Dehydroxylation of kaolinite to metakaolin—a molecular dynamics study

Abstract

The thermally induced transformation of kaolinite to metakoalin is simulated using molecular dynamics through a step-wise dehydroxylation approach. The simulation shows that the removal of structural water through dehydroxylation produces a distortion or buckling effect in the 1:1 Al-Si layers, which is due to the migration of the aluminium into vacant sites provided by the inter-layer spacing. The structural change is characterized by a loss of crystallinity and a concomitant change in aluminium coordination from octahedral to tetrahedral, with this study confirming the presence of 5-fold aluminium within the metakaolin structure. The degree and probability of Al migration is proportional to the amount of local disorder within the structure, which is governed by the degree of local hydroxyl group loss. This results in the formation of aluminium clusters within the layers. This study proposes that instead of a uniform structure, metakaolin exhibits regions of differing aluminium concentrations, which can have major effects in the reaction chemistry at those sites.

In situ high-temperature Raman and FTIR spectroscopy of the phase transformation of lizardite

“…The plateau E A corresponds to the recombination variant with the highest E A and the slight decrease indicates the increasing influence of transport in certain parts of the sample (large grains). This interpretation is compatible with DFT calculations (Molina-Montes et al 2008a, 2008bSperinck et al 2011), but not with experimental work by Gualtieri and Ferrari (2006) and Gualtieri et al (2012), who indicate one-dimensional diffusion of H 2 O molecules through the six-membered silicate rings. In the latter case, H 2 O-related modes should be visible in in situ Raman and FTIR spectra of partially dehydrated samples.…”

“…For heating rates similar to the one used here, the activation energy plateau is reached at about 50 C below the temperature at which the OH bands have lost almost 60% (at 639 C), respectively, 98% (665 C) of their intensity in the Raman spectrum. DFT calculations and almost all previous works (e.g., Gualtieri and Ferrari 2006;Molina-Montes et al 2008a, 2008bSperinck et al 2011) propose a recombination of neighboring proton and hydroxyl ions to H 2 O molecules as the primary reaction step, followed by diffusion of the products (H 2 O molecules) along the interlayer to the surface. Several distinct recombination variants are possible, which differ by the distance between the reacting hydroxyls.…”

Use of Brazilian Kaolin as a Potential Low-cost Adsorbent for the Removal of Malachite Green from Colored Effluents

“…From these curves, it can be seen the first endothermic peak around 50 ºC, which corresponds to the adsorbed water loss (about 1.3 wt.%). The second peak centered at 500 ºC corresponds to the mass loss (about 13.7 wt.%) due to kaolinite dehydroxylation toward the formation of a noncrystalline phase (metakaolinite) 62,63 . The particle size distribution curve of kaolin is shown in Figure 5.…”

Structural Performance of Alkali-Activated Soil Ash versus Soil Cement

“…In order to improve the conditions for its activation, the original raw material should be submitted to a previous thermal treatment. This will induce the loss of constituent water and the recoordination of the aluminium and oxygen ions, transforming the structure from crystalline to amorphous (Davidovits, 1991;Sperinck et al 2011;He et al, 2012). This structural modification creates an environment where chemical combinations are easier.…”