Transmission electron microscopy studies of calcium silicate hydrate (C-S-H) gels in portland cement have been performed. This is the first study which unambigiously demonstrates the presence of a mesostructure in the C-S-H gel phase. The mesostructure was found to consist of an amorphous matrix with a strongly variable composition which was embedded with nanocrystalline regions on the scale of $\sim 50Å$ or less each with a locally homogeneous composition and with short-range ordered regions on the scale of $\sim 10Å$ with a variable composition and structure. The mesostructure is believed to form due to the inability of normal long-range crystallinity to evolve because of gross fluctuations in the local Ca/Si ratio.

• Received 4 January 1996

DOI:https://doi.org/10.1103/PhysRevLett.77.952