DataSheet1_The influence of curing temperature on the strength and phase assemblage of hybrid cements based on GGBFS/FA blends.PDF

Hybrid cements are composites made of Portland cement or Portland clinker and one or more supplementary cementitious materials like slag, fly ash or metakaolin, activated with an alkali salt. To date, their hydration mechanism and the phase formation at various temperatures is insufficiently understood, partly due to the large variability of the raw materials used. In the present study, three hybrid cements based on ground granulated blast furnace slag, fly ash, Portland clinker and sodium sulfate, and an alkali-activated slag/fly ash blend were cured at 10 and 21.5°C, and subsequently analyzed by XRD, 27Al MAS NMR, and TGA. The compressive strength of the hybrid cements was higher by up to 27% after 91-day curing at 10°C, compared to curing at 21.5°C. The experimental results as well as thermodynamic modeling indicate that the differences in compressive strength were related to a different phase assemblage, mainly differing amounts of strätlingite and C-N-A-S-H, and the associated differences of the volume of hydration products. While the strätlingite was amorphous to X-rays, it could be identified by 27Al MAS NMR spectroscopy, TGA and thermodynamic modeling. The microstructural properties of the hybrid cements and the alkali-activated slag/fly ash blend as well as the compatibility between thermodynamic modeling results and experimental data as a function of curing temperature and time are discussed.</p

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