Aqueous dissolution of sodium aluminosilicate geopolymers derived from metakaolin

In dilute aqueous solutions, the elemental releases of Na, Al and Si from a metakaolin-based sodium aluminosilicate geopolymer were not very sensitive to pH in the range of 4–10 but increased outside this range, particularly on the acidic side. To minimise pH drifts, experiments were carried out using small amounts of graded powders in relatively large volumes of water. In deionised water, the Na dissolution rate in 7 days was dominant and increased by at least a factor of ∼4 on heating from 18 to 90 °C, with greater increases in the extractions of Al and Si. At 18 °C the elemental extractions in deionised water increased approximately linearly with time over the 1–7 days period. Further exposure led to a slower extraction into solution for Na and Si, with a decrease in extraction of Al. It was deduced that framework dissolution was important in significantly acidic or alkaline solutions, but that contributions from water transfer from pores to elemental extractions were present, even at low temperatures in neutral solutions. It was also deduced from the Na release data that the Na leaching kinetics of geopolymer in deionised water (dilute solutions) followed the pseudo-second-order kinetic model and the pseudo-second-order rate constant evaluated. Contact with KCl, KHCO3, and pH ∼6 and 10 potassium phthalate buffer solutions gave rise to a high degree of Na+ ↔ K+ exchange and rendered the framework ions less leachable in water.