Evolution of kaolinite morphology upon calcination and dissolution

The use of calcined clays as a supplementary cementitious material has been a subject of intensive research due to their high availability and their potential to be used as a low-CO2 alternative to Portland cement. Once calcined, kaolinite transforms into metakaolin, which can dissolve and react under alkaline conditions. This reactivity has been studied in terms of dissolution kinetics, but not much is known about the evolution of clay morphology upon dissolution. Here, we apply quantitative imaging approaches to statistically quantify the extent of morphological changes that occur in dissolving kaolinite and metakaolin at multiple scales. Firstly, at the micro-scale, we successfully capture in-situ exfoliation of clay particles in an alkaline solution using a novel experimental setup. We find a noticeable difference in the maximum relative Feret diameters (2.4 for kaolinite vs 1.5 for metakaolin) as well as the pattern in which these clays break apart (exfoliation for kaolinite vs disintegration for metakaolin). Secondly, at the nano-scale, we report ex-situ measurements and comparisons of individual layer thicknesses before and after dissolution. For the first time, to our knowledge, we report that kaolinite undergoes an average thinning of ~ 15 nm (from 110 nm to 95 nm) upon calcination to metakaolin, and consequently, metakaolin layers undergo an evident thinning of ~20 nm (from 95 nm to 75 nm) upon dissolution. Finally, at the nano-scale, using in-situ calcination of clay particles under TEM, we report a reduction of 2% to 5% of the area of kaolinite layers, showing that reactive surface area (in this case, the layer edges) are to be affected first. Moreover, we find that these nano-scale layers also show a reduction in their thickness, by measuring an increasing grayscale intensity of their images over time. These two main morphological changes account for the whole kaolinite layer volume, and by fitting an exponential model, it is estimated that 90% of the calcination occurs before 47 minutes. This also shows that morphological changes in kaolinite are related to its reactive surface area, and therefore, to the particle size distribution of clay powders. These new quantitative results on morphological changes in 1:1 clays upon dissolution could pave toward a fundamental understanding of clay reactivity as well as widespread usage in cementitious systems.

Name thesis:degree_name

M.S.

Level thesis:degree_level

Thesis

Discipline thesis:degree_discipline

Civil Engineering

Grantor

University of Illinois at Urbana-Champaign

Year dc:date

2022

Author dc:creator

• Romero Contreras, Pablo Alfonso

Contributors dc:contributor

• Garg, Nishant

• Kaolinite
• metakaolin
• dissolution
• image processing
• layer thickness

Statement dc:rights

• Copyright 2022 Pablo Romero Contreras

Language dc:language

en, eng

Handle dc:identifier

https://hdl.handle.net/2142/115958

OAI record oai:identifier

https://www.ideals.illinois.edu/oai-pmh?verb=GetRecord&metadataPrefix=etdms&identifier=oai%3Awww.ideals.illinois.edu%3A2142%2F115958