WHAT IS EMULSIFIED ASPHALT?
Emulsified asphalt is a pseudo-stabilization of asphalt cement (the “dispersed phase”) suspended in water (the “continuous phase”). The prefix “pseudo” (literally meaning “false”), is applicable in describing the thermodynamic stability of the emulsion-system (the propensity of an emulsion to remain stable or alternatively to “break” – separation of the asphalt and water-phases). Emulsified asphalt, by design, is intended to break upon interaction with aggregate but to remain stable during transport and storage at near-room-temperatures. As an analogy consider the tendency of fire-wood to burn but which is “kinetically hindered” to do so due to the lack of a spark (“activation energy”).
Once emulsified asphalt has been applied, the asphalt “falls out” from the emulsion and water is evaporated from the treated surface. The speed of the emulsified asphalt break upon mixing with aggregate is termed “set-speed”, this characteristic, along with emulsion viscosity, the softness or stiffness of the mother binder, the incorporation of solvents or additives such as fuel oils, antistrip agents, polymer additives, etc. is what produces the various grades of emulsified asphalt, each of which being uniquely tailored to a unique pavement preservation strategy.
EMULSIFIED ASPHALT – THE COMPONENTS
Asphalt cement is the residue from the vacuum distillation of crude oil – the “bottom of the barrel” so-to-speak of the crude of family. Hydrocarbons such as asphalt cement, are called so because they consist primarily of hydrogen and carbon with relatively few heteroatoms (e.g. nitrogen, sulfur, etc.). Hydrogen and carbon share the commonality of equal electronegativity – a measure of an atom’s affinity for electrons. Because of this, bonds occurring within hydrocarbons exhibit equal electron sharing known as “covalent bonds”. Such sharing mechanisms yield electronic structure free of sites of electron surplus or deficit (negatively and positively charged sites, respectively). Due to this, asphalt cement is said to be largely charge non-polarized (or simply non-polar substances).
In comparison to asphalt cement, wateris comprised solely of hydrogen and oxygen. And, unlike carbon (as found in hydrocarbons such as asphalt cement), oxygen is much greater in electronegativity than is hydrogen. This electronegativity-differential produces a surplus of negative charge on oxygen through an electron “hogging” effect, thus in-turn producing a deficit of negative charge on hydrogen. This mechanism of unequal electron sharing produces what are said to be charge polarized molecules. Charge polarized molecules align within electric fields and coordinate with other “polar” molecules so as to maximize favourable (positive-negative) and minimize unfavourable (negative-negative, positive-positive) interactions (much like the aligning of magnetized materials in a magnetic field).
These “polar opposite” electronic structures of asphalt and water is what leads to their mutual incompatibility and the hydrophobic (“water-fearing”) nature of asphalt.
EMULSIFIERS AND THE EMULSIFICATION PROCESS
Due to this asphalt-water incompatibility, to overcome coalescence of the asphalt binder mechanically sheared into the water-phase during emulsion production, emulsifiers are added to the water-phase to chemically stabilize the asphalt droplets. Emulsifiers, unlike asphalt or water, exhibit a hybrid of electronic structure, with one portion consisting of a hydrocarbon “tail group” and the other portion consisting of a chargeable “head group”. Due to these regional differences in emulsifier electronics, emulsifiers, once activated (charged) are able to act as “bridges” across which asphalt droplet facial tension is significantly reduced thus allowing for temporary suspensions of asphalt cement in water (i.e. an asphalt emulsion). Such suspensions are maintained via charge-repulsion among head-groups of adjacent droplets.
During emulsion production, emulsifiers are first activated/charged by the addition of small amounts of acid or base to yield positively or negatively charged head-groups, in the case of cationic and anionic emulsion, respectively (see illustrations 1 and 2 for examples of cationic soap production and anionic soap production).
Illustration 1: Cationic Emulsifier
Illustration 2: Anionic Emulsifier
Once activated and introduced to the asphalt-phase, such emulsifiers are able to bury their hydrocarbon tail-groups into the asphalt-phase while maintaining charge-polarized portions in the continuous/water-phase. It is these activated (charged) head-groups which produce a sphere of charge about the asphalt droplets thus maintaining the suspension through electrostatic repulsion – see illustration 3.
Illustration 3: Anionic Emulsion
Once used in the field, aggregate-emulsion reactions, working in synch with evaporation of water from the emulsion eventually forces adjacent droplets to come together providing an effective aggregate binder – see illustration 4.
Illustration 4: Evaporation of water from Emulsion producing Asphalt Binder