3 research outputs found
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)Âsilicate 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
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)Âsilicate 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
New Structural Model of Hydrous Sodium Aluminosilicate Gels and the Role of Charge-Balancing Extra-Framework Al
A new structural
model of hydrous alkali
aluminosilicate gel (N-A-S-H) frameworks is proposed, in which charge-balancing
extra-framework Al species are observed in N-A-S-H gels for the first
time. This model describes the key nanostructural features of these
gels, which are identified through the application of <sup>17</sup>O, <sup>23</sup>Na, and <sup>27</sup>Al triple quantum magic angle
spinning solid-state nuclear magnetic resonance spectroscopy to synthetic <sup>17</sup>O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate
gel predominantly comprises Q<sup>4</sup>(4Al), Q<sup>4</sup>(3Al),
Q<sup>4</sup>(2Al), and Q<sup>4</sup>(1Al) Si units charge-balanced
by Na<sup>+</sup> ions that are coordinated by either 3 or 4 framework
oxygen atoms. A significant proportion of Al<sup>3+</sup> in tetrahedral
coordination exist in sites of lower symmetry, where some of the charge-balancing
capacity is provided by extra-framework Al species which have not
previously been observed in these materials. The mean Si<sup>IV</sup>–O–Al<sup>IV</sup> bond angles for each type of Al<sup>IV</sup> environments are highly consistent, with compositional changes
dictating the relative proportions of individual Al<sup>IV</sup> species
but not altering the local structure of each individual Al<sup>IV</sup> site. This model provides a more advanced description of the chemistry
and structure of alkali aluminosilicate gels and is crucial in understanding
and controlling the molecular interactions governing gel formation,
mechanical properties, and durability