Protein enriched biowaste: A viable feedstock to make durable bio-binders for bituminous composites

Low resistant to water damage is a major concern in bituminous composites used in roads and roofs. Water damage is attributed to desorption of molecules which are pre-adsorbed to siliceous stone mainly due to the water molecules' higher affinity to active sites of siliceous substrates. This paper shows feasibility of engineering highly water-resistant bio-modifiers from a hybrid biomass feedstock containing a balanced combination of protein and lipid. In the present study, this concept is illustrated by synthesizing a novel bio-modifier referred to as Swilgae from coliquefaction of high-protein algae (A) with high-lipid swine manure (S) for use in bituminous composites. To examine efficacy of Swilgae bio-modifier to enhance bitumen's resistance to water damage, this paper specifically studies adsorption and de-wetting of Swilgae molecules from siliceous substrates as surrogates for quartz and granite stones used in bituminous composites. To do so, we use computational modeling geared toward the use of density functional theory (DFT) combined with laboratory experiments using a moisture-induced shear-thinning test and in-situ Fourier transform infrared (FTIR) spectroscopy. The study results showed that Swilgae bio-modifier is more effective at improving the asphalt resistance to moisture damage than those made from either algae or manure individually, owing to the synergy between lipid-rich swine manure combined with protein-rich algae. This was evidenced in bitumen containing 1A:1S Swilgae showing the least change in the moisture-induced shear thinning among all studied scenarios. This was attributed to the molecular structures of the Swilgae having both lower polarizability and higher adsorption energy per unit area of siliceous surface compared to those made from isolated feedstock. Additionally, Swilgae showed to passivate the active sites of silica and replace water molecules over the silica surface via a competitive adsorption with the water molecules on the hydroxylated silica surface that is consistent with the FTIR results. Therefore, 1A:1S Swilgae molecules build a well-structured bridge between bitumen and siliceous substrates, leading to improved adhesive forces and subsequently enhaced durability in the bituminous composites used in roads and roofs.