Experimental Investigation on Mechanical Properties and Failure Behaviors of Concrete for Ultra-Deep Shafts Using Acoustic Emission and Energy Evolution Characteristics

As coastal ultra-deep mine shafts advance to greater depths, shaft lining concrete may experience sustained humid–hot conditions. Elevated temperature is induced by geothermal heat and early-age hydration heat, while high humidity is maintained in water-rich underground environments, which can compromise long-term performance. Such late-age deterioration may increase maintenance demand and pose safety concerns for ultra-deep shaft construction and long-term service. This study experimentally evaluates a high-strength shaft lining concrete designed with a composite cementitious system and cured at 40, 60, and 80 °C (95% RH) for 30–180 days, considering the engineering scenario of the 2500 m shaft at the Sanshan Island Gold Mine. The selected temperature range was determined based on in situ temperature monitoring in the target shaft. P-wave velocity measurements and uniaxial compression tests were conducted, while acoustic emission (AE) monitoring and energy evolution analysis were used to interpret damage progression. P-wave velocity decreased with curing temperature, with the most pronounced reduction at 80 °C. Compressive strength increased at early ages and then declined at later ages; by 180 d, the strength loss relative to the peak level is more pronounced at higher temperatures. AE results show four typical damage stages, with activity increasingly concentrated near peak stress as temperature and age increase. AF–RA analysis indicates tensile cracking dominates, with a slight increase in shear-related events at higher curing temperatures and longer ages. Energy analysis further confirms that most input energy is stored as elastic strain energy prior to peak stress, and higher curing temperatures increase the proportion of input energy stored elastically, implying a higher tendency toward brittle failure. These results suggest optimizing curing regimes and toughness-enhancement strategies for durable shaft infrastructure.