Aggregate
Sand, gravel and crushed rock — by mass the most extracted solid material on Earth, and the one nobody thinks about until it runs short.
Aggregate is graded mineral particles — sand, gravel, crushed rock — and it is the bulk of concrete, asphalt, mortar and every road base. Around 70 to 80 per cent of concrete by volume is aggregate, and the cement is what holds it together.
It is by mass the most extracted solid material in the world, at a scale of tens of billions of tonnes a year, and it is almost entirely invisible in any account of materials because it is cheap, local and undifferentiated.
The grading is the engineering. A well-graded aggregate has a distribution of sizes such that small particles fill the gaps between large ones, which minimises the voids and therefore the amount of expensive binder needed to fill them. That is most of what a concrete mix design is about.
Processing
Dredged, dug from pits, or quarried and crushed, then washed and screened into size fractions. Its economics are dominated by haulage: aggregate is worth so little per tonne that transporting it more than a few tens of kilometres can cost more than the material, which is why quarries are where the demand is rather than where the best rock is.
Recycled aggregate from crushed demolition concrete is used increasingly, mostly in sub-base and lower-grade applications, because the old cement paste adhering to the particles absorbs water and weakens new concrete made with it.
Uses
Concrete, asphalt, mortar, road sub-base, railway ballast, drainage and fill. Between them those are the physical substance of the built environment.
History
Used as long as anything has been built. Its industrialisation is the story of crushing and screening machinery and of the lorry, which is what made a distant quarry viable at all.
Environmental impact
The sand problem is real and is not the one people assume. Desert sand is useless for concrete — wind-rounded grains do not interlock — so the sand a construction industry needs comes from rivers, lakes, beaches and the seabed. Extraction from river beds lowers water tables, undermines bridges and banks, destroys habitat, and in several countries is controlled by organised criminal networks with a documented death toll.
The volumes make even small impacts large. Quarrying is the largest earth-moving activity there is, and the transport of aggregate is a substantial share of all freight movement in most economies.
Recycled and manufactured aggregate reduce the demand and cannot replace it at current construction volumes, which is the honest position rather than a comfortable one.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
is a component of
- Asphalt concrete — material · 95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out
- Concrete — material · 70 to 80 per cent by volume — the cement is the expensive part and the stone is most of the material
is an input to
- Quarrying — process · and its economics are haulage: worth so little per tonne that a distant quarry costs more in transport than in rock
is used in
- Construction — industry · by mass the most extracted solid material on Earth, and invisible in most accounts of materials because it is cheap and local
is an alternative to
- Concrete — material · not a substitute — recorded because recycled concrete is crushed back into aggregate, which is the closest thing construction has to a loop at this scale
- Sand — material · the fine fraction against the coarse: they are the two size grades of the same job in a concrete mix, and neither substitutes for the other beyond the margins
is produced by
- Quarrying — process · crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing