The global greenhouse effect is escalating, leading to the progressive deterioration of ecosystems and climate worldwide. As the primary greenhouse gas, reducing CO₂ emissions is crucial for effectively mitigating this effect. Fly ash–CO₂ mineralization and sequestration technology represents a promising approach for carbon fixation and emission reduction. However, the low carbonation efficiency of fly ash–CO₂ remains the central constraint hindering effective CO₂ mineralization and sequestration. Thus, elucidating the microscopic mechanisms and key influencing factors of fly ash–CO₂ carbonation is essential toward addressing this limitation.

To investigate these issues, a custom-designed setup was used to conduct experiments, including conventional immersion, immersion–carbonation, and microstructural characterization tests to examine the effects of immersion time on alkaline metal ion leaching and diffusion, as well as the impact of the microstructures of fly ash and CO₂ on the carbonation efficiency.

Results show that the pH increase rate exhibited a negative exponential decay relative to immersion time. After 24 hours of immersion, the carbonation efficiency and sequestration capacity reached peak values of 12.855% and 12.91 kg/t, respectively, representing a 0.25-fold increase over non-immersed fly ash. Raw, unmineralized fly ash contains amorphous silica (SiO₂), mullite (Al₂SiO₅), and amorphous silica hydrate (SiO₂·xH₂O). No diffraction peaks were detected for calcium carbonate, calcium hydroxide, or magnesium hydroxide, confirming the absence of calcium carbonate in the original sample. Conversely, the mineralized sample contained phases such as amorphous silica hydrate (SiO₂·xH₂O), quartz (SiO₂), mullite (Al₂SiO₅), calcium carbonate (CaCO₃), and hydroxides (Ca(OH)₂ and Mg(OH)₂). The absence of the magnesite diffraction peak indicated that magnesium did not participate in the mineralization reaction. Furthermore, the observed low-intensity diffraction peaks were broad, indicating low sample purity and small crystal size, confirming the predominantly amorphous composition of the matrix. The diffraction-peak intensity of calcium carbonate initially increased and then decreased with increasing immersion time, with the maximum mass fraction (1.10%) observed at the 24-hour mark. Raw fly ash particles were spherical and dispersed. By comparison, the carbonated samples exhibited agglomeration and cementation, peaking at the 24-hour mark, with amorphous calcium carbonate deposited on the particle surface. Additionally, the carbon content of the carbonated fly ash increased with immersion time (within 24 hours), indicating a higher carbonation degree. The frequencies of larger and smaller particles increased and decreased, respectively, with the immersion time (≤ 24 h), suggesting positive and negative correlations between larger and smaller particles and the carbonation degree, respectively.

This study investigated the leaching and diffusion characteristics of fly ash–derived alkaline metal ions and their impacts on the microphysicochemical properties of fly ash, as well as their relationship with CO₂ mineralization efficiency. Through a series of experiments involving fly ash immersion across different durations, followed by mineralization tests and microstructural characterization of the mineralized fly ash, the influences of soaking time on the leaching and diffusion behavior of alkaline metal ions were evaluated. Additionally, the effects of leaching on the microstructure of fly ash, as well as the efficiency of CO₂ mineralization, were examined. Overall, these findings provide theoretical guidance for optimizing reaction parameters and enhancing mineralization efficiency in fly ash–CO₂ mineralization processes.

As the main energy source in China, coal can be considered the “ballast stone” in energy security. However, as the depth of coal mines increases, the gas pressure and content in the coal seam increase, and the risk of gas-related disaster increases. Current technologies for pressure relief and permeability enhancement (such as dense drilling, hydraulic punching/slotting/fracturing, etc.) face problems such as a large engineering volume and a limited permeability enhancement range. In contrast, hydraulic jet tree drilling can form a large-scale fracture network, which is effective for achieving uniform permeability of coal seams. Under laboratory conditions, it is the basis of the development of this technology to clarify the rock breaking mechanism of the stepped nozzle bit. This study is also of great significance for deepening students’ understanding of the theory of water jet rock breaking and its application in engineering, and improving their analytical skills in engineering and innovation ability.

A four-dimensional water jet comprehensive test system developed in-house was used to carry out the rotary impact rock breaking test using the stepped nozzle bit. This system integrates the rock bearing mechanism, which can accurately simulate the working conditions in the field at the millimeter target distance and realize the functions of fixed-point impact, rotary drilling, and transverse cutting of the drill bit. Standard sandstone samples were selected for drilling and treatment in the experiment. To meet the needs of experimental teaching, the speed is set by the control terminal of the rotary drilling device. When a stable speed is achieved, the high-pressure pump is opened and the pressure is set; when the pressure is stable, the sandstone sample is impacted for 30 s, after which the high-pressure pump is closed. By repeating the above steps, the characteristics of rock failure with variation of the parameters are revealed to assist students in understanding the mechanism of rock breaking using a water jet.

The influence of the rotation speed and pressure on the drilling efficiency is quantitatively analyzed. The rotation speed is optimal for a given pressure, drilling depth, diameter, and volume. The optimal rotation speed for the drilling depth under different pressures is 200 rpm; the optimal rotation speed is 150 rpm at pressures ≤ 12 MPa and 200 rpm at pressures ≥ 12 MPa. The optimal rotation speed for the drilling volume is 150 rpm at a pressure = 8 MPa and 200 rpm at pressures > 8 MPa. The sensitivity of boreholes with different geometries and sizes to the rotational speed is significantly different: the depth is the least sensitive, the volume is the second, and the diameter is the most sensitive.

This study introduces the cascade nozzle rotating impact crushing rock test system for experimental teaching of the course “Water Jet Theory and Practice,” and a teaching platform combining the practice and theory of water jet crushing coal and rock mass is built for application in engineering scenarios. The platform is used to explore the characteristics of self-rotating jet erosion and rock breaking, and to reveal the key influence of the pressure and rotational speed on the drilling efficiency. This platform effectively connects basic theoretical knowledge with complex engineering practice, provides support for scientific research and teaching of water jet rock breaking, creates favorable conditions for exploring the characteristics of jet impact rock breaking, and helps improve students’ professional identity, practical application skills, and innovative consciousness.