The present study assesses the hydration behavior of polymer modified and non-polymer containing calcium sulfoaluminate (CSA) cement pastes containing minor phase tri-calcium aluminate (C 3A) when cured at ambient laboratory temperature with 50% relative humidity. The particular CSA cement is unique as it offers two potential paths for ettringite formation. In the presence of sufficient calcium sulfate, these two potential paths stem from either hydration of yeelimite or hydration of tri-calcium aluminate. The study assesses the microstructural behavior of cement pastes utilizing powdered x-ray diffraction (XRD) and thermogravimetric analysis (TGA/SDT). Additionally, the study utilizes direct tensile strength testing according to ASTM C307, Standard Test Method for Tensile Strength of Chemical Resistant Mortar, Grouts and Monolithic Surfacings to quantify relationships between microstructural characteristics and mechanical property performance for representative mortars. The present study is significant as it illustrates differences in microstructural behavior for CSA cement materials with and without polymer cured at constant 50% relative humidity. The present study suggests ettringite decomposition occurs within non-polymer containing CSA cement paste samples cured at constant low humidity. The present study presents polymer modification as an effective means for mitigating ettringite decomposition and subsequent strength loss in materials containing CSA cement cured at constant 50% relative humidity and 23°C.