Thermal evolution of metakaolin geopolymers: Part 1 – Physical evolution

The physical evolution of materials during heating is a critical factor in determining their suitability and performance for applications ranging from construction to refractories and adhesives. The effect of different cations (sodium and potassium) on the physical evolution of geopolymeric materials derived from metakaolin is investigated for a range of specimens with Si/Al ratios between 1.15 and 2.15. It is observed that the effect of potassium is to reduce the thermal shrinkage, while thermal shrinkage increases with increasing Si/Al ratio in the presence of each alkali type. The thermal shrinkage behavior of mixed-alkali specimens is observed to change from a mean of the sodium and potassium specimens at low Si/Al ratio to behave similarly to sodium specimens at high Si/Al ratios. It is clear from this investigation that alkali cations only have a significant effect on thermal shrinkage of geopolymer at low Si/Al ratios (⩽1.65), while both Si/Al ratio and alkali cation have little effect on the extent of thermal shrinkage at Si/Al ⩾ 1.65.