Superabsorbent polymers (SAP) have been applied e.g. for frost resistance improvement, rheology modification, controlled release of admixtures, crack healing, surface curing and removal of concrete contaminants, and recently for reducing the coefficient of thermal expansion of concrete /1-4/. Probably the first /1, 2/, and currently one of the most promising application areas for SAP, is internal curing of High Performance Concrete (HPC) and Ultra-High Performance Concrete (UHPC). SAP are added in the dry state during mixing, whereupon they rapidly absorb part of the mixing water and form water-filled cavities (about 100-500 μm in diameter) in the fresh concrete /2, 3/. After setting, as part of the water in the capillary pores of the cement paste is consumed by cement hydration, SAP are progressively emptied as they provide water to the paste and compensate for the chemical shrinkage /5/. This study focuses on the effect of SAP on the kinetics of hydration of low w/c pastes.