Nanoscale Ordering and Depolymerization of Calcium Silicate Hydrates in the Presence of Alkalis

Increasing cement production and its substantial contribution to anthropogenic CO₂ emissions (∼5-8%) have led to the pursuit of alternative, sustainable cements. These sustainable cements often contain non-negligible amounts of alkalis (Na or K) which significantly influence the resulting material's performance. However, the precise mechanism(s) by which the atomic structure and thermodynamic stability of the primary binder phase is altered remains unknown. Here, we have synthesized 45 pure sodium-substituted calcium-(alumino-)silicate-hydrate (C-(N)-(A)-S-H) gels using a wide range of alkali concentrations (0.1-5 M). We report that the addition of higher levels of alkalis during synthesis of the gels results in a systematic reduction in basal spacing, mean silicate chain lengths, and, most importantly, degree of silicate polymerization. These changes to the gel's extent of polymerization, confirmed by X-ray pair distribution function and nuclear magnetic resonance (NMR) spectroscopy, may have implications on the long-term durability and stability of sustainable cements activated with high levels of alkali hydroxides or silicates. Finally, we compare the nanoscale ordering (between ∼5 and 40 Å) of our synthetic gels with real gels found in commercial systems and find that their nanoscale ordering is significantly different, where the real gels tend to be considerably more disordered.