Applied as a binder in road construction, bitumen is required to behave as a liquid during its application. Other than heating, there are at least two other ways with which bitumen can be made to behave as a liquid at lower ambient temperatures. One option is to dilute bitumen with petroleum solvents like kerosene and naphtha until it becomes fluid enough for application; the final strength of the material develops when the solvent evaporates. Another option is to emulsify bitumen wherein it is liquefied by dispersing in water. Bitumen emulsions are used widely in road construction, mostly in maintenance and repair works in the form of slurry seals and surface dressing, and in structural pavements in the form of prime and tack coats, cold mixes, recycled asphalt mixes and more. In road construction applications, bitumen emulsions provide a safer and more environment-friendly system than hot bitumen by avoiding the risks of fire and burns; using less energy in processes; and reducing emissions. What are bituminous emulsions? Bitumen emulsions are usually dispersions of minute droplets of bitumen in water. The droplets normally range from 0.1 to 20 microns in diameter. The bitumen content in bitumen emulsions are varied to suit different requirements and typically varies between 30 per cent and 70 per cent. Bitumen emulsions are usually made using a colloid mill. In the colloid mill, hot bitumen is sheared rapidly in water containing an emulsifying chemical agent (emulsifier). This results in the production of very small particles of bitumen dispersed in water (continuous phase). The bitumen particles in suspension do not readily coalesce owing to the presence of the emulsifier, which will get concentrated on the surface of the bitumen particles. It is necessary to apply mechanical energy in the right way to create small drops of bitumen; once formed, its coalescence shall also be prevented.The internal cohesion and viscosity of the bitumen and the surface tension forces resist the creation of a new interface that essentially opposes the emulsification process. Added to it is the tendency of droplets to coalesceor re-join. Thus, the droplets must be stabilised against coalescence by preventing them from getting very close together. The emulsifier added andadsorbed at the surface of droplets provides an electricaland steric repulsion energy barrier that helpsprevent close approach of the droplets. Even if this energybarrier is overcome and the droplets flocculate,the film of emulsifier on the surface still inhibitscoalescence. Flocculation and coalescence may be initiated by contactwith minerals or by settlement, boiling, etc. Bitumen emulsions will contain more emulsifier than necessary to fill the interface, known as ‘free’ emulsifier. This free emulsifier will help provide good stability and right performance properties and prevent coalescenceduring emulsification, storage and transport. Cationic vs.anionic bitumen emulsions The coating of the bitumen particles by the emulsifier gives them an electrostatic charge. Depending on the type of emulsifier, this can generally be either negative or positive. Emulsions in which the bitumen droplets are negatively charged are called anionic emulsions while emulsions in which they are positively charged are called cationic emulsions. The emulsifiers used to make anionic emulsions are typically prepared from fatty acids by reaction with a strong base such as sodium hydroxide or potassium hydroxide. The resultant fatty acid salt imparts a negative charge to the surface of the bitumen droplets in an emulsion. Figure 1: Bitumen droplet (anionic)© TransCalc Cationic emulsifiers are usually prepared from fatty amines by reaction with a strong acid such as hydrochloric acid. Fatty amine salts impart a positive charge to the surface of the bitumen droplets in an emulsion. pH has an important role in ensuring that the emulsion remains either anionic or cationic. Typically, an anionic emulsion has an alkaline pH of over 7 and a cationic emulsion has an acidic pH of below 7. Breaking and curing of bituminous emulsions Bitumen emulsions are required to remain stable so that they can be transported, stored and handled easily. At the same time, they must be made to separate or ‘break’ when they comeinto contact with aggregates or other pavement surfaces during road construction. This process of breaking iscrucial in enabling bituminous emulsions to become a continuous bitumen film to fulfiltheir role as a binder in road materials or as protective coatings. The speed ofthis setting and curing process dependon the reactivity of the emulsion, reactivity of the aggregate and environmental conditions such astemperature and humidity. Aggregates take up a characteristic surface charge that depends on pH and the nature of the minerals present. Silicious aggregates that are high in silica tend to take up a negative charge, whereas basic aggregates like limestone can take a positive charge.Thus, cationic emulsions react faster with an aggregate with negative surface charge. The first stage in the breaking and curing process involves the adsorption of emulsifier onto the aggregate surface. While free emulsifier gets adsorbed rapidly; emulsifier isabstracted from the bitumen water interface much more slowly. Cationic emulsifiers adsorb much more strongly on siliceous minerals than anionic ornonionic emulsifiers. The droplets of bitumen in the emulsion have a small charge and move towardsan aggregate surface with the opposite charge (electrophoresis), which results in concentration at the aggregate surface. This brings the particles closer together leading to flocculation, coalescence andspreading over the surface. The water eventually leaves the system by evaporation, resulting in the concentration of droplets and coalescence. Thesetting characteristics of emulsions are defined in terms of rapid (R), medium (M) and slow (S) settings. Rapid-setting emulsions set quickly in contact with clean aggregates of low surface area. Medium-setting emulsions set sufficiently less quickly that they can be mixed with aggregates of low surface area. Slow-setting emulsions will mix with aggregates of high surface area. Slow-setting emulsions are more stable than medium-setting emulsions which, in turn, are more stable than rapid-setting emulsions. Applications of bituminous emulsions Bituminous emulsions are used in a wide range of applications in road construction, varying from their use as bond coats to cold-in-place recycling and even in soil stabilisation applications. Bituminous emulsions are used as such without any addition of mineral aggregates as is the case with tack coats, prime coats and fog seals, and in pavement repair applications such as crack sealing, bond material during pothole repairs, etc. Bituminous emulsions in combination with graded aggregates, mineral fillers, additives and water are being widely used worldwide for various surface treatment applications such as single and double surface treatments, cape seals, slurry seals, micro-surfacing, seal coats, etc, and in soil stabilisation, construction of emulsion treated bases (ETB) and bitumen stabilised mixes (BSM). The use of bituminous emulsions in asphalt recycling applications is gaining popularity in recent years as a result of successful application in both in-situ and in-plant cold recycling projects including full depth reclamation (FDR) projects. About the author: Sreejith GHcomes with over 18 years’ experience inthe construction industry, primarily in highways and infrastructure projects in India and abroad in World Bank and ADB projects.