Electrical Resistivity Response and Mesoscopic Simulation of Concrete Containing a Composite Low-Freezing-Point Powder Under Subzero Conditions

Abrupt temperature drops during cold waves pose a threat to early-age concrete in cold regions. This study combined electrical resistivity monitoring, compressive strength testing, and mesoscopic electrical modeling to investigate the performance of concrete containing a composite low-freezing-point powder (CLFP). CLFP comprised calcium nitrate tetrahydrate and sodium nitrite at a mass ratio of 3:1. Five dosages of 0%, 2%, 4%, 6%, and 8% were investigated, and the specimens were cured at −20 °C for 3 d before being transferred to 20 °C. A random-aggregate model was used to link experimentally measured time-dependent mortar conductivity to the overall electrical resistivity of concrete, and the electrical responses were evaluated alongside subsequent strength development. All mixtures exhibited three resistivity stages: slow increase, rapid rise, and stabilization. Increasing the CLFP dosage delayed the resistivity-derived apparent freezing onset by up to 0.43 h and reduced the stabilized resistivity by 29.96%. After 28 d of subsequent curing at 20 °C, the 2% CLFP mixture achieved the highest compressive strength of 37.6 MPa, approximately 7.1% higher than the reference value of 35.1 MPa. The 4%CLFP mixture reached 35.4 MPa, whereas the 6% and 8% mixtures reached only 24.1 MPa and 25.3 MPa, respectively. The model reproduced the decrease in stabilized resistivity with increasing CLFP dosage, with relative errors of 1.5–8.4% for the 0–6%CLFP mixtures and 18.2% for the 8% mixture. Considering both delayed apparent freezing onset and strength retention, 4%CLFP was identified as a suitable dosage for balancing these two indicators under the tested conditions.