Influence of binder composition on the gel structure in alkali activated fly ash/slag pastes exposed to elevated temperatures

The present study investigated the influence of binder composition on the gel structure in alkali-activated fly ash/slag (AFS) paste exposed to elevated temperatures, and correlates the strength gain behavior with the phase transformation of binder gels. XRD, solid-state Si-29 and Al-27 MAS NMR, MIP were conducted to characterize the thermally induced microstructural and chemical changes of the binder gel phases. The XRD patterns of AFS paste exposed to 800 degrees C showed the presence of the main phases akermanite (Ca2MgSi2O7), gehlenite (Ca2Al[AlSiO7]), anorthite (CaAl2Si2O8), and minor quantity of nepheline ((Na,K)AlSiO4). The amount of crystalline phase formed after exposure to 800 degrees C is highly dependent on the Ca/Al ratio of the precursor, slag and fly ash. The pore structure of the AFS paste with a lower slag content after exposure to 800 degrees C became more porous, which is highly associated with that N-A-S-H, anorthite and nepheline formed by exposure to 800 degrees C are of significantly porous structure. The residual strength at 800 degrees C was highest at a lowest slag content (25%), which is likely due to the inherited high thermal stability of the highly cross-linked gel with Q(3) and Q(4) structures such as N-A-S-H, anorthite or nepheline, rather than the presence of C-(A)-S-H.