Influence of Mineralogy and Process Parameters on Microstructure Development and Mechanical Properties of Carbonated Compacts

Wet accelerated carbonation is a suitable treatment to reduce carbon dioxide emissions in the atmosphere and to valorize calcium-rich industrial alkaline residues derived from steel making plants. These materials are chemically unstable, so they tend to react relatively quickly with concentrated CO₂ and are available at low costs and in large quantities close to the main CO₂ emission points, avoiding also costs related to their transport.

This work shows the results of an experimental study of the effects of precursor properties and process parameters on the development of the carbonated microstructure. In particular, it focused on the influence of mineralogy and operating parameters on microstructure development and mechanical properties of carbonated compacts produced from steelmaking slags.

The experiments were carried out on Basic Oxygen Furnace (BOF) slags, collected downstream the iron steel recovery and on Electric Arc Furnace (EAF) slags. Beside, a model system was prepared employing commercially available minerals such as Wollastonite (CaSiO₃), Belite (Ca₂SiO₄), Alite (Ca₃SiO₅) and Portlandite (Ca(OH)₂), indicated in the cement chemistry respectively as CS, C₂S, C₃S and CH, considered as reference materials because of their high content in the analysed slags.

The starting materials were characterised from a physical and chemical point of view, in terms of the particle size, chemical composition, inorganic carbon content and mineralogical phases, before the carbonation treatment.

The sample, in the form of powder, was mixed with a fixed amount of water, equal to 10%, and left to react for 24 h in order to complete the hydration process of free lime impurities; after this time, the powder was pressed in pellets and put in a lab-scale carbonation reactor.

Subsequently, the wet-route for carbonation was performed using a liquid to solid (L/S) ratio of 1/10 by weight and operating at mild operating conditions, in particular the temperature was below 50°C and the gas pressure below 10 bar. The operating parameters during the tests were systematically varied to investigate their influence on the carbonation rate. The percentage of CO₂ in the gas flow ranged from 10% till 100%, the gas pressure was set at 1.3 bar or 10 bar, the temperature of the reactor was varied between 25°C and 50°C and the reaction time was extended from 15 min to 480 min, in order to reconstruct the carbonation kinetic of the material.

The products collected at the end of the test were characterized in terms of CO₂ uptake by TIC analysis and oven procedure, changes in the mineralogical phases by XRD analysis and morphology by SEM-EDX analysis and mechanical strength by compressive test.

The results indicate an important impact of the percentage of CO₂ in the gas flow, the reaction time and the mineralogy of the as-received slags. The maximum CO₂ uptake values obtained were 10-12%, i.e. 100-120 g CO₂/kg Slag, to which correspond a maximum compressive strength equal to 16 MPa and 60 MPa, respectively for the BOF and EAF slags, with a flow of pure CO₂, at 50°C and 10 bar, for 240 min.