Commonly investigated CCU technologies, produce either carbonated materials to be adopted in fresh mixtures (i.e. powders or aggregates) or carbonated pre-formed manufact (i.e. blocks produced via carbon curing). This study aims at exploring an innovative mineralisation process, still not deeply investigated in the literature, in which the accelerated carbonation is induced by injecting CO₂ gas in fresh mixture to produce pastes.

Cement of type 52.5 N is adopted as binder with 30% in weight substitution of Basic Oxygen Furnace (BOF) steel slag. An apparatus to directly inject the gas during the mixing process is implemented at laboratory scale and adopted to produce mineralised pastes. Carbonated samples, C-BOF-carb, are produced with a water-to-binder ratio of 0.5 and by mixing for 20 minutes with a constant CO₂ flow rate of 250 l/h. An improved configuration, C-BOF-carb+, was created by pre-carbonating BOF and by using a dispersant additive. The efficiency of the fresh mixture carbonation process is analysed by a comparison with reference samples, C-BOF-ref, both in terms of CO₂ uptake and mechanical properties of the hardened products. The experimental outcomes show a CO₂ uptake of the end-product up to 5.7%, and enhanced mechanical properties compared to non-carbonated reference samples. The promising results pave the way for further analysis aimed at improving the process and assessing its efficiency to produce mortar elements.

CO₂ emissions from the cement industry present a significant environmental challenge, prompting the development of Carbon Capture, Utilization, and Storage solutions. Among these, mineral carbonation stands out, driving the development of carbonation reactors that could enable the carbonation of Supplementary Cementitious Materials. The availability of reliable mathematical models describing the key phenomena occurring in a carbonation reactor would represent a valuable tool to simulate and optimise the behaviour of the unit operation and to provide quantitative insight on its performance. However, this is not trivial, due to the complexity of the phenomena taking place and to a general scarcity of experimental data which adds difficulty in model identification. In this study, we compare the fitting performance of different possible formulations for shrinking-core kinetic models with an approach to simplify these models, making identification easier while ensuring they remain accurate enough.

Shrinking-core models are based on particle-fluid processes and formulated by considering a network of resistance in solution-diffusion-reaction pathway. They provide a mechanistic insight into the carbonation process and are evolving based on particle geometry, rate limiting steps (surface reaction and film / product layer diffusion), and diffusion term (reaction time correlated, and non-Fickian diffusion). We used published kinetic data for wood combustion fly ash to identify these models. Our results show that R² can increase from 0.33 to 0.99 depending on the model being used – Fig.1 shows the best model performance.

A global sensitivity analysis (Sobol’s method) on model parameters reveals the dominance of activation energy across the entire reaction field, with other kinetic parameters exhibiting minor importance, regardless the models. In particular, this analysis shows a negligible sensitivity of models to pre-exponential factor, which is consequently difficult to estimate with limited experimental data. To ease model identification, we fix this parameter to nominal values for different evolutionary models and estimate the remaining parameters. Results show that the simplified model is still able to fit data with good accuracy.

In conclusion, mineral carbonation-related kinetic studies demand reliability and cost-effectiveness due to the expense associated with kinetic data. Our study demonstrates the potential to reduce the number of parameters without compromising the model reliability, and thus to exploit the information in experimental data more effectively.

Context

Cement production generates nearly 9% of global CO₂, driven by clinker burning. Substituting clinker with mineral additives (SCMs) is key to sustainable cements. Traditionally, fly ash from coal and slag were prominent SCMs, but industry shifts are reducing their availability, necessitating alternatives. Lignite fly ash (LFA), produced in significant quantities (5-10 million tons/year in Germany), emerges as a potential SCM. In addition, much larger quantities have been deposited in landfills, meaning that sufficient material will still be available beyond the planned coal exit in Germany. However, problems may occur due to free CaO, MgO, and varying sulphate contents. These components react in a cementitious environment and form damaging hydroxides.

Objectives

The ERA-MIN 3 project CO2TREAT tackles the previously mentioned challenges associated with the utilization of LFA in cement-based systems. Wet mineral carbonation is used to address the aforementioned challenges. The sample is carbonated under ambient conditions in an aqueous environment with dissolved CO₂, achieving high reaction yields. Alkaline earth oxides are transformed into carbonates, which permanently bind CO₂ and do not trigger harmful reactions in the cementitious system. In the wet process, sulfate components partially dissolve, which could reduce the formation of secondary ettringite in a cementitious environment.

However, the most significant advantage of mineral carbonation is the carbonation potential of silicate and aluminate-rich phases, such as the amorphous glass phase, akermanite-gehlenite, merwinite, melilite, ettringite, CSH, etc. In the context of carbonation, these phases can forme carbonates as well. Additionally, the remaining residues are amorphous aluminate and silicate gels that exhibit pozzolanic reactivity. This allows for the substitution of higher cement contents with fly ash, which could lead to a significantly higher CO₂ reduction potential in a further step.

Main results

A wet carbonation reactor was built capable of performing carbonation reactions between 30 °C and 70 °C, with a capacity of up to 1.8 L, and CO₂ concentrations ranging from natural conditions to 90 %. A partial factorial parameter study was conducted using this reactor principle to investigate the influencing factors to maximise CO₂ uptake. Phase development was analyzed using in situ investigation of pH, conductivity, carbon-sulfur analysis, X-ray diffraction, and R³ calorimetry. The results show that 50 to 140 kg CO₂/tonne of fly ash can be bound. Furthermore, the pozzolanic reactivity is increased by the carbonation reaction and the sulphate content is reduced.

Using steel slags as supplementary cementitious materials presents a promising opportunity to substantially reduce CO₂ emissions in the cement industry. Carbonation treatment of steel slag not only can activate its phases involving in early hydration but also has the function of carbon sequestration. This study tried to improve the carbonation efficiency of steel slag by different hydrothermal treatment methods. The chemical composition of mineral of electrical arc furnace steel slag (EAFSS) is presented in the figure. The EAF slag was put in a sealed reactor with a liquid to solid ratio of 5, and then the treatment was operated under three different hydrothermal conditions, including 24 hours at 80 ℃ (H_24h_80c), hydrothermal reaction under same temperature at 5 bar CO₂ pressure for 2 hours (LS5w_2h_80c) and the same condition in a solution of 2 mol/L acetic acid (LS5a_2h_80c). The composition of treated EAFSS was measured by Thermogravimetric analysis, X-ray diffraction and Scanning electron microscopy.

Results show that the hydrothermal reaction at 80 ℃ largely dissolved calcium from silicates, which leads to precipitation of portlandite. However, the RO (MgO-xFeO solid solution) phase is rarely dissolved even after 24 hours reaction. The hydrothermal reaction under the 5 bar CO₂ condition results in the formation of calcite. The addition of acetic acid appears to increase the dissolution of calcium silicates while inhibiting the precipitation of divalent metal carbonates, as indicated by the reduced mass loss between 600 °C to 800 °C in thermogravimetric analysis. Scanning electron microscopy images show a notable enhancement in the reaction degree of steel slag (SS) in the presence of acetic acid. The large mass loss between 300 ℃ to 500 ℃ seems to result from the decomposition of calcium and magnesium acetate. However, there is no corresponding peak detected in XRD for these two salts. Therefore, further investigation is needed to clarify the composition of hydrothermally treated steel slag under CO₂ atmosphere. Additional conditions will be explored to enhance the decomposition of the RO phase, aiming to activate these phases for use as supplementary cementitious materials.

Unlike granulated blast furnace slags, the crystalline nature of many steel slags largely limits their use as supplementary cementitious materials. One potential way of increasing their reactivity is mineral carbonation that will result in the production of Ca and Mg carbonates and a Si(Al)-gel with potential pozzolanic reactivity.
In this study a crystalline blast furnace slag and an electric arc furnace (EAF) slag were carbonated in a wet slurry reactor at 15 bar partial pressure of CO2 and 120°C for 14 days to evaluate their carbonation potential.
The mineralogic composition of the slags and CO2 uptake were determined before and after carbonation and the composition of the liquid phase was monitored during the reaction. Both slags contained significant amounts of (Ca, Mg)-(alumino)silicates. In the EAF-slag those were mainly C2S type minerals (larnite, bredigite, merwinite) with various Ca/Mg ratios (in total ~30 wt%) and around10 wt% of the alumino-silicate gehlenite. In addition, the EAF-slag contains ~25 wt% of wüstite type Fe-oxide containing also Mg and Mn. On the other side, the crystalline blast furnace slag mainly contained melilite group aluminosilicates (60 wt%), and approximately 10 wt% of pseudowollastonite.
After carbonation, the crystalline blast furnace slag contained 8.4 wt% of CO2 and the EAF slag 16.7 wt% of CO2, despite the higher alkaline earth content of the former (50 wt% and 37 wt% of CaO+MgO, respectively). This suggests that under the given conditions (Ca, Mg)-silicates are more readily carbonated than Ca-aluminosilicates. Furthermore, XRD analysis of the EAF carbonation products shows that not only Ca and Mg carbonates were formed, but also Fe containing carbonates (ankerite). Adding Fe significantly increases the pool of cations available for carbonation in EAF-slag. After carbonation the contents of larnite, bredigite, merwinite and wüstite were significantly reduced, showing that those phases were subject to carbonation. In contrast gehlenite content remained relatively constant. In the crystalline blast furnace slag only calcite was identified as carbonation product. After carbonation, the contents of melilite group endmembers (akermannite and gehleite) remained constant. Only pseudowollastonite and mixed melilite minerals appear to have been carbonated in this slag. The solution analysis showed that in both experiments, Ca and Si concentrations plateaued after 1 day, whereas Mg and Fe concentrations increased over a longer time. This suggests a need for longer reaction times to increase the solution concentration to a point at which these elements are incorporated in carbonates.
Overall, the results of the study show that Fe(II) contained in steel slags might be an essential constituent for carbonation, increasing the CO2 uptake potential of the slags and stabilizing mobile Fe(II). On the contrary, the presence of Al in many minerals of crystalline blast furnace slags appears to make its carbonation more challenging.

Lowering the direct CO2 emissions of cement production is a prime objective of the cement industry. Substitution of clinker, for instance with by-products from other industries, is a widely adopted means to reduce direct emissions. A bottleneck in lowering clinker content is the availability of known clinker-replacement materials, as beneficiation processes are often required to condition the materials for such use. A novel approach is to use mineral carbonation of steel slags as a beneficiation step for the conversion of the by-product into a supplementary cementitious material (SCM). The reaction with CO2 activates Ca-silicates by formation of calcium carbonates and reactive silica, immobilizes substances of concern and more importantly, it enables to sequester CO2 in stable mineral products.
In the ERA-MIN3 project CO2TREAT, a new direct (semi-)wet carbonation processes is developed for beneficiation of BOF steel slags. A dynamic treatment process was set up to study the main carbonation mechanisms and identify carbonation rate controlling parameters. The presentation will show the main outcomes of the process, focusing on carbonation degree and reactivity in function of carbonation time, CO2 pressure, relative humidity and temperature.