The use of calcined clays as supplementary cementitious materials (SCMs) may provide a valuable contribution to the reduction in CO2 emissions associated with cement production since calcined clays can be produced at significantly lower temperatures and do not involve a decarbonation reaction.The dehydroxylated form of the principal 1:1 clay kaolinite, metakaolin, represents an ideal model compound for an alumino-silicate rich SCM which has been widely studied in terms of its structure and reactivity.Less attention has been paid to thermal activation of the 2:1 clay, montmorillonite, although this mineral can be found in large deposits all over the world.