What is mineral filler?

Mineral filler is a very fine material made by grinding or crushing stone, most of which passes a 0.063 mm (63 micron) sieve. It is finer than the finest sand and about as fine as cement. The European standard for concrete aggregates (EN 12620) calls it filler aggregate. In Dutch it is known as steenmeel, literally “stone flour”.

Types

• Limestone filler: ground limestone with fairly rounded grains, by far the most common filler in concrete.
• Quartz flour (silica flour): ground quartz or pure quartz sand, with sharp, angular grains.
• Other rock fillers: often a by-product of crushing hard rock in quarries.

What it does

Filler completes the finest part of the grading. Fresh concrete becomes more cohesive, bleeds and segregates less, and the surface closes up better. That is why fillers are standard in self-compacting concrete and help in fair-faced concrete. Under the concrete standard EN 206, filler counts as a nearly inert (type I) addition. Whether it may be counted towards the cement content is decided by national rules where the concrete is used.

Application

1. Do not improvise a cement substitute. Adding your own stone dust to a mix does not make it stronger; the amount of filler belongs in the supplier’s designed mix.
2. Buy the right product. Filler for concrete must conform to EN 12620. Garden “rock dust” sold as a soil improver is something else.
3. Control dust. Quartz flour contains respirable crystalline silica. Use extraction and wear suitable respiratory protection (FFP3 for silica dust).
4. Fair-faced concrete is a package. Filler helps the surface, but formwork, release agent and compaction matter just as much.

Common mistake: confusing mineral filler with “stone dust” or crusher fines sold for laying paving. Those are much coarser, up to several millimetres, and do a completely different job.

Related terms

• Limestone
• Cement
• Portland cement
• Fair-faced concrete
• Crusher sand
• Fresh concrete
• Ready-mix concrete

Want to know more about building terms? Visit our knowledge base at fredsdiyplans.com.