Coal-bearing metakaolin (CMK) is a highly active volcanic material. In this study, the compressive strength of CMK cement soil was tested, along with five CMK samples cured in different concentrations of Na₂SO₄ solution. The compressive strength of the CMK cement soil first increased and then decreased with increasing Na₂SO₄ concentrations. When the Na₂SO₄ solution concentration was 18 g/L, the compressive strength of the CMK cement soil reached its maximum value. Additionally, X-ray diffraction (XRD) analysis of the chemical reactions between the CMK cement soil and Na₂SO₄ solution and scanning electron microscopy (SEM) were used to analyze the phase composition and microstructure of the CMK cement soil, respectively. Thus, the pore distribution of the CMK cement soil could be calculated and classified from a 500× magnification SEM image using Image-ProPlus6.0 analysis software. This study explains the mechanism of curing CMK cement soil in a Na₂SO₄ solution based on its phase analysis, microstructure, and pore distribution.