Although the understanding and modeling of water isotherm of activated carbons have been extensively researched, the mechanism of water adsorption is not yet fully elucidated to date and remains the object of discussion. In this regards, recently, an equation has been developed to adequately fit the low pressure range of the water adsorption isotherms which describes the phenomena of specific interactions of the water molecules occurring with the activated carbon surface and the subsequent cluster formation. This model enables the prediction of the first part of the water isotherm based on the amount of surface-oxygenated and -nitrogenated complexes present. However, when trying to validate its applicability to other kind of materials, it has been observed that the model should be further improved in order to account for samples with a high oxygen content (e.g. cementitious materials). Therefore, it is necessary to study if, for materials with an oxygen-rich surface Chemistry, the mechanism of micropore filling also proceeds through the formation of clusters of five water molecules, or whether the equation should be modified when a certain value of oxygen content is exceeded.