Alkali Activation and CO2 Curing for Bof Slag Valorization

The slag generated in steel manufacturing plants is an alkaline residue, mainly constituted by calcium or magnesium based alumino - silicates, that may contain also Ca hydroxides and thus typically presents latent hydraulic properties. Recently, experimental works on blast furnace slag, which is typically used in concrete manufacturing as a partial replacement of Portland cement, showed that alkali activation can prove to be an effective treatment to stimulate its latent pozzolanic properties and to obtain a concrete with improved mechanical performance. Alkali activation is a chemical process in which the raw material, usually composed of silicates and aluminates, is mixed with specific alkaline solutions (e.g. alkalis, carbonates, silicates) in order to enhance hydration reactions leading to the formation of a compact and strong product. The composition of the activators varies depending mainly on the treated material, although it has been generally observed that a mixture of sodium hydroxide and sodium silicate is the activating solution that provides the best formulation in terms of the strength of the product. Besides, as for all cementitious materials, a curing stage is also needed in order to allow the completion of the hydration reactions that leads to the strength gain of the material. This step may be carried out at different operating conditions, including CO₂ rich atmospheres, which were shown to provide results similar to steam curing.

This work investigates the performance of alkali activation and CO₂ curing on a Basic Oxygen Furnace (BOF) slag sample in terms of its effects on the mineralogical and mechanical properties of the obtained material and the extent of CO₂ storage. To this aim BOF slag, sampled downstream the milling unit for iron recovery of a steel making plant, was blended with two different alkali solutions: a) a mixture of sodium hydroxide and sodium carbonate and b) a solution of sodium hydroxide and sodium silicate. Different concentrations and molar ratios were tested. The isothermal conduction calorimetry at 20 °C was carried out on the obtained pastes in order to explore the hydration kinetics of the alkali activated BOF slag. The highest heat flow peaks and cumulative heat releases, indicating the occurrence of exothermic hydration reactions, were obtained either with a) 4M NaOH and 2M Na₂CO₃ solution, in a 3:1 w/w ratio, or b) 2M NaOH mixed in a 1:1 w/w ratio with a Na₂SiO₃ solution presenting a 3.4 silica modulus. These optimal formulations were then used to prepare BOF slag pastes and mortar cubes that were cured for 3, 7 and 28 days, in either a humidity chamber (RH=90% and T=20 °C) or a carbonation chamber (RH=75%, CO₂=5%) at both ambient (T=20 °C) and enhanced conditions (T=50 °C). At all tested conditions, the Differential Thermo-Gravimetric (DTG) analysis results showed the possible presence of calcium silicate hydrate and an increasing concentration of calcite in the solid product for increasing curing times, that was confirmed also by XRD analysis. In addition, significant occurrence of gaylussite (Na₂Ca(CO₃)₂·5H₂O) at early curing times and low curing temperature was observed mainly with the sodium carbonate solution. A maximum CO₂ uptake value of 60 gCO₂/kg was achieved after 28 days of curing under both ambient temperature and 50 °C for the BOF pastes treated with the mixture of sodium carbonate and sodium hydroxide. At ambient curing temperature, a comparable value (52 gCO₂/kg) was measured also for pastes treated with sodium silicate, although, in this case, a decrease of around 15% was detected at 50 °C. The compressive strength of the mortar cube activated with sodium carbonate showed to be negligible even after 28 days of curing in the carbonation chamber at ambient conditions. A compressive strength value of 2 MPa was obtained after 7 days of curing in the carbonation chamber at 50 °C for mortar samples prepared with the sodium silicate solution; a similar value was obtained also after 28 days of curing. SEM analysis of BOF slag paste samples and the porosimetry and leaching behaviour of BOF mortar cubes are currently being investigated.