CEAE PhD Defense: Oussama Khouchani - "INCORPORATION OF CELLULOSE NANOCRYSTALS IN PVA-REINFORCED AND POLYPROPYLENE-REINFORCED GEOPOLYMER COMPOSITES"

            Geopolymers have garnered immense interest within the construction materials research community as a prospective alternative to Ordinary Portland cement. This results from their lower carbon footprint and higher performance characteristics, including high compressive strength, high fire and acid resistance, low thermal conductivity, and low shrinkage. However, the large applications of geopolymer materials in construction have been limited due to several challenges. Foremost among these challenges is the issue of brittleness, due to their ceramic-like behavior. The incorporation of fiber and nanoparticle reinforcements emerged as an appealing strategy to overcome the inherent brittleness issues encountered in geopolymer materials. However, the high cost associated with nanoparticles (e.g. graphene, carbon nanotubes (CNTs)), combined with the limited improvements in toughness achieved through fiber incorporation (which are notably lower than those seen in fiber-reinforced OPC-based composites), present challenges in establishing geopolymer as a practical substitute to OPC. In this Ph.D. study, cellulose nanocrystals (CNC), cost-effective and eco-friendly nanoparticles, were exploited in fiber-reinforced geopolymer composites. The aim is to gain a deeper understanding of the interaction between CNC and fibers in geopolymer composites to further increase their mechanical performance. 

In the first part of this study, Polyvinyl alcohol (PVA)-reinforced metakaolin-based geopolymer composites with incorporated CNC were successfully synthesized. The flexural strength and fracture energy of the synthesized composites reached maximum enhancements of 190% and 325-folds respectively. The conducted analysis revealed that CNC incorporation increased the amount of formed geopolymer gel. Consequently, stronger interfacial bonding between the PVA and geopolymer matrix was achieved, resulting in an improvement in flexural strength and fracture energy. 

In the second part of the study, the interaction in the geopolymer mix between CNC and either fibers with high bonding characteristics (PVA) or fibers with low bonding characteristics (Polypropylene (PP)) was explored. The results based on the mechanical testing and microstructure analysis revealed that CNC is more compatible with fibers with high bonding characteristics, resulting in more enhancement in strength and toughness and enabling more toughening mechanisms. 

The third part of the study sheds light on the fiber/geopolymer matrix interface alterations by the incorporation of CNC. The pullout behavior of PVA and PP fibers in geopolymer matrices with and without CNC was characterized. Followed by mechanical testing of geopolymer samples with the same fibers to establish a correlation between the changes in the fiber/geopolymer interface and the observed increases in the mechanical performance of fiber-reinforced geopolymer composites. The enhancements of chemical debonding energy, pullout energy, and bond strength at the PVA/geopolymer matrix interface were notably greater than the PP/geopolymer matrix interface upon the incorporation of CNC. These enhancements can potentially justify the increases in the flexural strength, and fracture energy, and the enabling of more toughening mechanisms during loading.