What Happens to 5-year Old Metakaolin Geopolymers? the Effect of the Alkali Cation

In this study we report X-ray diffraction and dilatometry results of metakaolin (MK) geopolymers produced with Na, K, Rb or Cs silicate solutions, and cured for 7 days and 5 years, with the aim of identifying variations in structure and performance (dimensional stability at elevated temperature) over an extended curing period at ambient temperature. All of the geopolymers studied are mainly X-ray amorphous after 5 years of curing; however, in Rb-based and Cs-based geopolymers formation of aluminosilicate crystalline phases was identified. As the alkali cation radius increases, so does the thermal stability of the MKgeopolymer, potentially as a consequence of the combined effect of the higher degree of ordering of the geopolymer itself, which might retard the dehydration of the geopolymers upon heating, and the reduced energy of hydration of larger alkali cations. The 7-day and 5year cured samples produced with Rb and Cs silicate solutions do not exhibit significant dimensional changes above 300C, with a maximum shrinkage of  2% after exposure to 1100C. This shows that increasing the radius of the alkali cation during geopolymerisation of MK has an effect on the thermal stability of these materials, and promotes the formation of a highly densified and rigid structure at advanced curing ages.