Mineralization of CO2 using a Ca-rich metallurgical slag is known to be a promising approach to controlling CO2 emissions while converting slag by-products into valuable materials. However, untreated stainless steel slag (SSS) cannot straightforwardly be used for carbon capture and storage (CCS) or rather carbon capture and utilization (CCU) since there is a potential risk of toxic chromium release. For the dual goals of chromium immobilization and calcium recovery of SSS, four principal Ca-bearing phases of glass, dicalcium silicate, merwinite, and melilite were prepared and employed in order to investigate their decomposition behavior. Based on the experimental results, the morphology variations of the studied phases were revealed and the stability order was proposed as follow: glass > melilite > merwinite > dicalcium silicate. A MnO-based modification was conducted, and the effect of MnO content on the phase transformation and element distribution of SSS was investigated. Experimental results indicated that a proper increase of MnO content is beneficial for the calcium enrichment in target phases and chromium stabilization in a spinel phase. The structure and phases modification mechanism induced by added MnO is studied and reported in this work. (C) 2019 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.