The long-term suppression effects of fly ash and slag on the expansion of heat-cured mortar due to delayed ettringite formation

•The long-term resistance of mortar containing fly ash and slag against DEF expansion was confirmed over 10 years.•A substantial amount of unhydrated cement particles remained over long term for less-expanded mortar specimens.•The suppression of DEF expansion by fly ash and slag can be partly attributed to the elimination of moisture and ion transport in the mortar. The long-term resistance of mortar containing fly ash and slag against expansion due to delayed ettringite formation (DEF) was investigated. DEF expansion of heat-cured mortar containing fly ash and slag at various replacement levels was monitored over 10 years. The results indicate that fly ash and slag suppress DEF expansion effectively over the long term, even when there is excessive SO3 in the cement. In the DEF-expanded mortar, the cracks and gaps were filled with ettringite, and fully hydrated cement particles were observed. In contrast, a substantial amount of unhydrated cement particles and high S/Ca ratio of inner products remained over long term, since the incorporation of a sufficient amount of fly ash and slag possibly facilitated the development of a dense microstructure in the paste. A large amount of monosulfate was also present throughout the paste. These results suggest that fly ash and slag eliminate the transport of moisture and ions in the mortar, which considerably extends the latency time of DEF expansion.