Mixing the slurry components initiates a chemical process which leads to the curing of the seal.
The process can be considered in four stages:
Mix time:
During this period the slurry is brown, fluid and homogeneous and can be mixed. The slurry must remain workable and flowable during mixing and in the spreader box so it distributes easily over the road surface and the profile can be controlled with the strike offs.
Set time:
As the chemical reaction proceeds, the slurry builds viscosity, sometimes very rapidly and can no longer be mixed. Clean water separates from the bitumen and aggregate fractions. The process is irreversible but the seal is not strong enough to support traffic.
Traffic time:
The slurry builds sufficient cohesion that controlled speed traffic can be allowed on. The color is dark brown or black.
Full cure:
Trafficking and time complete the curing process by bitumen and the seal has the character of a hot mix overlay.
Mix time and temperature:
The mix time must be sufficient to allow the slurry to pass through the mixer and spreader box without problem. The residence time in the mixer is short only 10-15 seconds and 45 seconds or less in the spreader box would be typical. Different machines and boxes may require different minimum mix times. Too short mix time leads to “breaking in the box” which can completely stop production or lead to a buildup of hard mix in the corners of the box causing drag marks on the seal when it leaves. Hand work requires longer mix time.
The chemical reaction in the slurry is accelerated by heat. Hot weather or hot emulsion can lead to premature breaking and insufficient mix time. This can be corrected by the addition of chemical additives “dopes” or “break retarders” which are added to the water and act to deactivate the aggregate. On the other hand, the reaction in the slurry is slower in cold weather. The reaction in cold weather can be helped by the use of hot emulsion. With some materials, mix time and set time respond to filler content. Otherwise the emulsion recipe needs to be modified for cold weather paving.
One should be careful about the amount of break-retarder used in the mix; it is not good to use an excessive amount. It is best to test the slurry mixes with the expected amount of break-retarders used in the field to ensure that they are not significantly altering the performance properties of the binder and the slurry seal or micro surfacing mix.
The chemistry of the curing process by bitumen
The reaction of bitumen emulsion with aggregate in cold mix processes has been extensively studied. The mechanism of the setting and curing process by bitumen depends on the interaction of the emulsion and the other components of the seal. The droplets in bitumen emulsions are charged – positive in cationic emulsions or negative in anionic – and this charge is responsible for the stability of the emulsion. The charge is sensitive to pH – for example some cationic emulsifiers lose their charge at high pH. Moreover, bitumen contains natural acid components which may develop negative charge at high pH. In quick-set slurry systems and micro surfacing the ingredients are designed to react. Most often alkaline fillers like cement or lime are included to produce a large rise in pH, and to generate soluble calcium ions.
Slow-setting systems
Even without a chemical reaction between the emulsion and the aggregate and filler components, the seal will still cure slowly simply due to the evaporation of water in a similar manner to the drying of latex paint. As water leaves the system the droplets of bitumen and aggregate are forced closer and closer together and the concentration of soluble ions in the system also rises. The consequence is destabilization and coalescence of the emulsion, and sticking of the aggregate particles together. In practice it is hardly possible to have a system where there is no chemical interaction between the bitumen droplets and the aggregate or filler fraction.
Anionic quick-set systems
In anionic quick-set systems containing resin or sulphonate emulsifiers, cement is used to produce a high concentration of calcium ions which destabilizes the emulsion by neutralizing the negative charge on the emulsion droplets. The insoluble calcium soaps formed in the process are not effective emulsifiers and the system breaks rapidly.
The Redicote slurry system
The Redicote slurry system is based on cationic slow set emulsion stabilized with quaternary ammonium emulsifiers (Redicote E-11). The emulsion droplets have a positive charge and are relatively insensitive to pH changes. When mixed with fine aggregate these droplets bind closely with negatively charged mineral particles forming tightly bound assemblies which provide structure to the seal. The attraction between the bitumen and the aggregate is strong enough to squeeze water out of the system creating a high viscosity, cohesive mat of cold mix. Eventually water is displaced from between the particles and evaporates leading to full the curing process by bitumen. The emulsions typically pass the cement mix test, and in these systems cement and lime typically extend the mix time because the soluble ions they produce can reduce the interaction between the bitumen droplets and mineral particles.
Cationic quick-set and micro-surfacing systems
Developed in the 1960s, these systems use lime or cement filler in the mix and emulsifiers which require acid formulations to be cationic. The emulsifiers used are characterized by large head groups with multiple charged centers. When the slurry components are mixed the cement reacts with the acid. The acid is neutralized, the pH rises rapidly, and at the same time large quantities of soluble calcium ions are produced, and the charge on the emulsifiers is largely destroyed. The acid components in the bitumen can play an important role in the breaking process. At the high pH and calcium ion concentrations in the slurry the droplets of bitumen rapidly coalesce.
Phosphoric acid system
Phosphoric acid slurry systems were developed in Germany during the 1970s. The emulsions are formulated with phosphoric acid rather than hydrochloric. Phosphoric acid forms insoluble salts with calcium which limits or buffers the pH changes which occur in the slurry and the limits the concentration of calcium ions. The pH of the slurry never exceeds 9 and the charge on the emulsion droplets is maintained. Destabilization and setting are thought to occur though interaction of the bitumen droplets and oppositely charged mineral and filler particles. Because the filler particles are negatively charged at the slurry pH, they directly contribute to the breaking process.
