This study has been focused on the effect of pretreatment of FA and POFA after geopolymerization on the mechanical properties. The aim of this work was to simplify the pretreatment that it can easily industrial be applied, using existing technology. Previous work has shown that a reduction in particle size increases the mechanical properties. However, this method involves a milling process which is not applicable for a wide industrial application. Hence, FA and POFA particles have been heated to 300 degrees C, 500 degrees C, and 800 degrees C but applying different pretreatments: (i) predried at 110 degrees C (reference sample); (ii) as received; (iii) prewetted; (iv) prewetted and later quenched in cold water. It was found that during the treatment the particle size increased due to thermal stress cracking. During fast heating, trapped pore water cannot be removed as fast as it evaporates and hence the particles crack. This increase in particle size caused an increase in compressive strength. In addition, heating to 300 degrees C and 500 degrees C caused a dehydroxylation of FA and POFA. This dehydroxylation resulted in a higher initial reactivity, reducing the setting time and improved mechanical strength.