About the Technology Used to Produce Mineral Powders in Asphalt-concrete Production
A disperse component - mineral powder is of paramount importance in production of asphalt-concrete for road construction. It is a product of limestones, dolomites, dolomitic limestones and carbonate rocks, smelter slags, and industrial powder wastes finely ground to 2500–5000 specific surface area cm2/g.
The mineral powder in asphalt-concrete fills up the sand and crushed stone base cavities, increases the mineral aggregate density, and transforms oil bitumen into strong asphalt binding substance.
To perform as described above, mineral powder should have the following properties:
- it should not clod and form aggregates when mixed with bitumen;
- adhesion of bitumen with the mineral powder grain surface should be so strong to not let the water peel off bitumen throughout the entire specified lifespan of asphalt-concrete in the carpet;
- physicochemical interaction between the surface of mineral powder grains and bitumen should be strong enough to orient molecules in a thin layer of bitumen. However, the powder should not accelerate bitumen ageing process;
- the content of mineral powder in the mixture should be very minimal just necessary for asphalt-concrete to achieve specified density and strength.
The main method to improve the properties of mineral powder is their physicochemical activation during which the material grinding process is accompanied by processing of bitumen with surface-active agents (SAA) in 1:1–1:3 ratios.
As a result of the activation of mineral powders, they acquire a wide range of useful properties:
- hydrophilic surface of powder becomes hydrophobic surface, which simplifies powder transportation and storage;
- activated powders are not clodded, which allows decreasing grinding fineness during powder production;
- strong bitumen film on the powder surface significantly improves the conditions for subsequent interaction with bitumen during asphalt-concrete production;
- high quality of mineral powders provides the opportunity to produce asphalt-concretes with increased density, strength, water- and freeze resistance, and in some cases with shear and crack resistance. Such asphalt-concretes have the maximum amount of closed pores, which provides for lower water saturation at specified residual porosity and water permeability of the carpet;
- cold asphalt-concrete mixtures based on activated mineral powder are not caked during storage; the carpets made of such mixtures are formed faster being pressed by traffic;
- bitumen consumption for production of asphalt-concrete mixtures is 10 to 20% less than that for production of mixtures based on non-activated powder;
- the mixtures based on mineral powder are produced, placed, and compacted at the temperature which is 20 deg. C less than in the case with mixtures based on ordinary powder;
- the mixture exhibits improved processability during placing and compaction.
In view of the above, many road construction companies are now use activated mineral powder.
The Technology Used in Production of Mineral Powders
In the USSR, activated mineral powders were produced using ball-tube mills by dosing the preliminarily prepared activating agent on dried crushed stone into the auger upstream of the mill. Such equipment (for example, manufactured by the Samara Strommashina Plant) with 8–10 t/h capacity has installed output power of 450 kW, weight up to 200 t, and overall dimensions of 36х16х11 m. There are also versions providing for combination of drying and grinding operations with activation in a single unit - aerated separator mill of a hammer or paddle type.