Abstract Concrete admixtures influence the kinetic of cement hydration mainly during the dormant period. The dominant influence of admixtures seems to lie in different bound forces between dissociated ions in the pore water solution. Repulsive forces characterize the solvation process while attractive forces dominate during crystallization. These changes of ion bound forces lead twice to volume changes during phase transitions of hydration. Volume changes measured with an immersion weighing setup show clearly the effect of concrete admixtures on cement reaction. Retarder agents produce a volume swelling while accelerators force an immediate shrinkage behaviour. A mechanism as introduced by Le Chatelier involving a solution-crystallization step seems to describe the hydration process most adequately. As long as repulsive forces dominate, a volume swelling occurs and no strength gain can take place. The dormant period is defined by the length of the swelling process. Hardening and strength growth start at the point at which volume shrinkage appears.