Maximum likelihood estimation for nanoindentation on sodium aluminosilicate hydrate gel of geopolymer under different silica modulus and curing conditions

As an important inorganic material, geopolymer has been widely used for ceramics and sustainable cement in concrete. Sodium aluminosilicate hydrate (N-A-S-H) gel known as the zeolite precursor gel has the most critical impact on the performance of geopolymer. The nano/micromechanical properties of N-A-S-H have been investigated in several studies, but the resutls are always inconsistent. A novel compromise approach using Maximum Likelihood Estimation (MLE) for deconvolution of nanoindentation data is introduced to fundamentally further understand this issue in this study. Correlation and difference of different statistical techniques are compared to clarify the rationality of this method. Multiple characterization techniques including microstructure observation at micro-and nano-scale, element analysis, and crystal identification are applied to reveal the mechanisms. The results indicate that the elastic modulus and hardness of the N-A-S-H gel in geopolymer under different silica modulus and curing conditions vary in a small range from 10.50 to 14.30 GPa and from 0.40 to 0.57 GPa, respectively. When applying statistical nanoindentation in geopolymer, two kinds of spurious phases, mixed phases and sub-phases are unavoidable. For the MLE method adopted, the errors generated from analytical technique were estimated to be only 0.68 and 0.13 GPa for elastic modulus and hardness, respectively.