The Role of Al in Cross-Linking of Alkali-Activated Slag Cements

The structural development of a calcium (sodium) aluminosilicate hydrate (C-(N-)A-S-H) gel system, obtained through the reaction of sodium metasilicate and ground granulated blast furnace slag, is assessed by high-resolution Si-29 and Al-27 MAS NMR spectroscopy during the first 2yr after mixing. The cements formed primarily consist of C-(N-)A-S-H gels, with hydrotalcite and disordered alkali aluminosilicate gels also identified in the solid product assemblages. Deconvolution of the Al-27 MAS NMR spectra enables the identification of three distinct tetrahedral Al sites, consistent with the Si-29 MAS NMR data, where Q(3)(1Al), Q(4)(3Al), and Q(4)(4Al) silicate sites are identified. These results suggest significant levels of cross-linking in the C-(N-)A-S-H gel and the presence of an additional highly polymerized aluminosilicate product. The mean chain length, extent of cross-linking, and Al/Si ratio of the C-(N-)A-S-H gel decrease slightly over time. The de-cross-linking effect is explained by the key role of Al in mixed cross-linked/non-cross-linked C-(N-)A-S-H gels, because the cross-linked components have much lower Al-binding capacities than the noncross-linked components. These results show that the aluminosilicate chain lengths and chemical compositions of the fundamental structural components in C-(N-)A-S-H gels vary in a way that is not immediately evident from the overall bulk chemistry.