In pursuit of sustainable material design, novel blended cements with high levels of supplementary cementitious materials (SCMs) are being increasingly used. Although the hydration of these blended cements has been extensively studied in the literature, prediction of their hydration kinetics and phase assemblage remain poorly understood, especially in ternary and quaternary blended systems. This study proposes to determine the hydration kinetics and phase assemblages based on the isothermal calorimetry test, the theoretical framework proposed by modified Parrot–Killoh, and thermodynamic simulation. The isothermal calorimetry measurements were performed on various Portland cements blended with BFS (40% and 60%) and MK (30%) as well as a ternary blend (30% MK and 15% LS) at a water/binder ratio of 0.5. Additionally, quantitative X-ray diffraction was used to characterize the phase assemblages for the thermodynamic model. The result has shown that incorporation of BFS blends has led to the formation of hemicarbonate as the main AFm phase; however, the incorporation of MK blends has converted hemicarbonate to the formation of stralingite phase, with an increasing amount at later age.