The solution to the choice of mineral for sequestration must be one that is not mined or produced thermally and can be reacted ex-situ since this allows for validation of the storage capacity in the long term. The ZAP-C 3 process is designed to employ a waste mineral source that requires minimal processing: basic oxygen steelmaking (BOS) slag. 20 million tons of BOS slag is produced annually just in Europe as a by-product of a secondary steelmaking process when pig iron is refined into steel . Additionally, millions of tons of legacy slag are present at current and former steel works representing a major environmental hazard. The ZAP-C 3 process is positioned to consume a waste material, including legacy material, during CCS because BOS slag contains 30-50% CaO and 10% MgO making it an excellent mineral for sequestration . Previous approaches for the use of BOS slag in CCS have required increased temperature or a CO2 over-pressure to ensure significant reaction .