Though cement is a ubiquitous material with global production exceeding that of any other material of technological importance, the mechanism of its hydration and evolution of cement-water mixtures into gels of high compressive strength is poorly understood, despite extensive research over the past century. The present investigation, based on neutron-scattering measurements, aims at unraveling this enigma and outlines the evolution of the mesoscopic structure of the cement paste which exhibits temporal oscillations, strongly dependent on the scale of observation and on the medium of hydration (light or heavy water). While the formation of hydration products is synchronous for hydration with H2O , the process is nonsynchronous for hydration with D2O . The reason why morphological patterns of domains at different times look dissimilar, as seen before [S. Mazumder, D. Sen, A. K. Patra, S. A. Khadilkar, R. M. Cursetji, R. Loidl, M. Baron, and H. Rauch, Phys. Rev. Lett. 93, 255704 (2004)10.1103/PhysRevLett.93.255704; Phys. Rev. B 72, 224208 (2005)10.1103/PhysRevB.72.224208], for different hydration media emerges as a natural consequence of this finding. The present investigation also provides an explanation for disagreement with the hypothesis of dynamical scaling for hydration of cement with heavy water and is a step forward toward general understanding of hydration process.