Air-entrainment for tailoring multi-scale pore structures in shield synchronous grouting materials

This study systematically investigated the effects of different dosages of an air-entraining admixture (616 AEA) on the fresh and hardened properties of shield synchronous grouting materials. An appropriate 0.075% dosage significantly reduced apparent density by 7.4% and increased air content by 613.3%, while maintaining good flowability and anti-segregation. XRD and thermal analysis revealed the AEA did not alter hydration product composition/formation, indicating its mechanism was introducing air bubbles to modify the pore structure. At 0.075%, an ideal multi-scale pore structure formed, with 28-day of 7.7 MPa and water-to-land strength ratio of 89.6%, meeting engineering requirements. Advanced techniques like low-field NMR, mercury intrusion porosimetry, and ultra-depth imaging comprehensively characterized pore structure evolution, consistently demonstrating increased porosity and larger pores with higher AEA dosages. The findings elucidate the underlying mechanism by which the AEA optimizes the pore structure, providing guidance for mix design optimization and enhancing comprehensive performance for sustainable underground construction applications.