Amorphous calcium aluminate/anhydrite is a unique cement1tious material that otters superior nydratlon acuv1ty compared with crystalline calcium aluminates.It is useful as a rapid hardening additive and has been applied to many mortars and concretes, often in combination with ordinary Portland cement.Although the effective use of recycled fly ash is important, there have been no reports concerning the use of rapid hardening mortar with amorphous calcium aluminate/anhydrite in which a portion of the Portland cement is replaced with fly ash.Consequently, it is important to understand the characteristics of amorphous calcium aluminate/anhydrite-ordinary Portland cement-fly ash systems.In this study, the effects of fly ash replacement on the characteristics of rapid hardening mortar with added amorphous calcium aluminate/anhydrite were investigated.It was found that when adding no more than 0.9% of the regulated setting agent, there was no difference between the fly ash 0% and fly ash 20% specimens.However, at concentrations above 1.0%, the gelation time of the fly ash 20% was longer than that of the fly ash 0%.In addition, the 25-step flow values were the same regardless of the fly ash concentration.For curing times up to 28 days, the compressive strengths of specimens decreased as the fly ash concentration was increased from 0 to 20%, while the FA samples all exhibited reduced expansion at 28 days curing compared with the FA0% and the length change of the FA20% was equal to that of the FA 10%.