Evaluation of chemo-mechanical properties and microscopic characteristics of asphalt mastics with alternative geopolymer-based powders

Abstract

The transition to sustainable pavement materials requires innovative alternatives to traditional mineral fillers that can simultaneously deliver mechanical performance, durability, and reduced environmental impact. This study systematically investigates four geopolymer-based powders derived from fly ash (FAG), metakaolin (MKG), red mud (RM), and slag (S) as substitutes for conventional limestone fillers (WG and WG60K) in asphalt mastic. Rheological testing, chemical characterization, and microstructural analysis are conducted to evaluate their effects on the chemo-structural-mechanical behaviour of bitumen. Results show that FAG markedly enhances high-temperature performance, improving rutting resistance, thermal stability, and shear strength by up to 58%, 45%, and 62%, but exhibits poor fatigue resistance and limited stress-relaxation capacity. In contrast, slag and metakaolin powders offer a more balanced performance profile, with superior fatigue resistance, finer dispersion, and smoother surface morphology (Ra < 1.5 μm), making them promising candidates for durable pavements in warm and moderate climates. RM shows intermediate behaviour, providing good thermal stability but a rougher texture and stronger elastic stiffening. Although no chemical reaction is observed between powders and bitumen, physical interactions such as surface adsorption and alignment of aliphatic chains are found to stiffen the mastic and alter its temperature-dependent response. Surface roughness and dispersion quality are directly correlated with rheological performance, with coarser fillers (FAG, WG60K) enhanced rigidity but shortened fatigue life. Overall, Slag and metakaolin emerge as the most promising geopolymer fillers for durable asphalt pavements.