Effects of calcium on setting mechanism of metakaolin-based geopolymer

Geopolymer setting is seen to be substantially accelerated by addition of calcium and the objective of this study was to determine the mechanism for this effect by examining metakaolin geopolymers with and without calcium. Solid-state Al-27 NMR tests were used to examine the dissolution extent both qualitatively and quantitatively. Solid-state Si-29 NMR tests were conducted to determine the amount and structure of each phase. Prior to the quantitative tests, chemical extractions were used to facilitate assignment of peaks in each spectrum. On addition of calcium, it was found that both the rate and the extent of metakaolin dissolution were enhanced. Accelerating dissolution increases the Al concentration in solution, thus reducing Si/Al available for geopolymer gel formation and further accelerating the gel formation to cause faster setting. Although C-A-S-H was observed in the calcium mix, no evidence indicated that it is directly involved in setting.