Stabilization of sandy soil by cementitious materials is a widely used ground improvement technique. The typical binding material for this purpose is ordinary Portland cement (OPC). However, OPC would not be classified as eco-friendly material due to significant carbon dioxide emission, and calcium sulfo-aluminate (CSA) cement could be a suitable substitution for OPC. In this study, unconsolidated-undrained triaxial tests are conducted with 7% CSA cement content and 1, 3, and 7 days curing time. Based on the experimental testing, three-dimensional discrete element method (DEM) simulations are used to analyze the CSA cement-stabilized sands underlying microscale physics. A built-in linear-parallel bond model has been implemented in PFC3D-program to describe particle interactions in cemented media and calibrated by the brute force algorithm with the initial try error method to identify the appropriate range of microscale properties. In the simulations, cement contents are controlled by the radius multiplier parameter of the parallel bond model, and the bond stiffness and strength in different curing times are manipulated by a combination of bond effective modulus, stiffness ratio, tensile strength, and cohesion. The results show that displacement vectors in cement-treated soil particles could give the picture of failure plan. The results of the simulations would be employed to estimate the strength development of CSA cement-treated soils in the future.