Spent fluid catalytic cracking (FCC) catalysts, by-products from oil-cracking refineries, were evaluated as pozzolanic admixtures of concrete. In this study, the activity of two spent FCC catalysts, i.e., Ecat and Epcat, were examined and compared. The pozzolanic activity was indicated by their activity index in presence of Portland cement and the consumption of calcium hydroxide determined by DSC measurements. The effect of these two catalysts on the compressive strength of mortars was also investigated. The results were compared to those of silica fume. It was found that both Ecat and Epcat, like silica fume, show good pozzolanic activity and are reactive with CH. Furthermore, Epcat possesses much smaller particle size than the other waste catalyst. Therefore, Epcat provides a filling effect on the microstructure and enhances the compressive strength of the resulting mortars. The performance of Epcat is close to or slightly better than that of silica fume. Compared to the control mortar (W/B=0.42) cured at between 3 and 28 days, mixes with 5-15% cement replacement by Epcat increased the compressive strength by 10-36%.
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