Bitumen in different forms is used as a binder in road construction. At ambient temperature, bitumen is an extremely high viscous liquid that is not workable. It can be transferred into a workable state in three ways:
* by heating
* by blending with petroleum solvents (cutback)
* by emulsifying in water to form bitumen emulsion

The first alternative is normally used for hot mixes on medium and large size jobs, where equipment is available for heating, storing, transporting and applying the bitumen. This is, however, less suitable on small jobs or when equipment is not available. The second alternative, cut-back, is generally more expensive than hot mix since solvents, which do not play any part in the function of the binder, are often quite costly. Furthermore, the solvents pollute the environment and they are fire hazardous. The third alternative, bitumen emulsion, does not require heating when applied and it has the advantage over hot bitumen that it can be used with cold and even damp aggregate. Most emulsions have satisfactory adhesion properties as they are, especially cationic emulsions.

An emulsion can be defined as a dispersion of small droplets of one liquid in another liquid. Bitumen emulsions are an oil-in-water type of emulsion where the bitumen is dispersed in water. The size of the droplets is generally in the range 0.001 to 0.02 mm. The bitumen content depends on the intended application of the emulsion, but is rarely lower than 40% or higher than 70%.

The droplets will be packed so tightly that they will partly adhere to each other and eventually the water entrapped between the droplets will become water droplets. The result will be a water-in-oil emulsion, or an inverted emulsion. Such an emulsion has the nature of bitumen, with high viscosity.

The limit to bitumen content is in the range 70% to 80% and depends mainly on the size distribution of the particles.

To obtain a stable bitumen emulsion, an emulsifier has to be added. The bitumen droplets are kept apart due to the effect of the ionised emulsifier molecules, which orientates themselves to the surface of the droplets forming an electrostatic force field. The stability of the emulsion depends to a great extent on the strength of this force field. If the emulsifier is of cationic type, the droplets will be positively charged (cationic emulsion) while with an anionic emulsifier the charge will be negative (anionic emulsion).

Most countries have specifications for both anionic and cationic emulsions. These two categories of emulsions are divided into three classes, depending on the stability of the emulsion when in contact with aggregate or pavement surfaces, i.e. rapid, medium and slow setting emulsions. A rapid setting emulsion has little or no ability to mix with an aggregate, a medium setting emulsion is expected to mix with a coarse but not fine aggregate and a slow setting emulsion is designed to mix with fine aggregate.

Anionic emulsions were first developed in the early 1900s. They found their applications but the growth in use was relatively slow. In the mid 1940s cationic emulsions were introduced, which meant a major technical improvement.

Contents of cationic emulsions

Bitumen
Depending on the climate the bitumen used is from pen 50 up to 200. The quality of the bitumen very much affects the properties of the emulsion. Some bitumens give large particles in the emulsion. In some cases this problem can be solved by adding surfactant into the bitumen. Therefore, it is important to check the formulation in the laboratory before producing in the plant when changing to a new type of bitumen.

Solvent
Solvent can be used to give a temporary softening to the binder. The bitumen will then have an improved coating ability to the aggregate. The solvent also helps a prime coat emulsion to penetrate a dense base course.

When choosing a solvent there are two things to consider. First, the solvent has to be compatible with the bitumen, i.e. when mixing bitumen and solvent the mix should be homogeneous with no signs of separation. Second, the solvent has to be sufficiently volatile not to leave any residue in the bitumen which can give a lowered viscosity. White spirit is the most widely used solvent.

Dope in the bitumen
An emulsion produced of difficult bitumen can in some cases be improved by adding a surfactant to the bitumen. This is called "to dope the bitumen". This gives the emulsion more stability but does not affect the breaking.

Emulsifier
A careful choice of emulsifier is essential in order to get emulsions with the desired properties. There are many chemical compounds that can be used to emulsify bitumen, but for technical and economic reasons only a few have become widely used. These compounds can be used alone or combined with one or more of the other compounds. They can also be modified in different ways in order to obtain special properties. A good emulsifier should, apart from giving the emulsion its specified properties, also have a good cost-performance and preferably be safe and easy to handle.

The emulsifier normally consists of a long hydrocarbon chain, which terminates with either an anionic or cationic functional group. The paraffinic portion (the hydrocarbon chain) of the emulsifier ion orientates itself on the surface of the bitumen droplet, so that the hydrocarbon chain is firmly bound to the bitumen, with the ionic portion located at the surface of the droplet. Hence the droplets become electrically charged, positive charges for cationic and negative charges for anionic emulsions.

The true picture, however, is more complex. It involves all types of ions and molecules in the solution. A corresponding reaction takes place in an anionic emulsion. The properties of the double layer have a strong influence on emulsion stability and viscosity.

Cationic Emulsifiers
The cationic emulsifiers are generally based on long hydrocarbon nitrogen compounds, such as alkyl amines. The alkyl amines are powerful surface-active compounds with great influence on the surface tension. They can be modified in a number of ways to meet almost any requirement. Most emulsifiers need to be reacted with an acid before they can function and the pH of the emulsion will by this be lower than 7. The acid, in most cases hydrochloric acid, reacts with the nitrogen and forms an ammonium salt. In a batch plant this is often done by slowly adding acid and emulsifier to warm water under continuous agitation. When all emulsifier has been added and dissolved, the pH is adjusted by adding more acid until the correct pH is reached. In a continuous plant, the emulsifier is injected into the water line. Acid is added the same way and the reaction takes place before the water enters the mill. Liquid and easily dispersible emulsifiers are preferable in this type of plant.

Anionic emulsifier
Anionic emulsifiers are normally based on fatty acids. A fatty acid molecule consists of a long hydrocarbon chain and terminates with a carboxyl group. The emulsifier solution is prepared by reacting the anionic emulsifier with sodium hydroxide. This reaction is called saponification. The pH of an anionic emulsion is higher than 7 and the emulsion normally contains an excess of sodium hydroxide, which eventually reacts with any natural acids contained in the bitumen.

Cationic emulsifier			Anionic emulsifier

Stabiliser
When using a cationic system calcium chloride can be added to the emulsion to increase the stability and prevent swelling of the bitumen droplets caused by osmosis, i.e. when there is a high content of salt in the bitumen and the water is pulled into the bitumen droplets.

In an anionic system trisodium polyphosphate can be used for the same purpose.

Aggregate
Aggregates are classified as alkaline or acidic. The more acidic aggregates develop negatively charged surfaces in water over a wider pH range than alkaline aggregates, although at high pH all aggregates tend to become negatively charged. Aggregates rarely consist of one pure mineral and even a pure mineral may have both alkaline and acidic components. If aggregates are arranged according to their content of silica, as in the diagram, then those with the highest silica content are the most acidic with a strong tendency to adopt a negatively charged surface in water.

Many aggregates may also be contaminated with clay particles, which even in small quantities will increase the breaking rate.

Content of Silica, %

In most cases the aggregates are negatively charged and therefore the positively charged emulsifier is drawn electrostatically to the aggregate causing the emulsion to break. In addition, the emulsifier works as an adhesion agent at this point.

An anionic emulsion does not break chemically. The water evaporates and causes the emulsion to break. This is a much slower process and the adhesion is very poor.

Viscosity
For CRS and CMS emulsions the viscosity is important. When the CRS is used for surface dressing, the emulsion should have a certain viscosity. If the viscosity is too low, the emulsion will not stay thick. If the viscosity is too high, the emulsion will stay in lines on the surface.

The viscosity is controlled by bitumen content, temperature and emulsifier type. Some bitumen gives a higher viscosity than others. This is normally due to osmosis and can be corrected by adding CaCl2.

To correct the viscosity:
* Increase or decrease the bitumen content
* Add or take away the CaCl2
* Change emulsifier type

Flocculation is a process where the droplets start adhering to each other. Very often there is a large central droplet with smaller droplets surrounding it. Flocculated droplets can often be separated again by agitation.



Settlement
The bitumen has a slightly higher density than water and therefore, due to the gravity, the droplets tend to orientate themselves to the bottom of the container. If solvent is added to the bitumen, the bitumen can get a lower density than water and hence orientate itself to the surface. This is called "creaming".

Settlement can be reduced by:

· Improving storage conditions, for example keeping the emulsion at
· A higher temperature and agitating for short periods.
· Reducing the density of the bitumen by adding a solvent
· Preventing flocculation by changing types and concentrations of stabiliser and emulsifier or by changing the pH.
· Increasing bitumen content
· Increasing emulsion viscosity

· Insufficient emulsifier content
· Wrong type of emulsifier
· Wrong temperatures during production
· Wrong storage temperature
· Mechanical influence. (Circulated with a pump for a long time, transports).
· Unsuitable bitumen