Generalized Structural Description of Calcium–Sodium Aluminosilicate Hydrate Gels: The Cross-Linked Substituted Tobermorite Model

Structural models for the primary
strength and durability-giving
reaction product in modern cements, a calcium (alumino)­silicate hydrate
gel, have previously been based solely on non-cross-linked tobermorite
structures. However, recent experimental studies of laboratory-synthesized
and alkali-activated slag (AAS) binders have indicated that the calcium–sodium
aluminosilicate hydrate [C-(N)-A-S-H] gel formed in these systems
can be significantly cross-linked. Here, we propose a model that describes
the C-(N)-A-S-H gel as a mixture of cross-linked and non-cross-linked
tobermorite-based structures (the cross-linked substituted tobermorite
model, CSTM), which can more appropriately describe the spectroscopic
and density information available for this material. Analysis of the
phase assemblage and Al coordination environments of AAS binders shows
that it is not possible to fully account for the chemistry of AAS
by use of the assumption that all of the tetrahedral Al is present
in a tobermorite-type C-(N)-A-S-H gel, due to the structural constraints
of the gel. Application of the CSTM can for the first time reconcile
this information, indicating the presence of an additional activation
product that contains highly connected four-coordinated silicate and
aluminate species. The CSTM therefore provides a more advanced description
of the chemistry and structure of calcium–sodium aluminosilicate
gel structures than that previously established in the literature.