Effect of CO₂ pressure on carbon storage−mechanical properties of red mud−fly ash based grouting materials

The accumulation of solid waste and CO₂ emissions are two critical issues hindering the clean and efficient development of the energy industry. The red mud-fly ash base polymer grouting material was prepared by Bayer red mud and fly ash, which are by products of aluminum industry. The carbonation depth, carbonation area, CO₂ absorption, and uniaxial compressive strength of the materials under different CO₂ curing pressures were tested. Microscopic characterization techniques such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope-energy dispersive spectrometry (SEM−EDS), and low-field nuclear magnetic resonance (LF−NMR) were employed to reveal the carbon sequestration-mechanical performance evolution mechanism of the grouting materials. The results indicated that as the CO₂ curing pressure increased from 0.2 MPa to 1.8 MPa, the carbonation depth of the material increased from 2.07 mm to 4.27 mm, and the CO₂ absorption rate rose from 1.10% to 2.86%. After carbonation curing, the contribution of carbonates to the material's strength was superior to that of sodium/calcium aluminosilicate hydrate (N(C)−A−S−H) gel, leading to an increase in the uniaxial compressive strength with higher CO₂ pressure. When the curing pressure reached 1.4 MPa, the strength exceed that of the non-carbonated control group. During the continued curing process up to 28 days, the early carbonation reaction consumed free alkali as well as Ca²⁺ and Na⁺, which are essential for geopolymer formation, thereby restricting further geopolymerization. Additionally, the overall low-calcium system of the material hindered the sustained growth and development of calcium carbonate crystals. This ultimately led to an increase in material porosity, resulting in a deterioration of 28-day strength compared to the control group. This study provides a theoretical foundation for guiding the construction of CO₂ storage reservoirs in goaf areas and underground carbon sequestration.