Mineral filler is that portion of mineral matter passing a 0.075 mm sieve which can be derived as a portion of the coarse and fine aggregate grading, from the recycling of the dust extracted from the crushing and screening plant or from the dryers or drums of asphalt plants or as added material. The most common added fillers are:
- hydrated lime
- Portland cement
- cement kiln dust
- ground limestone
- ground slag
- fly ash
- baghouse dust.
Fillers can be used to achieve a range of requirements for asphalt mixes. They fill voids and, hence, reduce the optimum binder content. They enable the specified aggregate grading to be met. They increase the strength by stiffening the binder mastic but may also decrease workability. Some fillers, such as hydrated lime, improve the bond between the aggregate and bitumen.
The filler/binder ratio is defined as the percentage by mass of aggregate passing the 75 μm sieve to the effective binder content expressed as a percentage by mass of the total mix and is generally in the range 0.6 to 1.2%. These limits have been established to provide a balance between too little filler, which leads to a mix with low strength, and too much filler, which reduces the voids in the mineral aggregate to a level at which sufficient binder for a durable mix cannot be added. The appropriate proportion for a particular asphalt mix is, however, very dependent on the size, surface area and particle density of the filler.
Unless a particular filler material is specified, fillers are normally evaluated on the relative costs of purchasing, hauling and handling the material. Some fillers store and handle well whilst others cause problems, particularly in wet weather and where used spasmodically. A summary of the advantages and disadvantages of the various added fillers is shown in Table 3.2.
|Hydrated lime||Consistency of product, availability, ease of handling, improved resistance to stripping, high surface area||Relatively high cost, restricted availability in bulk, reduces workability|
|Portland cement||Consistency of product, availability||High cost, difficult to handle|
|Ground limestone||Easy handling, relatively low cost||Variable grading, restricted availability|
|Cement kiln dust||Relatively low cost||Restricted availability outside capital cities, variable bulk density|
|Fly ash||Low cost, easy to handle||Restricted availability, variable characteristics, promotes bitumen hardening in low binder content mixes|
|Ground slag||Low cost||Properties depend upon production process|
|Baghouse dust||Low cost, readily available||Variable characteristics|
The primary function of the filler is to increase the resistance of the binder mastic to flow, although hydrated lime has been shown to both improve resistance to stripping and the durability of the bitumen (Dickinson 1984).
The properties which are important when considering the use of added fillers in asphalt mixes are defined in Australian Standard AS 2150, Hot mix asphalt: a guide to good practice and include:
- particle size distribution
- moisture content
- particle shape (specific surface)
- water solubility
- loss on ignition
- clay content
- plasticity index
- compacted void content.
Other relevant properties are the particle density and the free lime content.
The grading of the filler is important in controlling the amount of oversize material including lumps. Also, the grading in association with the particle shape and texture influences the percentage of air voids in the compacted material and, hence, in the asphalt mix. Fillers with a finer grading tend to produce higher air voids on compaction and those with a high proportion of material finer than 20 to 30 μm may act as a binder extender rather than a filler.
Since the proportion of filler is controlled on the basis of a ratio of percentages by mass of filler to effective binder, fillers with low particle density will represent a greater volume of material.
The particle shape, as characterised by the surface area to volume ratio (specific surface), influences the binder film thickness and binder viscosity. Fillers with a high specific surface reduce the binder film thickness for a given binder content for adequate fatigue resistance of the asphalt mix and binder durability. The exception is hydrated lime which, despite having a high specific surface has been shown to improve binder durability. The improvement appears to stem from the action of the hydrated lime in absorbing the products formed as part of the bitumen oxidation process, thereby preventing further oxidation reaction. The effect of filler on binder viscosity has been shown to be related both to the specific surface of the filler and to its affinity for bitumen.
Some fillers, notably cement flue dust and fly ash, can contain a significant percentage of material which is soluble in water and likely to deteriorate in the pavement. Therefore, it is important that the percentage of such materials is limited.
Added fillers that have active lime are able to alter the surface polarity of aggregates and thus improve their bitumen affinity. The ‘active’ lime content is the free calcium oxide and hydroxide which occurs in hydrated lime, cement flue dust and Portland cement and will readily hydrolyse to form hydroxides.
Most of the commercially available materials used as added filler are produced to standards that reflect their primary use for other applications, or are by-products of other processes. Specific requirements for use in asphalt are included in AS 2150 as well as some asphalt specifications.
An outline of the test used for the evaluation of filler properties is provided in Appendix A.