Mussel-inspired polydopamine functionalization of recycled plastic aggregates for enhancing mortar performance

Abstract Plastic waste accumulation and growing natural sand shortages have spurred a wealth of research on recycled plastics as sustainable fine aggregates in cement mortar. However, unmodified recycled plastics have inherently inert and hydrophobic surfaces that form weak bonds with cement paste and cause large drops in mechanical strength. To solve this problem, we developed a mussel-inspired surface modification method that deposits polydopamine (PDA) coatings on recycled polypropylene particles (RPP) through alkaline oxidative self-polymerization. The resulting PDA coating creates a reactive surface layer rich in hydrophilic hydroxyl and amine groups, lowering the water contact angle from 109.5 ± 5.3° for unmodified RPP to 60.8 ± 4.2° at the highest dopamine concentration used in this study. Catechol groups in the PDA coating may interact with calcium ions (Ca2+) released during cement hydration, potentially strengthening the plastic-cement interface. Among the dopamine concentrations tested (1, 2, and 3 g/L), mortars containing 40 vol% RPP aggregates modified with 2 g/L dopamine showed the highest measured mechanical performance. At 28 days, flexural and compressive strengths increased by 45.7% and 47.8%, respectively, relative to mortars with unmodified RPP, recovering 86% of the compressive strength of plain cement mortar. The use of excessive dopamine (3 g/L) during particle modification resulted in incomplete polymerization and surface agglomeration, producing small decreases in both strength metrics. These performance improvements are consistent with a proposed PDA-Ca2+ interaction mechanism that may help densify the interfacial transition zone (ITZ) and promote heterogeneous nucleation of hydration products on the RPP surface. This bio-inspired surface modification approach improves the interfacial compatibility of recycled polypropylene in cement mortar and recovers most of the mechanical strength lost due to the addition of plastic aggregates.