The effects of supercritical carbonation on fly ash (FA) separated according to particle size and raw cement powder were investigated. ASTM Class C fly ash containing a high content of CaO produced from a fluidized bed-type boiler and normal portland cement were used as raw materials. This type of FA is used to carbonate mortar under supercritical conditions for CO2 fixation because it cannot be recycled and simply reclaimed because the high CaO content may adversely affect environmental friendliness as a result of its volume expansibility. Specimens of various particle sizes of FA and cement (the particles were separated in steps that varied from as received to less than 45 mu m in 4 steps), were prepared with distilled water, aged for 3, 7, and 28 days, and subsequently, carbonated in a supercritical CO2 atmosphere at 80 kgf/cm(2) pressure and at 40 degrees C for 60 min. As a result, an improvement in the mechanical properties of the specimens was expected because CaCO3 was produced in the specimens, which fills the cracks, leading to an enhancement in the mechanical strength. However, because the vacant sites were filled with the reaction products, the average particle size plays an important role in the final compressive strength of the specimen that was carbonated under the supercritical condition. This study investigates the relationship between the average particle size and the mechanical strength after carbonation.

Keywords: Fly ash; Supercritical condition; Carbonation; Particle-size separation