Atomic and nano-scale characterization of a 50-year-old hydrated C₃S paste

This paper investigates the atomic and nano-scale structures of a 50-year-old hydrated alite paste. Imaged by TEM, the outer product C–S–H fibers are composed of particles that are 1.5–2 nm thick and several tens of nanometers long. ²⁹Si NMR shows 47.9% Q₁ and 52.1% Q₂, with a mean SiO₄ tetrahedron chain length (MCL) of 4.18, indicating a limited degree of polymerization after 50 years' hydration. A Scanning Transmission X-ray Microscopy (STXM) study was conducted on this late-age paste and a 1.5 year old hydrated C₃S solution. Near Edge X-ray Absorption Fine Structure (NEXAFS) at Ca L_3,2-edge indicates that Ca²⁺ in C–S–H is in an irregular symmetric coordination, which agrees more with the atomic structure of tobermorite than that of jennite. At Si K-edge, multi-scattering phenomenon is sensitive to the degree of polymerization, which has the potential to unveil the structure of the SiO₄⁴⁻ tetrahedron chain.