OBSERVATIONS OF PEAK STRENGTH BEHAVIOR IN CSA CEMENT MORTARS

The purpose of this study was to assess the mechanical property performance behavior of calcium sulfoaluminate(CSA)cement mortar when cured at ambient laboratory temperature of~23℃(73°F)and constant 50%relative humidity for an extended period of time.Four CSA cement mortars were tested.Three CSA cement mortars contained equivalent mass amounts of calcium sulfate;whereas,the fourth mortar contained double the amount of calcium sulfate.The three CSA cement mortars containing constant mass amounts of calcium sulfate differed as the specific type of calcium sulfate varied across the three formulations-one mortar containing solely anhydrite,one mortar contained half anhydrite and half gypsum while the other mortar solely contained gypsum.The fourth mortar contained double the amount of calcium sulfate when compared with the others while having a 1/1 blend of anhydrite and gypsum.Specific mortars were either tested for direct tensile strength according to ASTM C307 or for compressive strength according to ASTM C109.All tested mortars displayed statistically significant strength loss trends versus time when cured at constant 50%relative humidity.Cement paste samples were analyzed with TGA/SDT and XRD in an effort to identify microstructure changes corresponding to observed strength loss.Cement paste analysis suggests strength loss within the tested CSA cement mortars occurred as a result of ettringite decomposition.

INFLUENCE OF LATEX POLYMER ADDITION ON THE BEHAVIOR OF MATERIALS CONTAINING CSA CEMENT CURED AT LOW HUMIDITY

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(C3A)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℃.