic ettringite was chosen as model system to investigate the impact of different levels of low-pressure treatment (4 mbar, 60 μbar, 2.8 nbar) on its chemical composition, crystal structure, and morphology during characterization measurements and the drying process
Summary
Ettringite (Ca6 Al2 (SO4 ) (OH)12 ·26H2 O) is formed as small needle-shaped crystals on the surface of cement particles during the early stages of hydration [1,2,3,4,5,6] and is of significant technical importance, as it influences the rheological properties of fresh cement suspensions. Previous works proved that the crystal structure of ettringite consists of dodecahedrally coordinated calcium ions and octahedrally coordinated aluminium ions forming face-linked hexagonal prisms along the c-axis [1,9,10,11]. The pioneering works of Skoblinskaya et al revealed prominent changes in the crystal structure: a shrinkage in the cell parameters at different hydration stages using isobar treatment at 8.0 μbar in a range of 25 ◦ C to 325 ◦ C was observed [5,6]. Characterizations of the changes in ettringite’s crystal structure were performed after the treatment at ambient pressure in a range of 25 ◦ C to 200 ◦ C for up to 7 h [12]. The above-mentioned works have already demonstrated that the bound crystal water of ettringite can be effectively removed by low-pressure or elevated temperatures
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