Surface-Modified Wheat Straw for the Production of Cement-Free Geopolymer Composite: Effects of Wheat Variety and Pre-Treatment Method

The development of new composite materials with specific properties and reduced environmental pollution can be achieved by the incorporation of agricultural residues, whose morphology is strongly affected by their variety and growing conditions. Herein, the functional properties of a cement-free geopolymer composite reinforced with straw from two wheat varieties (Ada and Malibu) were investigated through different straw pre-treatment methods and their surface modification with silane coupling agents. The characterization of the wheat surface and the geopolymer composites involved SEM-EDS, TGA, FTIR, and gas physisorption analysis methods supplemented with mechanical strength and moisture ingress measurements. Mild (23 °C) and severe (100 °C) physical pre-treatment methods with chemical soaking in 7.3 M isopropanol solution were applied on wheat straw. Tetraethoxysilane (TEOS) with octadecylamine was employed for chemical surface modification. The set of geopolymer compositions was prepared with untreated, pre-treated, and modified straws. The results revealed the hot pre-treatment method caused a higher degradation of siliceous layers of straw, especially in the Ada variety. The modification with TEOS resulted in irregular silane coating formation regardless of the wheat variety and pre-treatment method. Despite good interfacial bonding of the modified straw with the geopolymer matrix, the mechanical strength of the composites was reduced, although the resistance to water ingress slightly increased. Comparing both varieties, Ada wheat showed better performance than Malibu.