Core-shell modification of expanded polystyrene for tailoring interfacial microstructure and macro-properties of lightweight cementitious composites

EPS particles tend to distribute unevenly during concrete mixing owing to their surface hydrophobicity. The weak interfacial bonding between EPS particles and the cement paste further deteriorates the interfacial transition zone (ITZ). These issues result in inferior mechanical performance and limit the wider application of EPS lightweight concrete. However, efficient and controllable modification strategies remain scarce. This study develops a two-step approach to prepare surface-modified EPS (SM-EPS) and core-shell structured EPS (CSM-EPS). The hydration behavior and hydrophilicity were characterized using XRD, TGA, SEM, and contact angle measurements, with key parameters including water content, curing regime, modification method, and shell thickness optimized. Alkali-activated concretes (AAC) with a similar density grade were fabricated using EPS, SM-EPS, and CSM-EPS as lightweight aggregates. Results show that CSM-EPS specimens exhibited a 30.78% increase in compressive strength and a 3.66% lower thermal conductivity than the reference group. Microstructural analysis indicates improved interfacial bonding and densified ITZ. This core-shell modification strategy provides a potential approach for developing lightweight cementitious composites with improved mechanical performance while maintaining thermal insulation properties.