Speaker
Description
Quantification of the metastable hydrate phases CAH10 and C2AHx is essential in understanding and evaluating the conversion progress of CAC pastes for scientific and industrial research. Mainly XRD or TGA are used for phase quantification, whereas XRD proved to be more precise. While structural data exist for CAH10, they do not yet exist for C2AHx, as the proportion of interlayer H2O in the unit cell of this AFm-phase varies during hydration. Pure synthesis is strongly dependent on the relative humidity and made even more critical by the risk of CO2-uptake by C2AHx and transformation to monocarbonate. Thus, in the initial hydration of CACs either x = 8.2, 8 or 7.5 interlayer H2O are described, resulting from reduction of interlayer distances. By further drying at elevated temperatures x = 5 or even 4 is reported.
An evaluation of the synthesized phase contents of C2AHx phases was achieved by multiple synthesis routes, back-quantification of minor phases that have been precipitated as impurities and modeling of an hkl-model according to the PONKS method.
The suitability of this structure model was verified by in-situ XRD of an accelerated system (CA hydration at 40°C, with a maximum crystallinity of C2AHx) and various measurements of synthetic and industrial calcium aluminate cements, e.g. in applications for refractories or binders.
Factor F was determined as the product of the squared volume V of unit cell and the density ρ of C2AHx, (F=V2 * ρ) for the use within the inp-dataset corresponding to this hkl-model for quantification by Topas. This model can be used to determine the quantity of C2AHx in any hydrated CAC sample by XRD.
Error analysis including the influence of the external standard, synthesis inaccuracy and the interlayer H2O variation of C2AHx resulted in an error of only 3.5 rel% of the determined C2AHx quantity.
Affiliations
Hilti Entwicklungsgesellschaft mbH, Hiltistrasse 6, 86916 Kaufering, Germany
julian.goergens@hilti.com
| Title | Quantification of the metastable calcium aluminate phases C2AHx using the extended PONKS method |
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| Keywords | CAC, XRD, Conversion, Metastable hydrates, Quantification |