Accelerated aging studies can be used to assess the degree of deterioration and predict the failure of specimens after exposure to known climatic conditions. In this paper, we investigate the freeze-thaw deterioration factor with X-ray computed microtomography (μCT) analysis to compare the deterioration due to freeze-thaw cycles for cement mortar samples with and without 5% biochar as a partial cement replacement. The samples were exposed to a standard temperature profile from −20 °C to +20 °C and tap water or 3% NaCl solution on the top surface. After the exposure, samples were transferred to the μCT to obtain a 3D image of the inner structure. The sample with biochar as a partial cement replacement showed less freeze-thaw deterioration and crack propagation than the reference sample under the effect of the NaCl solution, while under the water influence deterioration was rather similar.