Multi-scale optimisation of high internal phase emulsified asphalt: microstructure, thermal stability and rheology

High internal phase emulsified asphalt (HIPEA) is a promising low-energy binder for sustainable pavement construction, yet its macroscopic performance is strongly influenced by the microstructure controlled during preparation. This study aims to optimise the preparation parameters for HIPEA and establish quantitative relationships between preparation conditions, microstructure and macroscopic performance. An L9(3⁴) orthogonal experimental design was employed to systematically investigate the effects of asphalt content (75%–89%), emulsifier dosage (2%–6%), mixing speed (600–1800 rpm) and mixing time (5–15 min) on HIPEA properties. Centrifugal stability tests and multiple linear regression modelling were used to identify dominant factors and optimise formulation. Fluorescence microscopy, combined with random forest image segmentation, quantified droplet microstructure evolution under thermal loading (60 °C and 90 °C), while dynamic shear rheometer (DSR) tests characterised high-temperature rheological behaviour using the Carreau–Yasuda and Herschel–Bulkley models. Range analysis revealed asphalt content as the dominant stability factor (40.8% contribution), followed by emulsifier dosage (24.1%), mixing speed (21.9%) and mixing time (13.2%). The optimised formulation (89% asphalt, 6% emulsifier, 1800 rpm, 11 min) achieved zero bleeding (P = 0%) and the finest droplet size (2.52 μm), maintaining exceptional thermal stability with minimal droplet growth at both 60 °C (2.74 → 3.44 μm) and 90 °C (2.86 → 4.15 μm). This formulation exhibited superior rheological performance, including the highest gel point (66.69 °C), zero-shear viscosity (3743.88 Pa · s at 60 °C) and yield stress (9.92 Pa at 60 °C). Strong negative correlations (|r|≥0.99) between droplet size and rheological parameters confirmed the microstructure-performance linkage. The established preparation-microstructure-performance relationship provides a multi-scale framework for designing high-performance HIPEA in sustainable pavement engineering.