Carbon emissions in the construction industry are increasing now. Against the backdrop of environmental factors and low-carbon economy, the research on reducing carbon emission in cement industry has garnered widespread attention. During the cement production procedure, the decomposition of carbonates during the calcination of raw materials is the main source of CO₂ emissions. Therefore, reducing the calcium content in cement clinker is expexted to significantly reduce the carbon emissions, thereby achieving energy conservation and emission reduction in the industry. Unlike the C₃S mineral in ordinary Portland cement clinker, CS, C₃S₂, and γ-C₂S are three types of low calcium silicate minerals, but they are all non hydraulic minerals that cannot obtain a certain mechanical strength through traditional cement hydration. Former research revealed that the carbonation activity is higher than the hydration activity of above three low calcium minerals, and cement products can be prepared through carbonation curing.

To solve the problems of high energy consumption and high CO₂ emissions in the production of ordinary Portland cement, low calcium silicate minerals (CS, C₂S, and C₃S₂) with carbonation activity are used to replace C₃S minerals. The strategy could not only reduce carbon emissions in cement production, but also enhance utilization of carbon. Research has found that incorporating β-C₂S into the low calcium system of γ-C₂S can significantly enhance the mechanical properties of the system. In most reported research work, the single minerals are often blended after firing, and then the carbonization properties of the mixed minerals are studied. How to burn the C₂S minerals with the optimal crystal ratio in one step and to achieve the symbiosis of γ-C₂S and β-C₂S in the system remains a challenge. This work focuses on C₂S minerals. By changing the calcination temperature, C₂S minerals with different crystal ratios are treated in one step, and the relative content of γ-C₂S and β-C₂S in low calcium cement clinker is regulated. The symbiotic mechanism of the two and the influence of different mineral contents of β-C₂S and γ-C₂S on the carbonization performance of the material are studied. The carbonization products and mechanisms are explored, providing a theoretical basis for the application of low calcium fixed carbon cement in carbonization and new ideas for the carbonization research of C₂S minerals. which has certain guiding significance for energy conservation and emission reduction in the cement industry.

This work used limestone and sandstone to obtain low calcium cement clinker with different proportions of γ-C₂S and β-C₂S by adjusting the calcination temperature. The carbonization and hardening mechanism of low calcium cement clinker was analyzed using thermogravimetric analysis, X-ray diffraction, pH value, conductivity and other testing methods. The results showed that the molar ratio of γ-C₂S/β-C₂S was between 0.19 and 2.63. As the ratio of γ-C₂S/β-C₂S decreased, the compressive strength of low calcium cement clinker after 24 hours of carbonization gradually increased. When the calcination temperature was 1340 ℃ and the ratio of γ-C₂S/β-C₂S was 0.19, the compressive strength of the system reached 163.32 MPa. The change in CO₂ absorption of the sample is related to the carbonation activity of calcium silicate. The early carbonization reaction of the experimental group, mainly composed of γ-C₂S minerals, is severe. There is a phenomenon of incomplete reaction in the test block. The experimental group mainly composed of β-C₂S minerals has a longer duration of exothermic reaction, which is more conducive to the progress of carbonization reaction. Among them, the heat released by γ-C₂S during the carbonization process promotes the easier dissolution of Ca²⁺ from β-C₂S, facilitating the carbonization reaction of β-C₂S. The addition of a small amount of γ-C₂S (i.e. 8.13%) has a positive effect on the carbonization degree and strength of β-C₂S. In addition, the dissolution of β-C₂S leads to a gradual increase in the pH value of the suspension, changing the liquid phase environment and accelerating the dissolution rate of γ-C₂S in water under alkaline conditions. The excellent compressive strength of the sample is attributed to its high degree of carbonization and dense microstructure. The higher the content of β-C₂S minerals (γ-C₂S/β-C₂S ratio=0.19), the better crystallized calcite type calcium carbonate can be observed after carbonization, presenting a stacked and dense morphology. The samples mainly composed of γ-C₂S minerals have weak particle bonding and loose structure after carbonization, showing amorphous calcium carbonate with many pores.

Therefore, it can be concluded that the synergistic carbonization of γ-C₂S and β-C₂S mainly exists in two processes: the dissolution process before CO₂ is introduced, and the carbonization reaction process after CO₂ is introduced. Due to the action of water, β-C₂S preferentially dissolves Ca²⁺, changing the liquid-phase environment around γ-C₂S particles and promoting Ca²⁺ dissolution. After the introduction of CO₂, the carbonization reaction of γ-C₂S becomes more intense, releasing a large amount of heat that accelerates the dissolution of β-C₂S minerals and promotes their carbonization reaction. The final carbonized product is mainly composed of stacked calcite, with a small amount of high polymer silica gel interspersed to bond the calcite. Therefore, the high content of β-C₂S carbonized product shows a more tightly aggregation of particles and thus better performance.

Carbon dioxide emissions from cement kiln flue gas constitute a significant contributor to global climate change and environmental pollution. As global emission reduction targets progress, research and application of carbon capture, utilization, and storage (CCUS) technologies have increasingly become critical strategies for mitigating CO₂ emissions. Among these technologies, the calcium cycle method demonstrates substantial potential due to its low cost, high efficiency in CO₂ trapping, and broad applicability, particularly in the context of cement kiln flue gas. However, traditional calcium-based materials, such as limestone, face challenges including high precalcination temperatures and resource wastage, highlighting the need for more efficient and sustainable alternatives.

Calcium carbide slag, an industrial by-product, has shown significant promise as a material for the calcium cycle method. It contains a high proportion of reactive CaO, along with inert components such as alumina and magnesium oxide. These inert components enhance resistance to sintering after preburning. However, repeated use of calcium carbide slag leads to pore structure collapse, particle agglomeration, and sintering, severely diminishing its adsorption performance and posing a significant barrier to practical application.

To address these challenges, various modification techniques have been explored to improve the absorption efficiency and sintering properties of calcium carbide slag, as well as to mitigate the impact of SO₂ and NO_x in flue gas on CO₂ absorption.

Organic acid modification has been identified as an effective strategy to enhance the adsorption performance of calcium carbide slag. Through reactions with calcium hydroxide to form organic calcium salts, pyrolysis at high temperatures releases small molecules that disrupt the original structure of the slag. This process increases porosity, specific surface area, and CO₂ adsorption capacity. Different organic acids, due to their distinct molecular structures, yield variations in the molecular weight of organic calcium salts and the types and quantities of small molecules released during pyrolysis. Consequently, modifying calcium carbide slag with specific organic acids allows tailoring of its pore structure to enhance CO₂ adsorption performance.

Doping modification technology is another effective approach. By incorporating various substances into the slag, the properties of calcium carbide slag can be optimized through different chemical reaction mechanisms. Inert materials serve as structural frameworks, inhibiting CaO grain migration and growth to improve anti-sintering performance. Doping with oxygen-deficient materials facilitates CO₂ diffusion and O^2- migration, while potassium and sodium salts increase defect concentrations in the CaCO₃ product layer, enabling more efficient Ca²⁺ migration and enhanced CO₂ absorption.

Hydration processes also play a critical role in influencing sintering. While CaCO₃ typically decomposes at high temperatures, introducing water vapor during calcination reduces the partial pressure of CO₂, promoting its conversion to CaO. Water vapor also shortens decomposition residence time and slows sintering, delaying sintering deactivation and improving stability. By optimizing calcination temperatures and water vapor concentrations, the CO₂ adsorption performance and stability of calcium carbide slag can be enhanced over multiple cycles.

In the context of cement kiln flue gas treatment, SO₂ and NO_x pose additional challenges to the CO₂ trapping performance of calcium carbide slag. SO₂, being strongly acidic, preferentially reacts with CaO to form dense CaSO₄ layers, which accumulate over cycles, diminishing adsorbent activity. Research suggests that specific modification methods or adjustments in the absorption sequence (e.g., absorbing CO₂ before SO₂) can mitigate these effects. Regarding NO_x, calcium carbide slag inherently lacks reductive properties and cannot remove NO_x through traditional calcium cycle methods. Doping with reducing substances, such as copper, iron, and other metal oxides, can enable NO_x reduction by promoting reactions that convert NO_x into harmless nitrogen and oxygen.

The future of calcium carbide slag modification technology is expected to move towards systematic and refined strategies. These strategies can leverage the synergistic effects of multiple modification techniques to optimize performance across different reaction stages. As research progresses, the multifaceted applications of calcium carbide slag will gain increasing recognition. Beyond its role in CO₂ capture, it holds significant potential for processes such as desulfurization and nitrogen removal.

From an environmental perspective, utilizing calcium carbide slag addresses resource wastage while aiding the cement industry in achieving a green transformation by reducing greenhouse gases and pollutants such as CO₂, SO₂, and NO_x. Continuous innovation in modification technology will ensure that calcium carbide slag not only plays a pivotal role in CO₂ capture but also in mitigating pollutants, contributing to the attainment of global emission reduction goals. The enhanced versatility and cyclic stability of modified calcium carbide slag will further support the green transition of the cement industry and other high-emission sectors, contributing to efforts toward a sustainable future.