Kraft Lignin as a Partial Filler Replacement in Asphalt Mixtures with Nanosilica-Modified Bitumen: Volumetric, Mechanical, Moisture-Related and Rheological Properties

Kraft lignin, an aromatic biopolymer from pulping, has mostly been studied as a binder modifier. Its use as a mineral-filler replacement has received little attention at the mixture level. This study evaluated the effect of kraft lignin replacing 25% and 75% of the limestone filler in a dense-graded asphalt mixture (dry process), alone and combined with 0.5% and 1.0% nanosilica blended into a 50/70 bitumen (wet process) as a modifier of the lignin effect. Volumetric properties, Marshall stability and flow, indirect tensile strength (ITS) before and after AASHTO T 283 conditioning, and recovered-binder rheology were measured. The materials were also examined by Fourier-transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Lignin lowered the voids in the mineral aggregate (VMA), effective binder content and air voids, and raised unconditioned ITS from 864 kPa to 1204–1232 kPa, but the tensile strength ratio (TSR) fell from 93.2% to 60.4% and 44.0%. Combined with nanosilica, the lignin mixtures gained a further 10–20% in unconditioned ITS, and at 75% replacement their TSR rose to 58.0–60.3%, a significant gain, though still below the control value. Recovered-binder rheology, determined once per binder, responded non-monotonically to nanosilica dosage. Lignin thus increased unconditioned strength at the expense of moisture resistance, and nanosilica offset only part of this loss.