Strength Development of Cemented Paste Backfill with CaCl₂ and NaCl in Above-Zero Underground Environments

Cemented paste backfill (CPB) is prevented from freezing by antifreeze during transport in subzero temperatures. However, there is yet to be a study on the mechanical properties of CPB with CaCl₂ and NaCl antifreeze. This study investigated the effects of antifreeze (CaCl₂ and NaCl) on the strength development of CPB curing in above-zero underground environments with a pre-cured at − 12 °C for 1 h. CPB samples with CaCl₂ and NaCl antifreeze were prepared and cured under above-zero underground temperatures (2, 20, and 35 °C). Compressive strength tests were implemented after specific curing time (1, 3, 7, and 28 d). Moreover, microstructural analyses and monitoring experiments were performed. The strength of the sample with 15 g/L CaCl₂ and NaCl decreased with curing at underground temperatures 20 °C for 1, 3, and 7 d, and increased at 28 d, which was attributed to the competition between the CaCl₂ and NaCl antifreeze-induced strength enhancement, that is, increased hydration products, and strength weakening, that is, coarsened pore structure. The development of the CPB strength with the curing time depends on the concentrations of CaCl₂ and NaCl and the above-zero underground environments. The findings elucidates on antifreeze CPB in engineering practice.