Effect of temperature and alkaline concentration on metakaolin leaching kinetics

This study aims to advance in the understanding of how alkaline concentration and temperature affect metakaolin dissolution and hydrolysis and the characteristics of the reaction products formed. Two working temperatures, 25 and 85 °C, and two alkaline concentrations, 5- and 8-M NaOH, were studied. The findings showed that metakaolin dissolution is a three-stage process. A very short period of rapid incongruent MK dissolution resulting in [Al]>[Si] is followed by a process of moderate speed and duration in which [Al]≤[Si], denoting congruent, synchronised dissolution and lastly by a slow, later age process in which, under very vigorous conditions, [Si]>[Al], indicative of incongruent dissolution. The results also revealed that soluble species do not necessarily reflect the ultimate structure of the reaction products formed. The activator concentration and especially temperature affect process kinetics as well as thermodynamics, although much less intensely.