Effect of aggregate surface clay contamination on the moisture susceptibility of hot-mix asphalt

Naturally adhering clay contamination on aggregate surfaces is an unavoidable consequence of quarrying and aggregate processing, yet its influence on asphalt mixture performance has received considerably less attention than aggregate mineralogy or intentionally added mineral fillers. Herein, we investigate the effects of aggregate surface clay contamination on the adhesion performance and moisture resistance of asphalt mixtures prepared with five limestone aggregates and two conventional paving asphalt binders (#70 and #90 penetration grades). The aggregates and adhering clays were characterized using X-ray fluorescence (XRF) and quantitative X-ray diffraction (QXRD), while asphalt–aggregate interactions were evaluated through surface free energy (SFE) analysis, indirect tensile strength ratio (TSR), a modified boiling-water test with digital image analysis, and Cantabro loss. The results show that naturally adhering clay contamination significantly deteriorated moisture resistance, despite increasing the calculated dry adhesion work because of its highly polar surface. Even at a relatively low level of surface-adhering clay contents (1.03–2.36 wt%) recovered from aggregate particles retained on the 300 μm sieve, the dry and moisture-conditioned indirect tensile strengths decreased by 7.20–19.09% and 15.63–40.88%, respectively, accompanied by reductions of 5.7–22.0 %age points in TSR. Notably, the presence of this relatively small amount of surface clay contamination was sufficient to cause asphalt mixtures containing limestone, an aggregate generally regarded as highly moisture resistant, to fail the commonly accepted TSR criterion. The deterioration is attributed to the hydrophilic nature of the adhering clay, which readily adsorbs water and facilitates moisture ingress to the asphalt–aggregate interface, thereby weakening interfacial bonding and accelerating asphalt stripping and particle loss. These findings demonstrate that aggregate surface cleanliness is a critical factor governing moisture resistance and should be considered in aggregate quality evaluation and asphalt mixture design.