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Multiscale Characterization of Cementitious Interphase Regions Using an Integrated Microstructural-Mechanical-Chemical Approach
Effective properties and structural performance of cementitious mixtures are substantially governed by the quality of the interphase region because it acts as a bridge transferring forces between aggregates and a binding matrix and is generally susceptible to damage. In spite of advancements made over the last several decades, understanding the interfacial region of cementitious mixtures still presents important challenges. This is primarily due to its small length scale and the complexity raised by mixture components and design. As non-traditional additives such as recycled aggregates and alternative binding agents are more often used today, there is a growing need of fundamental knowledge to uncover interphase formation mechanisms and a resulting model to predict interphase properties. This study aims to characterize the interphase region in cementitious mixtures and provide key information and better understanding of interphase formation mechanism due to the interaction between concrete constituents. The microstructural, chemical, and mechanical properties of the interphase region formed due to the interaction of two different types of binding materials (i.e., fly ash-based geopolymer and ordinary Portland cement) with three different aggregates (i.e., limestone, quartz, and recycled concrete aggregate) were examined. To this end, microstructural characteristics using scanning microscopies, nanomechanical properties by nanoindentation tests, and spatial mapping of chemical components based on the energy dispersive spectroscopy were integrated to identify and investigate the interphase region formed by the case-specific interactions between the binding matrix and aggregate. The microstructural-nanomechanical-chemical mapping was effective to better understand links between material-specific properties of cementing phases. More specifically, the fly ash-based geopolymer paste was usually well bonded to the aggregate surface with a rich formation of sodium aluminosilicate hydrate (N-A-S-H) gel, while interfacial debonding was often observed between aggregate surface and paste in ordinary Portland cement concrete. It was found out that the thickness of interfacial debonding is more dominantly influenced by moisture absorption capacity, while the surface chemistry of the aggregates did not significantly affect the characteristics of the interphase. In contrast, the interphase between aggregates and fly-ash based geopolymer seemed to be affected by the chemical composition of the aggregates. In addition, it was observed that geopolymeric materials can reach the pre-existing incomplete interphase within recycled concrete aggregate and create hydration-geopolymerization products that combine calcium-silicate-hydrate (C-S-H) and N-A-S-H gel.
Khedmati, Mahdieh, "Multiscale Characterization of Cementitious Interphase Regions Using an Integrated Microstructural-Mechanical-Chemical Approach" (2019). ETD collection for University of Nebraska - Lincoln. AAI27666213.