Reactivity of Metakaolin in Alkaline Environment: Correlation of Results from Dissolution Experiments with XRD Quantifications

Abstract

Systematic investigation of filtrates and filter residues resulting from a 24 h treatment of metakaolin in different alkaline solutions were performed. On filtered metakaolin particles, inductively coupled plasma-optical emission spectrometry (ICP-OES) measurements reveal an enrichment of iron and titanium, which suggests an inhomogeneous distribution of these cations. Since the SiO2/Al2O3 ratio remains constant in all filter residues examined, the dissolution of the Si and Al monomers is congruent. Structural differences, identified by attenuated total reflection–Fourier transform infrared spectroscopy (ATR-FTIR) as a consequence of alkali uptake, influence the X-ray scattering contribution of metakaolin, and thus quantifications with the partial or no known crystal structure (PONKCS) method. This leads to deviations between the degree of reaction calculated from Si and Al solubility from filtrate and that quantified by quantitative powder X-ray diffraction (QPXRD) using the filter residue. Nevertheless, the described changes do not cause a shift in the X-ray amorphous hump in case of congruent dissolution, and thus allow the quantification of the metakaolin before and after dissolution with the same hkl-phase model.

Hydration of cubic tricalcium aluminate in the presence of calcined clays

“…For the system substituted with 20 wt% calcined kaolinitic clay, the reaction degree of metakaolin was varied in a range which is consistent with data from literature. 37 The lower chemical reactivity of calcined illite as well as the higher ratio of Si to Al ions in comparison to calcined kaolinite is in line with previous publications. 32,55,56 The slower reaction rate of CKc compared to CKf can be explained by lower specific surface area and degree of disorder in the kaolinite structure.…”

“…Phase assemblage of the reference system was calculated depending on CS̅ content. For the system substituted with 20 wt% calcined kaolinitic clay, the reaction degree of metakaolin was varied in a range which is consistent with data from literature 37,54 …”

“…The eluate was filtered and acidified to a pH of 1.0 after the corresponding time. The concentration of silicon and aluminum ions was measured using ICP‐OES (Varian ICP‐OES 720 ES) as described by Scherb et al 37 A multi‐point calibration with an external standard was carried out beforehand. Assessment of chemical reactivity according to current state of the art was performed by determining the evolved heat during the first 72 h using the R 3 calorimetry test 38,39 …”

“…For the system substituted with 20 wt% calcined kaolinitic clay, the reaction degree of metakaolin was varied in a range which is consistent with data from literature. 37,54 3 | RESULTS AND DISCUSSION…”

Mechanically activated mine tailings for use as supplementary cementitious materials

“…Considering that the pH of the 0.5 M KOH in the modified R 3 test is 13.5 at room temperature, it is possible the modified R 3 test is driven by similar silicate/aluminosilicate dissolution behavior. At any rate, these studies are among the few that show direct links between SCM dissolution and reactivity in model systems [40,42,43].…”