The hydration of slag-blended cement is studied by considering the interaction between the hydrations of slag and Portland cement clinker. Three reaction models for the slag-blended cement are developed based on stoichiometric calculations. These models correlate the compositions of the unhydrated slag-blended cement with the quantities and compositions of the hydration products. The model predictions are further used to calculate some properties of hydrating slag cement pastes, including the molar fractions of products, the water retention, chemical shrinkage and porosities of pastes. The models are validated by comparing the model predictions with the measurements and proven to be successful in quantifying the hydration products, predicting the composition of the main hydration product (C-S-H) and calculating the properties of the hydration process. 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, but the chemical shrinkage can vary in a wide range, depending on the slag hydration degree in the cement.