Heated Montmorillonite: Structure, Reactivity, and Dissolution
The dehydroxylated form of the principal 1:1 clay, kaolinite, known as metakaolin, has been widely studied in terms of its structure and reactivity. However, detailed information on the dehydroxylation of the abundant 2:1 clay, montmorillonite, is lacking in this respect. Three montmorillonites, calcined at various temperatures have been characterized by solid-state 29Si MAS NMR. The dehydroxylation (600 – 800 °C) results in progressive distortion of the SiO4 tetrahedra, followed by crystallization of inert, stable phases at higher temperatures. The dissolution kinetics of a structurally pure montmorillonite, SAz-2, calcined at two different temperatures are found to be in good agreement with its pozzolanic reactivity established in an earlier study. It is also found that SAz-2, calcined at its optimum calcination temperature of 800 °C, undergoes incongruent dissolution reaching a molar Si/Al ratio of 3.7 in a 0.1 M NaOH solution after one day of dissolution. It has been reaffirmed that both the degree of dehydroxylation and the type of structural phases (Q3/Q4) have a significant impact on the reactivity of the calcined montmorillonite. The clear identification of inert phases and reactive sites by solid-state NMR may have major implications in the utilization of not only montmorillonites but also other calcined clays.
KeywordsDissolution Kinetic Octahedral Sheet Supplementary Cementitious Material Incongruent Dissolution Optimum Calcination Temperature
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The Danish National Advanced Technology Foundation is acknowledged for the financial support to the SCM project.