This paper discusses the variation of pH and the solution of CaO and SO3 in the sea water, into which the hardened concrete specimens are immersed. The experimental factors are as follows; kind of cement, water cement ratio, exposed period of specimens to the air, ratio between the sea water volume and the surface area of specimens and so forth.
　Test results indicated that pH of the sea water increased to the following order, that is, normal portland cement, A-type blast furnace slag cement, B-type blast furnace slag cement. Though pH came up to the maximum value within shorter days in the case of concrete of lower water cement ratio, the maximum value was not distinguishable provided that water cement ratio ranged from 45 to 62 percent. The exposed period showed more influencial factor rather than concrete materials and
mix proportioning. When the sea water volume per the concrete surface area was 8.9 cc/cm2, the maximum value of pH decreased to 8.8 under the condition that the sea water changed every day. It was, therefore, likely that the hardened concrete immersed in the sea water did make little contribution to the increase of pH at the natural condition.