Improvement of Core–Shell Lightweight Aggregate by Modifying the Cement–EPS Interface

To improve the interfacial compatibility between cement matrix and expanded polystyrene (EPS) in core–shell lightweight aggregates (CSLA), the effects of sodium silicate, polyvinyl acetate (PVA) emulsion, vinyl acetate–ethylene (VAE) emulsion, acrylic acid, and acetic acid on the cement–EPS interface were investigated. The density of the interface was studied by scanning electron microscopy (SEM), and the effect of interfacial agents on the hydration process of cement was studied by the heat of hydration and induction resistivity. The macroscopic properties of the interface of the CSLA were characterized by the “leak-white” rate, drop resistance, and numerical crushing strength. The results show that the sodium silicate densifies the interface by generating hydration products on the EPS surface. At the same time, organic acid enhances the interfacial properties of EPS and cement by increasing the surface roughness, and allowing hydration products to grow in the surface micropores. In terms of the cement hydration process, both interfacial agents delay the cement hydration. Above all, with comprehensive interface properties, “leak-white” rate, and mechanical properties, VAE emulsion and sodium silicate can achieve the best performance with a final crushing resistance of 5.7 MPa, which had a 46% increase compared with the reference group.