This paper presents the strength characteristics and porosity of concrete mixed with seawater and tap
water incorporating Fly Ash (FA) with a water binder ratio of 40%, 50%, and 60%. The effect of seawater
mixing, FA, and curing conditions such as tap water curing (TC), seawater curing (SC), and air curing
(AC) on the strength performance of concrete was evaluated. Moreover, the total pore volume (porosity)
and thermogravimetric-differential thermal analysis (TG-DTA) were also observed to verify the results of
compressive strength of concrete cylinder specimens. Eight series of concrete mixtures using tap water or
seawater as mixing water were prepared by OPC and FA as a binder specified in Japan Industrial
Standard. Concrete specimens were demolded at 24 hours after casting, then cured in tap water, in
seawater, and in air (20ºC, R.H. 60% controlled room). After certain curing of 28 days and 365 days,
compressive strength was measured in accordance with Japan Industrial Standard. Based on the
investigation result, it was obtained that seawater mixing improved the strength of OPC concrete and FA
concrete up to 365 days in all curing conditions, in TC, SC, and AC. The Porosity of seawater-mixed
OPC concrete and FA concrete was decreased compared to tap water-mixed concrete. It was also found
that the mass change (weight loss) of hydration product (CH) of seawater-mixed OPC concrete was
higher than that of tap water-mixed OPC concrete in TC and AC.