The hydration of slag in Portland cement is studied by considering the interaction between the hydrations of slag and Portland cement clinker. The slag hydration heat value that the different dosage of slag is in Portland cement is measured with isothermal calorimetry method as quartz sand sample contrast. The kinetic analysis was used to obtain parameters, which were employed to find out the influence of the slag content on the reaction rate constant value. The reaction stages have been analyzed and explained in accordance with kinetic models. The model predictions show that as the slag proportions in the blended cement changes, water retention in the hydration products changes only slightly if compared to that of Portland cement, The investigations have shown that the addition of silica fume of more than 30 mass% reduces the duration of the phase boundary interaction as a rate-determined process resulting in the fast diffusion rate-determining process. The results of this study have also revealed evidence of the accelerator effect of slag during the first 24 h of hydration when it still exists as chemically inert filler.