Asphalt emulsion is a mixture of tiny asphalt droplets suspended in water, stabilized by a chemical emulsifying agent. It allows asphalt to be applied at much lower temperatures than traditional hot-mix methods, and in many cases without the petroleum solvents that older “cutback” asphalts relied on. A typical emulsion contains 55 to 70% asphalt cement, with water making up most of the remainder and a small percentage of emulsifier holding it all together.
How Asphalt Emulsion Is Made
Production starts by heating asphalt and water separately to target temperatures, then feeding both into a colloid mill, a high-speed mechanical device that shears the asphalt into extremely small droplets. The mill’s rotor spins at 1,000 to 6,000 revolutions per minute with a gap of just 0.25 to 0.50 millimeters. The result is droplets smaller than the diameter of a human hair, ranging from about 0.001 to 0.010 millimeters across.
An emulsifying agent, dissolved in the water phase before milling, coats each droplet and gives it an electrical charge. That charge keeps the droplets from clumping back together, the same principle that keeps milk from separating on the shelf. The temperature of the emulsion leaving the mill must stay below the boiling point of water to prevent the mixture from flashing to steam, unless a heat exchanger is used to cool it down immediately.
Cationic vs. Anionic Emulsions
The emulsifier determines whether the asphalt droplets carry a positive or negative electrical charge, and this distinction matters because aggregates (crushed stone, gravel, sand) also carry surface charges. For the emulsion to bond well with an aggregate, the charges need to attract rather than repel each other.
Cationic emulsions give the asphalt droplets a positive charge. They work with most aggregate types, which is why they dominate the market. The minerals in the water itself can affect stability: calcium and magnesium ions help cationic formulations stay stable in storage, and calcium chloride is often added for that reason. On the other hand, carbonate and bicarbonate ions in the water can destabilize cationic emulsions by reacting with the emulsifier.
Anionic emulsions carry a negative charge. They’re less commonly used but perform well with certain limestone and siliceous aggregates. Interestingly, the water chemistry that helps one type can hurt the other. Carbonate and bicarbonate ions actually benefit anionic emulsions through a buffering effect, while the calcium and magnesium ions that stabilize cationic versions can cause anionic emulsifiers to form insoluble salts and fall out of solution.
Setting Speed Grades
Beyond charge type, emulsions are classified by how fast they “break,” meaning how quickly the asphalt separates from the water and begins binding to aggregate. The three main categories each suit different jobs.
Rapid-setting (RS) emulsions react almost immediately on contact with aggregate. They’re designed for surface treatments like chip seals, where stone is spread onto freshly sprayed emulsion and needs to stick right away. High-viscosity RS grades resist running off sloped surfaces.
Medium-setting (MS) emulsions don’t break on contact, giving workers time to mix them with graded aggregate before placement. Depending on the formulation, the mix can stay workable anywhere from a few minutes to several months. These are common in cold-mix patching and stockpile mixes that crews keep on hand for pothole repair.
Slow-setting (SS) emulsions offer the longest working time and rely mainly on water evaporation to cure. They’re formulated for dense-graded aggregate mixes, where thorough coating of tightly packed stone requires extended mixing. Slurry seals and certain recycling operations use slow-setting grades.
A fourth category, quick-setting (QS), is designed specifically for slurry seal and microsurfacing work where roads need to reopen fast. A freshly placed microsurfacing layer can typically handle traffic in less than an hour.
Common Applications
Asphalt emulsion shows up across nearly every stage of road construction and maintenance. One of the most routine uses is as a tack coat, a thin layer sprayed between lifts of asphalt pavement to bond them together. Caltrans guidelines recommend application rates of 0.05 to 0.15 gallons per square yard, with rates near 0.10 gallons per square yard producing the most uniform coverage.
Slurry seals blend emulsion with very fine crushed aggregate, water, and additives to resurface worn pavement. Polymer is commonly added to the emulsion for better durability. Microsurfacing uses a similar mixture but includes chemical additives that trigger a controlled chemical break rather than relying on sun and heat to evaporate the water. This makes microsurfacing viable in cooler weather and shaded areas where a standard slurry seal would cure too slowly.
Other uses include prime coats on newly graded base courses, fog seals to rejuvenate oxidized surfaces, and cold in-place recycling where existing pavement is milled up, mixed with emulsion on site, and re-laid without a hot plant.
Environmental Advantages Over Cutback Asphalt
Before emulsions became widespread, cutback asphalts were the standard way to thin asphalt cement for field use. Cutbacks dissolve asphalt in petroleum solvents (gasoline, kerosene, or diesel-range oils) that evaporate after application. Those evaporating solvents are volatile organic compounds (VOCs), a significant source of air pollution and smog precursors.
EPA data shows the scale of the difference. A rapid-cure cutback with 25% solvent by volume loses about 17% of its total weight as VOC emissions. At 45% solvent, that figure jumps to 32%. Even slow-cure cutbacks release 5 to 10% of their weight as VOCs. Asphalt emulsions, which use water instead of solvents, are now used in place of cutbacks specifically to eliminate those emissions. They also save energy because the water phase doesn’t require the production, transport, and recovery costs associated with petroleum diluents.
Storage and Handling
Asphalt emulsion should be stored between 50°F (10°C) and 185°F (85°C). Exceeding 185°F risks boiling off the water and destroying the emulsion. Freezing is equally damaging: ice crystals rupture the emulsifier film around each asphalt droplet, causing the asphalt and water to separate permanently. Once an emulsion freezes and breaks, it cannot be reconstituted.
Storage tanks need insulation and weather-resistant covering, especially in climates with cold winters. Gentle recirculation or periodic agitation prevents the heavier asphalt droplets from settling to the bottom during long storage periods. Before use, the emulsion should be mixed thoroughly, as some separation is normal and doesn’t indicate a defective product, provided it hasn’t been caused by freezing.
Industry Standards and Testing
In the United States, ASTM D2397 is the primary specification for cationic emulsified asphalt, covering seven grades. The standard requires testing for viscosity, storage stability, particle charge, coating ability, water resistance, and the ratio of asphalt to water determined by distillation. These tests ensure the emulsion will perform as expected for its designated setting speed and application. Anionic emulsions fall under a separate ASTM specification, D977. State departments of transportation often add their own requirements on top of these national standards, tailoring performance thresholds to local climate and aggregate sources.