Early warning of CO ₂ sequestration based on crack evolution in sandstone–concrete composites under Sc-CO ₂

During CO₂ sequestration in abandoned mines, concrete isolation walls or lining layers and surrounding rock may suffer damage or failure during CO₂ injection and long-term storage, increasing leakage risk. To investigate the damage–failure behaviour, crack evolution and sequestration prediction of sandstone–concrete composites under Sc-CO₂ conditions, specimens with interface inclinations of 0°, 30°, 45°, 60° and 90° were immersed in Sc-CO₂ for 0, 12 and 24 h. Uniaxial compression and acoustic emission (AE) tests were conducted, and constitutive and sequestration prediction models were established. Results show that: (1) at the same immersion time, specimens with a 60° interface inclination exhibited the lowest peak strength (15.82–18.53 MPa) and cumulative AE ringing counts (1.8×10³–4.1×10³); (2) for a given inclination, peak strength increased while cumulative AE ringing counts decreased with immersion time; (3) permeability first decreased and then increased with interface inclination, reaching a minimum at 60°, which was 31.2%–69.9% lower than that of other inclinations; and (4) CO₂ sequestration states were classified into steady, controllable, warning and critical levels. Based on strain and cumulative AE ringing counts, crack constitutive and sequestration prediction models were developed, with prediction errors within ±8%. These findings provide theoretical guidance for safe CO₂ sequestration in abandoned mines.