Modification by chemical modifiers

The chemical modifiers, such as sulfur, lignin and certain organo-metallic compounds, do not really modify the bitumen but, rather, the properties of the asphalt mixture. The sulfur or the lignin extend the properties of the bitumen and modify the properties of the asphalt. Hence, the sulfur and the lignin are called bitumen extenders. The organo-metallic compounds modify the asphalt mixture by their catalytic action. 

The bitumen extenders replace the bitumen needed in an asphalt mixture while the organo-metallic compounds are added to the bitumen. The typical range of bitumen replacement seems to be 30% to 40%, when sulfur is used, and 10% to 12% when lignin is used. When organo-metallic compounds are used, approximately 2% to 3%, by mass of bitumen, is added. 

The concept of using a sulfur extender/modifier is very old, but it was not until the 1970s, during the advent of sulfur-extended asphalts, that a process was developed and started to be used on project sites (Beatty et al. 1987 Denning and Carswell 1981b Kennepohl et al. 1975). 

The high sulfur cost in comparison to bitumen during that period, the blending and storage problems encountered and the health/safety concerns had affected the use of sulfur in asphalt works. The development of solid flour pellets, added directly to the asphalt mixing process, and changes in sulfur/bitumen cost eliminated the abovementioned problems. 

Another bitumen extender product based on crumb rubber, the so-called elastomeric asphalt extender, has been recently introduced in the market under the trade name RuBind. 

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