Dispersion, stability, and performance of nanomaterials in cementitious systems
Cement-based composites such as concrete perform satisfactorily in practice; however, their deterioration with time is concerning. Hence, efforts are always made to improve their strength and impermeability to enhance service life. Improving the packing density of constituent materials and densification of the microstructure of composite are considered significant parameters in this context. Recently, nanoparticles (NPs), such as nanosilica, nanolimestone, nanoalumina, carbon nanotubes, graphene, and graphene oxide, are increasingly used in cementitious composites for this purpose. However, due to their high surface reactivity and specific surface, NPs tend to heavily agglomerate in the composites, which might act as potential sites for stress concentration, thereby causing adverse effects.This dissertation presents the studies conducted to – (1) effectively disperse NPs (with a focus on nanosilica) in the cementitious systems; (2) develop a methodology for quantifying the dispersion of NPs in the hardened cement matrix; and (3) analyze the effect of various NPs on the hydration kinetics and temperature sensitivity of cement. The results are presented in the form of four journal papers. The efficacy of different anionic (SDS and PCE) and nonionic (Tweens and Tritons) surfactants in dispersing nanosilica particles in aqueous and cement pore solution was investigated in the first paper. In general, strong correlations between the critical micelle concentration of nonionic surfactants and the average hydrodynamic size and polydispersity index of nanosilica particles in solution were established. Following this, the nonionic surfactant Triton X-405 exhibited the highest efficacy in deagglomerating and stabilizing nanosilica. The effect of surfactants on the properties of nanosilica-modified cement paste was evaluated in the second paper. The nonionic surfactants reduced the flow and accelerated the hydration of the paste. The highest compressive strength increments (33%, 41%, and 54% at 1, 3, and 28-day, respectively) were obtained using Triton X-405. Further characterization tests such as S(T)EM, TGA, and XRD were performed. The results revealed enhanced nucleation of outer product CSH gel and densification of its regions due to the formation of calcite nanocrystals that improved the dispersion of nanosilica particles and strength of paste containing Triton X-405. In paper 3, a novel SEM-EDS and image analysis-based methodology is presented to quantify the dispersion of nanosilica in the hardened cement matrix. The developed methodology showed a relatively small mean absolute error (1.6 volume %) and can serve as a guide to develop a similar one for other NPs such as nanoalumina. Besides, for the first time, the agglomeration and dispersion of nanosilica in the hardened cement paste were characterized using various quantitative descriptors such as Delaunay triangulation, free-space length, and gap statistic. The effect of particles of nanosilica (dry powder and colloidal), nanoalumina, and nanolimestone on the hydration kinetics and apparent activation energy of cement was analyzed in the fourth paper. Different variables, such as NP replacement levels (0, 0.5, and 5 wt. %) and testing temperature (10-40 ̊C), were evaluated. For a given type of NPs, the acceleration effect on cement hydration increased with increasing NP replacement level, but it reduced with increasing testing temperature. Apparent activation energy increased at low replacement (0.5%) while it reduced at high replacement level (5%). Considering both the size and replacement level of NPs, a unified index (area multiplier) was also introduced in this paper.