On the hydration of water-entrained cement–silica systems: Combined SEM, XRD and thermal analysis in cement pastes

► A new model of hydration in SF-modified cement-based systems is presented. ► Internal curing in cement-based materials is described by the thermogravimetric technique. ► Pozzolanic activity is limited in low water and low porosity systems. ► Internal curing by superabsorbent polymers results in enhanced hydration. The work described in this document focuses on the hydration of low water and low porosity SF-modified cement-based materials. The hydration of the clinker compounds was followed by X-ray diffraction (XRD), differential thermal analysis (DTA) and also by means of the thermo-gravimetric technique (TGA). This study was performed in three systems, each with a different composition namely a plain cement paste, a silica fume (SF)-modified cement paste and a water-entrained SF-modified cement paste with superabsorbent polymers (SAPs). In addition to the previous experiments, the microstructure of the systems was accessed by means of the scanning electron microscopy technique (SEM). This was primarily done with the purpose of supporting some ideas that have emerged when determining the hydration of these complex systems using the former techniques. However, in this manuscript only the results found through the TGA/DTA technique will be shown. Thus, in respect to the quantification of the CH phase developing in the system, the results taken by the TGA/DTA technique enable a more feasible description of the hydration of low water and low porosity SF-modified cement systems, including systems with water-entrainment by superabsorbent polymers. The results show that for cement-based materials with the physical nature of the systems that have been studied in this work, the pozzolanic activity is limited due to lack of water and/or space to accommodate additional hydration products, and as a consequence, a surplus of silica fume is to be found in the mature material. Due to either physical or chemical constraints, the system is not able to fully convert the calcium hydroxide into calcium silicate hydrate during the first month of hydration in sealed conditions. Additionally, in systems with water-entrainment by means of superabsorbent polymers, it is possible to observe the internal curing activity being promoted by this addition, which is translated by enhanced hydration of cement reactants. A model has been constructed from the empirical knowledge developed on the previous framework, which may be used in further research activities to study different formu