Nanoindentation of Alkali-Activated Fly Ash

Low-calcium ground fly ash was activated with a sodium silicate solution and cured under ambient and heat conditions. Nanoindentation was employed for the characterization of reaction products, mainly alumino-silicate (N-A-S-H) gel. Samples were indented by a series of grids consisting of hundreds of indents each. The intrinsic Young's modulus of the N-A-S-H gel was found to be around a mean value of 17.7 GPa regardless of the curing procedure. Other characteristic phases were also identified, namely partly activated slag, nonactivated slag, and fly ash particles. Results of pore size distributions measured by mercury intrusion porosimetry were systematically observed depending on the curing procedure. Additional measurements using the same methodology were performed on cement paste samples. The synthesized N-A-S-H gel was found to be mechanically close to calcium silicate hydrate gels in matured cement pastes.