Valorising South African lignin as a sustainable bitumen additive in pavement engineering

The construction industry is one of the largest contributors to global carbon emissions; therefore, the demand for sustainable alternatives is constantly increasing. There is a large focus on finding material alternatives that fit within the context of a circular economy. One such material is lignin. Lignin is a biopolymer found in the cell walls of plants. Lignin-rich spent pulping liquors are produced as waste streams from the paper and pulp industry. Lignin is not widely utilised in higher value applications; however, literature has shown promise with regard to using lignin as a partial substitute for bitumen due to chemical similarities. Bitumen is a viscoelastic liquid obtained from the distillation of crude oil and is typically used to bind aggregates to produce asphalt. This study investigated the performance of four different lignin types (Sappi kraft, Mondi kraft, Sappi soda and Sappi lignosulfonate) sourced from various pulp and paper mills around South Africa. The kraft and soda lignins were separated from their respective spent liquors using acid precipitation, while the lignosulfonate spent liquor was spray dried. The lignins were characterised, and they were then blended with bitumen at a 10 wt% blending ratio. This study found that Sappi soda lignin increased the resistance to permanent deformation (69 %) and the elastic recovery percentage (586 %). This can be explained through the lower content of phenolic hydroxyl groups and lower S/G ratio in Sappi soda lignin, limiting the hydrogen bonding capacity. This increases the elasticity of the soda lignin-modified bitumen (LMB) blend, while the other types of lignins form a stiffening network in the blends. Soda LMB also showed a significantly higher critical high temperature (2 °C), as well as wet indirect tensile strengths (ITS) of the associated asphalt specimens (19.7 %). Both types of kraft lignins also generally improved the performance of the bitumen, while the lignosulfonates generally did not statistically significantly change the performance of the binder. This study thus concluded that all types of lignin tested are feasible as bitumen modifiers at a 10 wt% blending ratio, with the Sappi soda LMB blend performing the best. The soda lignin was then blended with bitumen at higher blending ratios (20 wt% and 30 wt%). The 30 wt% blends experienced phase separation, limiting the practical feasibility of this blend. The 20 wt% blend significantly improved the dry ITS of asphalt specimens made from the bitumen (26.7 %); however, the drawbacks of poor storage stability hinder the feasibility of this blend. This study thus concluded that Sappi soda lignin is feasible as a partial bitumen substitute up to a 10 wt% blending ratio. This thesis concluded that all types of lignins tested are suitable as bitumen modifiers at a 10 wt% blending ratio. Further studies should investigate the industrial feasibility of this process through techno-economic analyses and life cycle assessments. Further work is also required regarding the processing of the spent pulping liquor (spray drying versus acid precipitation) to understand how this effects the properties of the LMB.

Grantor dc:publisher

Stellenbosch : Stellenbosch University

Year dc:date.issued

2026

Author dc:creator

• Vermeulen, Ryan

Advisors dc:contributor.advisor

• Gorgens, Johann Ferdinand
• Tyhoda, Luvuyo
• Goosen, Elaine Simone

Language dc:language.iso

en

Repository record dc:identifier.uri

https://scholar.sun.ac.za/handle/10019.1/135620

OAI identifier oai:identifier

oai:scholar.sun.ac.za:10019.1/135620