Aggregate Washing: Why Is Washed Material Better?

In an aggregate production line, washing is the stage that directly affects product cleanliness. Material with suitable grading still performs poorly in concrete and asphalt if its particles carry a coating of clay or silt.

Why impurities cause problems

Clay and silt particles have three undesirable effects:

• Higher water demand: the large specific surface of fine particles takes up more water.
• Weaker bond: a coating of fines on the stone surface stops cement paste or bitumen adhering to the particle.
• Interference with admixtures: some clays absorb part of a plasticizing admixture and reduce its effect.

This is why standards limit material finer than the 75 µm sieve. In ASTM C33, for example, the limit for concrete fine aggregate is 3 or 5 percent depending on abrasion conditions.

Common washing equipment

The choice of equipment depends on the material and how contaminated it is:

• Rinsing screen: removes surface contamination from coarse aggregate.
• Screw or bucket wheel sand washer: washes and dewaters sand.
• Log washer: for material with sticky clay that needs stronger scrubbing.
• Hydrocyclone: separates fine particles and recovers fine sand that simpler methods lose.

Where requirements are stricter, two-stage washing is used. Double-washed sand is one example of such a product.

Managing water and sludge

Washing needs water and produces muddy water. Processing units treat it in settling ponds or with thickeners and filter presses so that it can return to the circuit. Managing this well lowers water use and turns sludge into something that can be handled.

Measuring the effect of washing

The result of washing is measured with a few simple tests:

• Material finer than the 75 µm sieve (ASTM C117)
• Sand equivalent (ASTM D2419)
• Clay lumps and friable particles (ASTM C142)

More on these methods is in the quality and standards section. Regular monitoring of these indicators shows whether the washing line matches its feed or needs adjusting.