<p>This study investigates the combined influence of calcium-sulfo-aluminate (CSA)-based expansive agents and polypropylene (PP) micro-fibers on the compressive strength of M20 grade concrete. Although CSA-based expansive agents and fiber-reinforced concrete have been widely studied individually, their combined behavior in normal-strength concrete remains limited. To address this gap, eight concrete mixes were prepared with varying proportions of CSA expansive agent (0, 5, 10, and 15%) and polypropylene fibers (0 to 0.1%). In addition to compressive strength testing, microstructural characterization using scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) was conducted to examine the interactions between fibers and expansive agent products within the cement matrix. The results indicate that CSA-based expansive agents significantly improve compressive strength in mixes without fibers, whereas the combination of fibers and expansive agents results in a reduction in strength, due to the potential microstructural conflicts. However, the inclusion of fibers plays an important role in regulating expansion and the optimal mix was identified as PP0.05E10, containing 10 expansive agent and 0.05% polypropylene fibers. This study provides new insight into the interaction between CSA expansive agent and polypropylene fibers in normal-strength concrete, contributing to the optimization of mix proportions for practical and sustainable low-carbon concrete construction applications.</p>