The study of hydration kinetics by in-situ X-ray powder diffraction can provide fundamental details on the time evolution of the phase assemblage in hydrating cement pastes. The main limit of the technique is the lack of quantitative information about the amount of C-S-H and unbound water, which cannot be measured directly by conventional quantitative phase analysis procedures based on X-ray diffraction, due to their X-ray amorphous nature. Here, a mass balance algorithm, which can be used to determine the amount of both C-S-H and capillary water, is presented and compared with methods based on standards. This method can also provide information about the stoichiometry of C-S-H formed by the reaction of C3S, hydrated in the presence of gypsum, suggesting the incorporation of 0.3mol of sulfate per mole of C-S-H precipitated. In addition, the results show a significant increase in the rate of C3S hydration, when gypsum is added to the system.