C-S-H seeding is a relativley new approach to accelerate cement hydration.C-S-H nanoparticles are added to the cement lime to stimulate the formation of C-S-H nuclei during very early stage of cement hydration, which will accelerate the C-S-H formation during the continuing cement hydration.In the present work two different approaches to prepare C-S-H seeds are shown: a mechanochemical and a sol-gel method.In both cases the way the seeds are prepared, directly influence their ability to accelerate cement hydration.A factorial design of experiments setup was used to optimize the mechanochemical method in order to achieve the best acceleration.The method itself uses commercial available materials and simple grinding technology.For the optimization the accelerating effect of the C-S-H seeds was analysed by heat flow calorimetry and four factors could be determined to be significant for the characteristics of thus prepared seeds.When the most efficient seeds were added to ordinary Portland cement an earlier and faster setting could be determined as well as higher early strength, even at low concentrations of 0.25 wt.% of C-S-H seeds.The accelerating effect of C-S-H seeds could also be verified in cements, which were blended with fly ash or ground granulated blast furnace slag.