# Laboratory Study on the Stiffening Phenomena Caused by Aging and by the Addition of Kraft Lignin at Low Dosages Measured by Physico-Chemical and Rheological Tests on a Soft Bitumen

## TL;DR

This study examines how aging and adding Kraft lignin affect the stiffness of soft bitumen used in cold climates.

## Contribution

The paper provides new insights into the limited anti-aging effects of low-dosage Kraft lignin in bitumen.

## Key findings

- Long-term aging significantly increases bitumen stiffness, while lignin causes only moderate stiffening.
- Kraft lignin does not provide noticeable anti-aging or antioxidant benefits to bitumen.
- Lignin behaves like an inert filler with minimal impact on bitumen's viscoelastic properties.

## Abstract

This study investigates the stiffening phenomena caused by aging and low-dosage Kraft lignin addition on a soft bitumen (PG58S–28)- used in cold climate regions. Through a combination of physico-chemical and rheological analyses, including Fourier-transform infrared spectroscopy (FTIR), Brookfield rheometer viscosity (BRV), dynamic shear rheometer (DSR), multiple stress creep recovery (MSCR), bending beam rheometer (BBR), and complex shear modulus (G*) tests, the impacts of lignin modification and thermo-oxidative aging are evaluated. In particular, the anti-aging potential of lignin is scrutinized. The results indicate that while the carbonyl index effectively tracks bitumen aging, the sulphoxide index is less reliable due to high initial S=O bond content in Kraft lignin and greater repeatability variability. Standard rheological tests (BRV, DSR, MSCR, and BBR) show that long-term aging significantly increases bitumen stiffness, while lignin modification leads to a moderate stiffening effect but does not exhibit any noticeable anti-aging properties. The G* analysis confirms that aging strongly influences bitumen rigidity, particularly at low and intermediate equivalent frequencies, while lignin acts similarly to an inert filler, with minimal effects on linear viscoelastic (LVE) behaviour. Overall, the study concludes that the addition of Kraft lignin at low dosage does not alter the fundamental aging mechanisms of bitumen, nor does it provide significant antioxidant benefits. These findings contribute to the ongoing discussion on bio-based bitumen modifiers and their role in sustainable pavement materials.