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%.
There is an upper limit to the bitumen content of an emulsion, which mainly depends on the
relative volume of the two phases. When reaching this limit there is insufficient room for
more droplets without deforming them.
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.
Physical and chemical properties of white spirit:
Boiling point
150-200°C
Flash point
+38°C
Density
0.75 kg/litre
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.
In a cationic emulsion the positively charged ions orientate themselves to the surface of the
bitumen droplets. The negatively charged chloride ions are attracted to the surface of the
droplets by the positive charges and an electrical double layer is formed by the ions in the
emulsion.
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
Acid
Acid is used to form a salt from the emulsifier and for lowering the pH to a predetermined
level. When using a cationic system hydrochloric acid is used. In a anionic system sodium
hydroxide is used.
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, %
Emulsion properties
Breaking
The emulsion should be stable during storage and transport but should break quickly when
applied. This quality is most easily achievable with cationic emulsions since such emulsions
react and break chemically in contact with most aggregates.
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.
The breaking can be controlled by the choice of emulsifier and emulsifier content. Other
factors that influence the breaking are temperature, humidity and type of bitumen.
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
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
Coalescence
When droplets merge and form larger particles, the emulsion will eventually break. This
often starts with flocculation, which can be influenced by the following:
· 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
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