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 ¹⁷O, ²³Na, and ²⁷Al triple quantum magic angle spinning solid-state nuclear magnetic resonance spectroscopy to synthetic ¹⁷O-enriched gels of differing Si/Al ratios. The alkali aluminosilicate gel predominantly comprises Q⁴(4Al), Q⁴(3Al), Q⁴(2Al), and Q⁴(1Al) Si units charge-balanced by Na⁺ ions that are coordinated by either 3 or 4 framework oxygen atoms. A significant proportion of Al³⁺ 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^IV–O–Al^IV bond angles for each type of Al^IV environments are highly consistent, with compositional changes dictating the relative proportions of individual Al^IV species but not altering the local structure of each individual Al^IV 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