Micro-mechanical properties of alkali-activated fly ash evaluated by nanoindentation

•Nanoindentation combined with SEM-EDX were applied to study the micro-mechanical properties of AAFA.•Three phases were identified, corresponding to three distinct peaks in the indentation results.•The micro-mechanical properties of the gel phase were greatly affected by the pores between nano to micro scale.

In this study, the micro-mechanical property of different phases in alkali-activated fly ash (AAFA) pastes was investigated by nanoindentation. The mechanical information of the gel phase in AAFA was then linked to the chemical and microstructural information provided by SEM-EDX, ATR-FTIR and N2 adsorption, respectively. Three phases, i.e. aluminosilicate gel, residual fly ash particles and pores, were identified from the microstructural analysis of AAFA, corresponding to three distinct peaks in the indentation modulus frequency plots. The peak corresponding to the gel phase varied with curing age and activator content. The elastic modulus of aluminosilicate gel measured by nanoindentation was affected by the pores between nano to micro scale. The results of the chemical bonds of AAFA, however, cannot correspond well with the micromechanical results.