Direct carbonation of porous materials produced from self-hardened paper mill fly ash

A high-Ca fly ash generated from a paper mill using a fluidized bed boiler was used as the potential medium to sequester CO2 via the direct carbonation process. In this study, the fly ash samples were exposed to accelerated carbonation to produce hardened pastes. The influence of the porosity controlled by the amount of foam in the matrix for maximizing CO2 sequestration was determined. The effect of porosity on carbonation was evident. The samples prepared with 40% foam and a water-to-fly ash ratio of 0.79 recorded porosities of 56.1% (carbonated) and 66.8% (naturally carbonated (NC)) with the lowest mechanical strength of 2.1 and 1.1 MPa, respectively. These naturally carbonated (NC) samples have been cured naturally without using high CO2 pressure. Meanwhile, the samples prepared with 10% foam and a water-to-fly ash ratio of 0.5 recorded porosities of 41.1% (carbonated) and 52.9% (NC) with the highest strength at 10.2 and 4.6 MPa, respectively. The highest carbonation efficiency determined by thermogravimetric analysis was obtained from the carbonated samples with 40% foam. Scanning electron microscopy–energy-dispersive spectroscopy depicted the areas with high Al/Si content but low Ca–O content (dark areas) and carbonated areas with high Ca–O content (bright areas). Mercury intrusion porosimetry was used to determine the pore size distribution, whereby the samples were dominated by large capillary pores, resulting in their high porosity in this study.