Expansion of clay minerals in expansive soil, such as montmorillonite (MMT), is critical to a broad class of problems in science and engineering. Cement is one of the most widely used materials in the treatment of expansive soil. However, at present, the expansion mechanism of the soil treated by cement has not been thoroughly investigated at the nanoscale level. In this paper, four types of cement-stabilized MMT (Na-MMT, K-MMT, Ca-MMT, and Mg-MMT) molecular models with different interlayer cations and surface charges are established and validated by experimental data. The maximum adsorption energies of the four different cement-stabilized MMT systems are then investigated through molecular simulations, and the optimum water ratios are proposed for the four cement-stabilized MMT systems to achieve proper volume stability. The numerical simulations also show that the volume change of cement-stabilized MMT depends not only on the water-absorbing capacity but also on the CSH-MMT interface energy as well as the competitive absorption of CSH and MMT. For different types of expansive soil, Mg2+ or K+ modification may be used to improve the efficiency of the expansion control.