Steel slags contain high calcium and silica content and are reactive with carbon dioxide.This characteristics can be utilized to develop strength.On the other hand, production of ordinary Portland cement (OPC) is an energy-intensive and CO2-emitting process.If steel slag can be activated to serve as a cementing binder to replace OPC, its building products will show significantly reduced embodied energy and process emission.This paper examines the potential of using carbon dioxide activated steel slags to produce high strength low-emission cementing binder.Steel slag from KOBM (Klockner Oxygen Blown Maxhutte) process was examined.It was found that de-ironized KOBM slag was more carbon-reactive than the traditional slag with higher iron.Two-hour carbonation can promote carbon uptake to 12% based on dry slag mass.Two-hour carbonation strength was comparable to that of OPC.It was 80 MPa in carbonated KOBM slag versus 88 MPa in carbonated OPC.However the early hydration strength was very low in hydrated slag.The one-day hydration strength was 1 MPa in ground slag against 48 MPa in OPC.Nevertheless the slag could gain significant hydration in subsequent hydration after 2h carbonation.The combined strength at 28 days was 109 MPa in ground slag versus 94 MPa in OPC.The slag could develop more strength than OPC in subsequent hydration.Quantitative XRD showed that de-ironized KOBM slag comprised of 66% C2S, of which 46% was γ-C2S and 20% β-C2S.QXRD also indicated that carbonation consumed most of γ-C2S, leaving β-C2S for subsequent hydration.The excellent performance of carbonated steel slag was attributed to the deironization of the slag.Carbonation of slag is a CO2 uptake process.Carbon-activated steel slag binder reduces the embodied energy, reduces carbon footprint, preserves natural resources and possibly leads to a carbon-negative clinker-free concrete product.