Nanoscale attraction between calcium-aluminosilicate-hydrate and Mg-Al layered double hydroxides in alkali-activated slag

Calcium-aluminosilicate-hydrates (C-A-S-H) and Mg-Al layered double hydroxides (LDHs) are widely identified as the two main hydrated phases formed in alkali-activated slag (AAS), regardless of reactant composition. In addition to the precipitates of microscale crystals, Mg-Al LDHs can be intermixed particularly with the C-A-S-H in the inner product of AAS. This study aims at revealing the intermixing behaviors of C-A-S-H and Mg-Al LDH down to the nanometer scale using multiple advanced characterization techniques. The results show that the nanocrystalline C-A-S-H and Mg-Al LDH grains are thoroughly distributed in the background of an amorphous matrix. It is observed that Mg-Al LDH nanocrystalline grains with a size of about 15 to tens of nanometer strongly attach to the C-A-S-H nanocrystalline grains. The findings support the hypothesis that the positively-charged LDH layers are strongly attracted to the negatively-charged C-A-S-H layers during alkali-activation. This study contributes to a better understanding of the alkali-activated process as well as phase formation and distribution in this system.