Fly ash is a pozzolanic material an

Fly ash is a pozzolanic material and a by-product from combustion of pulverized coal in an electricity power plant. Characteristics of fly ash vary due to coal type and combus-tion condition. In Thailand, it is estimated that since 2008, more than 3.5 million tons of fly ash has been produced an-nually. However, only half of this has been utilized. The majority of fly ash is from pulverized coal combustion from Mae Moh Power Station in the north. Recently, however, a 684 Int. J. Miner. Metall. Mater., Vol.17, No.6, Dec 2010 number of fluidized bed coal combustion power plants have been put into service, and as a consequence, its fly ash is in-creasing. Utilization of this fly ash is still limited due to the lack of understanding on the characteristics of fly ash itself and the properties of fly ash concrete. Although many re-searchers investigated the influences of fly ash on the poros-ity and air permeability of blended cement pastes, few re-searchers were found dealing with the fineness and shape of fly ash and their influences on the properties of hardened cement pastes. Understanding the effects of the fineness and shape of fly ash on the porosity and air permeability of ce-ment pastes will lead to an increasing use of fly ash in con-crete. 2. E