Why do we use fly ash in mass concrete especially in dam construction?

Fly ash is used as SCM (supplementary cementious material) in large proportion (high dose 30-60%) together with portland cement in mass concrete in dam construction because of technical, environmental and economical considerations. It improves durability and strength of the mass concrete at lower cost with better temperature properties but has longer setting time which could be a problem under wet environmental conditions impacting schedule finish time and its long term performance