Life cycle assessment of upcycling waste slag via CO2 pre-treatment: Comparative study of carbonation routes

Accelerated carbonation of industrial by-products such as yellow phosphorus slag (YPS) and basic oxygen furnace slag (BOFS) is an effective method to convert waste into valuable construction materials. Accelerated carbonation or CO2 sequestration can be conducted directly under dry or aqueous conditions. However, the slow reaction and low CO2 fixation efficiency of dry carbonation hinder the successful development of accelerated carbonation, whereas water consumption is the key challenge of aqueous carbonation for industrial applications. Therefore, this study evaluated the environmental impact of carbonation of YPS and BOFS via dry and aqueous routes, and upcycling these CO2-treated waste slags as cement replacements. The environmental impacts, including midpoint and endpoint impact, were quantified using life cycle assessment (LCA) by applying the ReCiPe methodology. The LCA results show that aqueous carbonation generates lower carbon emissions (11.3% and 214.0%) and human carcinogenic toxicity (2.4% and 0.2%) than dry carbonation for YPS and BOFS, but is accompanied by more serious impacts on mineral resource scarcity and water consumption. On the other hand, aqueous carbonation of BOFS generates lower (or even negative) carbon emissions than aqueous carbonation of YPS, and the same results can be found for human carcinogenic toxicity, mineral resource scarcity and water consumption. Sensitivity analysis shows that prolonged carbonation, particularly in the case of dry routes, results in very high carbon emissions. Apart from carbon emission benefits, using 20% aqueous carbonated YPS and 20% aqueous carbonated BOFS as cement replacements exhibited minimal impact of mechanical strength reduction, as compared with pure cement paste samples.