The ball‐milling hydration of highly reactive β‐Ca2SiO4synthesized from hillebrandite, Ca2(SiO3)(OH)2, was investigated and compared with its static hydration under a water/solid ratio of 10. In static hydration, the hydration is completed in 8 d to yield C‐S‐H with a Ca/Si ratio of 1.81 and Ca(OH)2. There is no major change after this period. In the case of ball milling, the hydration is completed in 2 d to yield C‐S‐H with Ca/Si = 1.81 and Ca(OH)2. After this, the two products react to form a C‐S‐H(II)‐like monophase hydrate having a Ca/Si ratio of 1.98. The morphology and structure of this hydrate are different from those of the earlier hydrate, and afwillite is not formed.29Si MAS NMR measurements indicated the C‐S‐H to be a mixture of dimers and single‐chain structures. Dimeric species are the main species up to completion of the reaction, but polymerization progresses very rapi