Nanoindentation assisted investigation on the viscoelastic behavior of carbonated cementitious matrix: Influence of loading function

This paper presents an evaluation on the viscoelastic behaviors of the microscopic phases present in the carbonated wollastonite (CaSiO3) matrix using nanoindentation test method. Wollastonite forms silica gel (SiO2·nH2O) and calcium carbonate (CaCO3) during the reaction with CO2 in the presence of moisture. Thus, the carbonated wollastonite matrix used in this study was composed of silica gel, calcium carbonate, unreacted grains, and porosity. Nanoindentations were performed on this system with different peak loads (2000-8000 μN) and holding durations (15-200 s) to study the creep behaviors. The silica gel showed the highest creep effect among all of the microscopic phases. Performances of the standard solid, Burger's, logarithmic, and stretched exponential models to capture the creep performance of the silica gel phase were evaluated by fitting these models with the experimentally obtained nanoindentation depth-time data. Although these models were able to produce good fits with the experimental data set, the fit parameters of only the standard solid model and logarithmic model showed good correlation with the elastic modulus of silica gel. The fitting parameters of all these models were found to vary with the holding durations and peak loads. Specifically, the viscoelastic deformation of silica gel was observed to increase with both, peak loads and holding durations. Finally, the self-consistent homogenization scheme was utilized to predict the creep behavior of the overall carbonated wollastonite paste from the nanoindentation data for different peak loads. Larger portion of silica gel within the tested area was found to result in a higher creep effect of the paste irrespective of the peak load used for testing.