The compressive strength, the amount of Ca(OH)_2 and the pore structure of the concrete which contained large amount of fly ash as a part of fine aggregate were investigated experimentally to provide useful information on the establishment of the rational mix designing for high volume fly ash concrete. The experimental results showed that the compressive strength of the concrete increased in early age as well as in long term age with the increase of the content of fly ash which filled the voids between particles and prevented Ca(OH)_2 from big crystallization. The compressive strength of the oncrete was highly correlated with the cement-to-water ratio in the age of 28 days, on the contrary with the binder-to-water ratio in the age of 1 year. The increase of the compressive strength of the concrete in long term age, which was caused by pozzolanic reaction, was considered to result from the reduction of the amount of Ca(OH)_2 rather than the reduction of the pore volume
