Effect of accelerated carbonation on AOD stainless steel slag for its valorisation as a CO2-sequestering construction material

• AOD slag can be valorized as building material by mineral carbonation.
• Depending on the process strengths of more than 30 MPa can be achieved.
• XRD analysis show formation of amorphous carbonation products.
• SEM analysis shows different morphology of calcite at different pressures.
• Leaching of several heavy metals is reduced on carbonation.

Non-stabilized Argon Oxygen Decarburisation (AODNS) slag in powdered form was examined for its carbon dioxide sequestration capacity and for its potential utilisation in the fabrication of high value building materials. The curing of the sample was carried out in two accelerated carbonation environments: (i) in a carbonation chamber, maintained at atmospheric pressure, 22 °C, 5 vol.% CO2 and 80% RH; and (ii) in a carbonation reactor, where the CO2 partial pressure (pCO2) and temperature could be further increased. In the carbonation chamber, an average compressive strength of over 20 MPa, on a 64 cm3 cubic specimen, was obtained after one week of curing, which is sufficient for many construction applications. Further carbonation resulted in a linear increase of strength up ∼30 MPa after three weeks. The CO2 uptake followed a similar trend, reaching a maximum of 4.3 wt.%. In the reactor, the compressive strength improved with an increase in pCO2 up to 8 bar, temperature up to 80 °C, and duration up to 15 h where the maximum CO2 uptake was 8.1 wt.%. The reduction in porosity in the carbonated specimens was approximately in line with the strength gain in the samples. Phase analysis by X-ray powder diffraction and inspection by scanning electron microscopy showed the precipitation of calcite and formation of significant amounts of amorphous material after carbonation. Infrared spectroscopy also pointed to the presence of aragonite and vaterite. In the carbonation chamber, the calcite morphology was uniform throughout the specimen. In the reactor, however, the calcite crystals near the outer edges of the cubes had different morphology than those near the core. Carbonation of the slag resulted in the reduction of basicity by up to one pH unit, and contributed to controlling the leaching of several heavy metals and metalloids.