Nanoscale Charge-Balancing Mechanism in Alkali-Substituted Calcium–Silicate–Hydrate Gels

Alkali-activated
materials and related alternative cementitious
systems are sustainable technologies that have the potential to substantially
lower the CO2 emissions associated with the construction
industry. However, these systems have augmented chemical compositions
as compared to ordinary Portland cement (OPC), which may impact the
evolution of the hydrate phases. In particular, calcium-silicate-hydrate
(C–S–H) gel, the main hydrate phase in OPC, is likely
to be altered at the atomic scale due to changes in the bulk chemical
composition, specifically via the addition of alkalis (i.e., Na or
K) and aluminum. Here, via density functional theory calculations,
we reveal the presence of a charge balancing mechanism at the molecular
level in C–S–H gel (as modeled using crystalline 14
Å tobermorite) when alkalis and aluminum atoms are introduced
into the structure. Different structural representations are obtained
depending on the level of substitution and the degree of charge balancing
incorporated in the structures. The impact of these substitutional
and charge balancing effects on the structures is assessed by analyzing
the formation energies, local bonding environments, diffusion barriers
and mechanical properties. The results of this computational study
provide information on the phase stability of alkali/aluminum containing
C–S–H gels, shedding light on the fundamental atomic
level mechanisms that play a crucial role in these complex disordered
materials.