Understanding the dissolution of metakaolin in sodium hydroxide solutions

The dissolution kinetics of metakaolin are examined at room temperature in NaOH solutions at concentrations of (0.1, 5, or 10) mol L⁻¹ and at liquid-to-solid mass ratios (L/S) of 50 or 1000. Both the dissolved Si and Al concentrations as well as the morphology and structure of the solid are reported at various times. The dissolution rate is approximately constant for about an hour, (0.0043–0.1030) µmol m⁻² s⁻¹, depending on alkalinity and L/S. Some precipitates are observed within the first ten minutes but do not affect the dissolution rate during that first hour, implying that the precipitates form in the solution and do not block or alter the metakaolin surfaces significantly. The dissolution rate during the first 45 min increases with alkalinity and decreases with L/S. The metakaolin exfoliates at higher NaOH concentrations, exposing more surface area to the solution. The effect of exfoliation increases the apparent dissolution rate and promotes more congruent dissolution. These observations not only improve our understanding of the metakaolin dissolution mechanisms but also provide insights into the reactivity of calcined clays that are becoming increasingly important to the sustainability of the cement and concrete industry.