Low-Carbon and Sustainable Fly Ash–Slag-Based Geopolymer/SBS-Modified Emulsified Asphalt Composites: Compatibility and Physicochemical Properties

Abstract The fly ash–slag-based geopolymer (FASGG)/SBS-modified emulsified asphalt (SBS-EA) composite demonstrates significant potential for advancing low-carbon and sustainable cold recycling technologies for asphalt pavements. However, its compatibility and interaction mechanisms remain insufficiently understood. In this study, molecular dynamics simulations and a series of macro- and microscale experiments are conducted to investigate the compatibility between FASGG and SBS-EA and the physicochemical characteristics of the resulting composite. N-A–S–H demonstrates superior compatibility with SBS-EA compared with C-A–S–H and APTES further improves this compatibility. Ca2+ and Na+ in FASGG exhibit strong coordination or ionic interactions with oxygen-containing functional groups in asphalt, resulting in combined chemical–physical adsorption. FASGG accelerates the demulsification of SBS-EA, and the fusion process evolves through four stages. Fluorescence microscopy reveals a dense, dual-phase interpenetrating organic–inorganic structure, in which the continuous SBS-EA phase dominates and FASGG interweaves with SBS-EA to form a cross-linked network. The inorganic gel fills the SBS-EA matrix and forms a three-dimensional entangled network, thereby improving overall compactness. However, excessive FASGG increases surface roughness and tearing, potentially reducing crack resistance. These findings provide theoretical support for improving cold-recycled asphalt pavements and offer insights into the development of high-performance green pavement materials.