This paper discusses particle size effects on laboratory and field performance in microsurfacing using an optimized emulsifier system consisting of amido-amine and immidazoline emulsifiers. The main conclusions that could be drawn are that particle size (PS) and particle size distribution (PSD) have an effect on microsurfacing and quick set slurry break, coating of aggregate, cure time and traffic time. This finding was applied to adjusting the mix designs and later to equipment technology for other customers. The variables that affect emulsion and end use performance are many, but an optimized balance can be achieved and even shortcomings of some systems overcome e.g asphalt quality (4). This information is useful in scaling to a full-scale production with a colloid mill that is unfamiliar.

• Break
• Coating and film formation
• Cure

Each if these are discussed below.

2.1 Break

This is the flocculation and coalescence within the emulsion and the reaction with the aggregate surface. The PS and PSD could have several effects here. Firstly, on coalescence and flocculation, if the emulsion is already coarse the floccs will be large and create large coalesced particles. Such particles will have the charge spread over a larger particle and the charge density at the point of contact with the particle will be reduced; hence, as the contact angle is reduced and the rate of reaction is reduced, the thermodynamic break rate will be reduced. The kinetic effect of access to the aggregate surface could be significant. That is, larger particles interfering with each other and flocculating and coalescing within the emulsion rather than reacting with the surface. This may give the appearance of a fast set but be more likely to create false slurries.

2.2 Coating and Film Formation

This refers to the binder evenly coating all particles and forming a coherent film. The above kinetic effects will affect the coating of the particles and, as we are dealing with graded fine aggregates in slurry surfacing aggregates, a tendency to break on the larger surface area fines and not coating large particles at all. Coherent films require even coalescence on the particle surface and entrapped water will interfere with this, leading to a less cohesive mix. PSD is an important factor in film formation and a range of sizes that fit together will assist this process.

2.3 Cure

This is the steady loss of water from the system and the stiffening of the total mix as cohesion increases. Entrapped water will clearly inhibit cure. Rejection of water from the aggregate surface is both a thermodynamic and kinetic effect. The thermodynamics relate to the energy differences between the emulsifier and the aggregate charge, the kinetic to the diffusion controlled loss of water through the coalescing binder. So the potential effects on performance are that coarse emulsions with wide PSD will be more likely to give poor coating, false slurries, and poor cure rate expressed as cohesion build up or traffic time.

3. CONTROLLING PARTICLE SIZE

Several papers have been written describing methods to improve particle sizing of emulsions by methods of formulation and adjustment of asphalt chemistry (3,4). The methods usually involve improvement of chemical systems, doping of asphalt with surface active agents, tailoring asphalt composition and optimization of manufacturing conditions. The approach is basically to improve dispersion of the asphalt in the mill and to stabilize the resulting emulsion against early flocculation and coalescence. However the physical act of milling is the main determinant of initial particle size for a given asphalt and emulsifier system. The particle size is determined by the shear in the mill and the residence time (1,5)

This can be expressed by :
Shear Rate = (2 R V / 60 E) ………(1)
where R = Colloid Mill Radius
V = Rotation speed (rpm)
E = gap dimension (microns)

That is, the particle size is a function of mill diameter, gap and peripheral speed(1). A correlation between this shearing and the d50 value and , a correlation between PSD and the initial particle sizes in the mill has been reported (6).However, mill configurations internally are quite different, with different tooling and effective gap sizes created by this. For this reason the relationship is changed to:

Shear Rate = (2 R V / 60 e) Mf ……………… (2)
where Mf is the mill factor.
The mill factor is the increase or decrease in shear created by the mill configuration and tooling. Various mills with different Mf values were used in this study to achieve different shear rates.(8) as shown in Table 1.

Mill (Mf)	Nominal Shear Rate s-1
1 (1.2)	120,000
2 (0.8)	80,000
3 (1)	100,000

• Particle size and distribution affects emulsion physical properties
• Control of particle size may be carried out by control of mill configuration (Mf)
• Mixture properties that are controlled by coating and reaction rate between aggregate and emulsion are improved by reducing d50 and the narrowing distribution.

1. Durand,G ,Piorier,J.E (1996) AEMA International Symposium On Asphalt Emulsions Washington D.C.
2. Booth,E.H, Gaughan R, G, Holleran, G (1994) Australian Road Research Board International Conf Perth.
3. Holleran, G (1999) AEMA International Symposium on Asphalt Emulsions Washington D.C
4. Holleran, G (1999) International Slurry Surfacing Meeting Puerta Villarta Mexico.
5. Province, R (1986) Workshop on Bitumen Emulsions Melbourne Australia
6. Holleran US Patent 5,518,538
7. Holleran, G (1999) ISSA/AEMA Meeting Amelia Island Florida.
8. Holleran G, Reed, J,R (2002) Effect Of Emulsion Particle Size and Distribution On Microsurfacing Applications, ISSA International Congress March 2002 Berlin