Characteristic of CO2 sequestration in steel slag wastes at elevated temperature and longer contact time

Global warming is one of the most pressing environmental problems that the world is
currently facing. Iron and steel production is a significant source of carbon dioxide (CO2) emissions,
which is a major contributor to global warming. Steel industry is responsible for approximately 8-10% of
all CO2 released into the atmosphere. The primary form of waste produced during steel manufacturing
is solid waste, specifically steel slag. Steel slag has been repurposed in many different industries for
various applications. For example, it is utilized as a substitute for raw materials in cement clinker
production, as a silicate fertilizer in the agricultural sector, and even as a material in road construction.
Furthermore, steel slag has been widely used in recent years for CO2 mineralization, and this is
believed to be a potential solution for the stabilization of solid waste. The process of CO2 sequestration
through the use of industrial solid waste offers three key advantages. Firstly, these materials tend to
have high levels of calcium content. Secondly, they generally come from sources that also emit CO2,
resulting in reduced transportation and raw material costs. Lastly, solid waste typically has low chemical
stability, meaning that the reaction phase with alkaline silicates, oxides, and hydroxides occurs readily
without the need to extract reaction ions from the solid matrix. Meanwhile, mineral carbonation using
solid wastes is considered to be an efficient method to capture and store CO2 from power plants and
industry. This research aims to study the potential of steel slag waste for direct carbon capture from the
air so as to lock it away permanently through mineral carbonation with the goal of achieving net-zero
emissions.
The methodology applied entails the testing and evaluation of two historic steel slags, Ground
Granulated Blast-furnace Slag (GGBS) and White Slag (WS) for mineral carbonation. The effects of
carbonation time, temperature, particle size, the ratio of solid to liquid, and pressure are considered,
and their influence on the amount of CO2 sequestration is examined. Both quantitative and qualitative
studies are conducted. The quantitative study analyses data from Calcimeter and thermogravimetric
analysis (TGA), which are crucial in assessing CO2 sequestration, while the qualitative study analyses
data relating to the surface properties, crystalline phases, and functional groups of steel slags using
scanning electron microscopy (SEM), X-ray diffraction analysis (XRD), and Fourier-transform infrared
spectroscopy (FT-IR). Finally, a comparison between GGBS and WS is conducted in terms of their
capacity for carbon capture and storage.
The quantitative results demonstrate that the amount of CO2 sequestered was positively affected by
increased temperature and contact time between the CO2 and steel slags. In addition, reduction in
particle size to equal to or smaller than 63 μm led to an increased rate of and overall capacity for CO₂
sequestration. Conversely, higher pressure resulted in a decrease in CO₂ sequestration. In terms of the
duration of carbonation, the WS sample showed a higher percentage of CO₂ sequestration compared
Proceedings SARDINIA 2023.  2023 CISA Publisher. All rights reserved / www.cisapublisher.com
to the GGBS sample on the first day. However, the degree of carbonation of the GGBS sample rose as
the carbonation period was extended, whereas the WS sample was not notably affected by the duration
of the carbonation process and its level of carbonation remained relatively constant over extended
periods of time. For example, from one to four days, the carbonation of the GGBS slag sample
increased by approximately 60%, reaching a value of 4% compared to an initial value of 2.5%. In
contrast, for the WS sample there was a relatively moderate increase of up to 22% during the same
period, resulting in a final value ranging from 23% to 28%. Regarding the effect of temperature, the
lowest levels of sequestration occurred at 20 ± 2 °C, with the GGBS and WS samples recording
sequestration values of 3.5% and 27.5% respectively. However, when the temperature was increased
to 90 ± 2 °C, levels of sequestration rose to values of 13% and 29% for the GGBS and WS samples
respectively. A negative relationship between pressure and CO2 sequestration was also found, and the
findings indicate that both GGBS and WS slags attained maximum CO2 sequestration of 14% and 30%
respectively at a pressure of 10 bar. However, when this was raised to 50 bar, sequestration levels fell
to 13% and 26% for GGBS and WS respectively.
The qualitative analysis showed that the structure of GGBS and WS changed, indicating the formation
of magnesium or calcium calcite. SEM images of the steel slags before carbonation reveal a rough and
heterogeneous microstructure with a dense surface. However, following the carbonation process, the
surface of the slag particles becomes smoother due to the development of a coating of CaCO3 and
MgCO3 layers characterized by cubic structures on the surface. With respect to the FT-IR spectrum, the
two main functional groups of aluminosilicate and carbonate were assigned to both GGBS and WS
samples. The symmetric stretching of the O-C-O bonds of the carbonate group at 1420-1520 cm-1
gradually increased after carbonation, representing the existence of calcium and magnesium calcites in
the system. The findings of the study were supported by XRD analysis, showing that calcite peaks were
present in the carbonated samples. The XRD analysis conducted before carbonation indicates that the
GGBS and WS slags are complex and amorphous, with many mineral compounds present. The silicate
compounds identified included Ca2SiO4, Ca2Al(AlSiO7), and Ca3Mg(SiO4)2, which are known to be
commonly found in slags. Additionally, the samples contained sulphates such as CaSO4.2H2O and
K2Ca(SO4)2·H2O.
The outcomes of this research offer important knowledge about how to enhance the process of CO2
sequestration utilizing steel slag waste, and this will aid in the development of sustainable technologies
that can minimize greenhouse gas emissions from the steel industry.