Intrinsic differences in atomic ordering of calcium (alumino)silicate hydrates in conventional and alkali-activated cements

The atomic structures of calcium silicate hydrate (C-S-H) and calcium (-sodium) aluminosilicate hydrate (C-(N)-A-S-H) gels, and their presence in conventional and blended cement systems, have been the topic of significant debate over recent decades. Previous investigations have revealed that synthetic C-S-H gel is nanocrystalline and due to the chemical similarities between ordinary Portland cement (OPC)-based systems and low-CO₂ alkali-activated slags, researchers have inferred that the atomic ordering in alkali-activated slag is the same as in OPC-slag cements. Here, X-ray total scattering is used to determine the local bonding environment and nanostructure of C(-A)-S-H gels present in hydrated tricalcium silicate (C₃S), blended C₃S-slag and alkali-activated slag, revealing the large intrinsic differences in the extent of nanoscale ordering between C-S-H derived from C₃S and alkali-activated slag systems, which may have a significant influence on thermodynamic stability, and material properties at higher length scales, including long term durability of alkali-activated cements.