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Lignin is abundant in nature. The use of lignin in the asphalt pavement industry can improve pavement performance while effectively optimizing pavement construction costs. The purpose of this paper is to study the effect of lignin on the anti-aging properties of asphalt. Commercial lignin was selected to prepare a lignin-modified asphalt binder. The properties of lignin-modified asphalt were studied by rheological experiments. The high-temperature rheological properties of two kinds of base asphalt and modified asphalt samples with different contents of lignin under three conditions of original, rolling thin film oven (RTFO) aging, and pressure aging vessel (PAV) were tested and analyzed with temperature sweep, frequency sweep, and multiple stress creep recovery (MSCR) tests. By comparing the variation laws of evaluation indicators, such as complex shear modulus G*, phase angle δ, anti-aging index, cumulative strain, and viscous component Gv, we found that lignin could effectively improve the high-temperature stability of base asphalt, but it had a negative impact on the compatibility issues of base asphalt. Meanwhile, lignin played a filling role in the base asphalt, and the increase in viscosity was the fundamental reason for improving the high-temperature stability of the base asphalt. The research results indicated that lignin could effectively improve the anti-aging performance of asphalt and play a positive role in prolonging the service life of pavement.

Lignin is a major waste product of biofuel and paper industries that can be used as a modifier to improve the relevant properties of asphalt. To investigate the effect of lignin and formic acid lignin wood incorporations into asphalt and the effect on asphalt binder that was unaged and aged for 85 min, 5 h, and 10 h, a series of tests were conducted, including high- and low-temperature rheological tests by a dynamic shear rheometer (DSR), followed by Fourier transform infrared spectroscopy (FTIR) tests, and finally by gel permeation chromatography (GPC). The test results show that the additions of lignin and formic acid lignin could improve the high-temperature performance and fatigue capacity of asphalt. In addition, at the glass-transition temperature, it was observed that the additions of lignin and formic acid lignin into asphalt can effectively improve cracking at low temperatures; however, the quantity of lignin and formic acid lignin should be controlled. Fourier transform infrared spectroscopy tests showed that the purity of lignin treated with formic acid decreased, and degradation and formylation of the same formic acid-treated lignin occurred, indicating that the lignin underwent chemical changes following acid treatment. The analysis of the results by gel permeation chromatography (GPC) showed that, with aging, the average molecular weight (Mw) of lignin-modified asphalt decreased. The reason was that lignin and formic acid lignin were cracked during aging, which reduced their molecular weights.

The production process of the traditional paper making process produces a large amount of waste water, known as paper black liquor. It is needed to explore new ways of reusing this waste and replacing part of the base bitumen to reduce the consumption of non-renewable resources, such as petroleum, thus obtaining better environmental, economic, and social benefits. This paper analyses the feasibility of using paper black liquor, which contains a large amount of lignin, as a modifier for bitumen in the paper industry. Samples of modified bitumen were prepared with 15%, 30%, and 45% of the base bitumen replaced by paper black liquor, and a control group of base bitumen was prepared for testing. The samples were subjected to an 85 min short-term ageing test, FTIR scanning test, thermogravimetric test, frequency scanning test, MSCR test, and LAS test. The analysis of the FTIR and thermogravimetric tests showed that the paper black liquor was mainly composed of lignin and some cellulose, and contained a small amount of salts with Na ions; based on the results of the frequency scan, the compatibility analysis of the vGP curve showed that the modified bitumen was more compatible in the high-temperature range after short-term ageing, thus inferring that the water content of the concentrate had an influence on the compatibility, making it necessary to further investigate different optimum water contents to achieve the best performance and benefit. The incorporation of paper black liquor improved the rutting resistance and fatigue resistance of the modified bitumen, and also gave the paper-black-liquor-modified bitumen a better ageing resistance than the base bitumen. While demonstrating the feasibility of using paper black liquor as an bitumen modifier, this study also helps to provide a basis for theoretical applications of biomass materials in the field of road engineering.

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