Apply 29Si, 27Al MAS NMR and selective dissolution in identifying the reaction degree of alkali activated slag-fly ash composites

This paper characterizes the phase composition and reaction degree of alkali activated slag, fly ash and their blends by using 29Si, 27Al MAS NMR and selective dissolution; with special emphasis on the effect of activator modulus and the usage of blended binders. The results revealed that in alkali activated slag systems, the increase of the activator modulus leads to a reduction of slag reactivity, together with significantly increased Q3(1Al) and reduced Q2(1Al) groups. Selective dissolution contributes to distinguish the unreacted fly ash and reaction products; 29Si, 27Al MAS NMR analyses were carried out on both components and their compositions were quantified in detail. Both polymerized Q4 structures and Q3 to Q0 sites were formed, and increasing the activator modulus resulted in an increased content of Q4 groups. When a blended system was applied, the slag reactivity was reduced with the increased fly ash reaction degree, the blended binder exhibited higher contents of newly formed Q4 groups with reduced Si/Al, reduced Q0 sites, shifted mean chain length and Si/Al of the formed C-A-S-H type gels, indicating gel interaction. Applying these two testing methods together brings new insights in understanding the structural composition of alkali activated slag, fly ash and their composites