Carbonation-Granulation of Alkali Activated Steel Slag for CO2 Storage and Production of Recycled Aggregates

Large amounts of alkaline solid residues are generated during steel manufacturing; among these, BOF slag, a by-product of the conversion of iron into steel in Basic Oxygen Furnace units, is one of the most abundant. Commonly, this material is ground for metal recovery and then landfilled or limitedly reused for low-end applications because some of its properties (i.e. high free lime content, fine particle size and excessive mobility of specific elements) may negatively affect its technical performance and/or environmental behaviour. Several studies have indicated that accelerated carbonation may be an effective treatment for improving the environmental and mechanical properties of an alkaline material such as steel slag. However, a specific grain size distribution is required in order to reuse carbonated steel slag as aggregates in civil works. To this aim, the coupling of carbonation with a granulation treatment may prove to be a particularly interesting process for producing recycled aggregates from industrial residues, while also storing some of the CO₂ emitted by the industrial process in a solid and stable form. Granulation represents in fact a particle size enlargement process, which is achieved by contacting a mixture of a dry powder and a liquid binder in a dynamic device, such as a disc granulator or a rotating drum.

Indeed, previous findings of our research group indicated that the combination of carbonation with a granulation process employing only water as binder applied to Basic Oxygen Furnace (BOF) steel slag was able to produce aggregates with mean diameters ranging from 4 to 10 mm. These tests were carried out in a laboratory-scale granulation device equipped with a lid and a CO₂ feeding system. In each test, humidified slag (liquid/solid ratio of 0.12 l/kg) was treated at ambient temperature for reaction times from 30 to 120 min under 100% CO₂. Even under the mild conditions applied, a significant CO₂ uptake (120-144 gCO₂/kg) was attained after 30 min treatment for granules with a diameter below 10 mm and after a reaction time of 60 min for the coarser fraction. The analysis of the mineralogical modifications of the slag occurring upon the treatment indicated that calcium hydroxide was the main reacting phase with CO₂ and calcite the primary reaction product. As for the environmental behaviour of the treated material, particularly for the coarser granules, a noteworthy pH reduction, a decrease of Ca and Ba mobility and an increase of V leaching compared to the untreated slag were observed. Overall, hence, the process appeared to be a promising treatment for steel slag valorization; however, the most critical factor for the use of the manufactured granules in construction applications proved to be their poor mechanical strength compared to that of natural aggregates.

The current work aims to investigate the effects of the above mentioned treatment employing as binder a sodium silicate and sodium hydroxide solution with the aim of enhancing the mechanical properties of the obtained aggregates by alkali activation. The tests were carried out in the same set-up employed for the previous experiments and treating another batch of ground BOF slag sampled from the same steelmaking plant, but presenting a higher content of Ca-silicate phases and a lower concentration of calcium hydroxide compared to the previously tested slag. In each test, around 500 g of dry slag was mixed with different concentrations of alkaline solution at set liquid to slag ratios in the granulation device under atmospheric air or a CO₂–rich atmosphere for reaction times of 30 min or 1 h. The effects of the treatment were assessed in terms of the physical properties, CO₂ uptake, mineralogy, leaching behaviour and mechanical properties of the obtained granules; the latter evaluated applying the Aggregate Crushing Value (ACV) test. In addition, the effect of applying different curing conditions (including high CO₂ concentrations) on the mechanical strength of the granules was also investigated. The results of these tests will be presented and discussed in the full paper.