Does nano basic building-block of C-S-H exist? - A review of direct morphological observations

Despite significant advancements in microstructural characterization methods, the interconnections between nanostructure and morphological diversity of calcium-silicate-hydrate (C-S-H), the primary binding phase in modern concrete, remain unclear. This review delves into the state-of-the-art experimental findings of the morphology of C-S-H and comprehensively analyses the various influencing factors. The focus here is to address the long-standing debate: whether there are fundamental structural units, either fractural globule or nano sheet, that assemble to form diverse C-S-H microstructures. We critically assess that C-S-H formation involves the structured assembly of layered nanostructure with an approximate size of 4-10 nm, rather than occurring randomly. Such nanostructure may have substantial heterogeneity and deformability, often blurring the distinction between the globular and sheet models. Finally, this paper offers perspectives on future research directions aimed at further unravelling the intricate structure of C-S-H.