Crushed stone

Crushed stone, also called aggregate, is made by blasting rock loose in a quarry, breaking it in crushers and screening it into sizes. Unlike river gravel, it is shaped by machines, not by nature, so its pieces are angular with freshly broken faces. It is made mostly from limestone and dolomite, and also from hard rocks such as basalt, andesite and granite. It is the main material of asphalt, concrete, road bases and railway ballast, and the raw material of cement and lime works. In Türkiye it is commonly known as “mıcır”.

Road building: The base and sub-base layers of roads are made of compacted crushed stone, and most of an asphalt surface is crushed stone too. Concrete: Mixed with sand and cement, it is the coarse aggregate in concrete. Railway ballast: The bed of angular stones beneath the rails holds the sleepers in place and lets water drain away. Cement and lime: Crushed limestone is the raw material for cement and lime works. According to the USGS, about 72 percent of crushed stone in the United States is used as construction aggregate, 17 percent for cement and 6 percent for lime.

The pieces are angular and sharp-edged, with fresh-looking broken faces. River gravel, by contrast, is rounded and smooth. The pieces in one heap are all about the same size, because they have been screened. They are often coated with fine rock dust. The colour gives away the rock: crushed limestone is light grey or white and fizzes when a drop of vinegar is added; crushed basalt is dark grey or black and noticeably heavy in the hand; crushed andesite is grey, pinkish or brownish. Not every heap of crushed stone is natural rock: broken old concrete and asphalt are also recycled as aggregate. These pieces show traces of cement mortar, brick or black bitumen.

On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Crushed stone is a low-value raw material sold by the tonne but used in vast quantities. Transport often costs about as much as the stone itself at the quarry, or more, which is why quarries are usually opened close to cities and large building sites. Its value depends on the hardness and wear resistance of the rock (especially for asphalt and ballast), the shape of the pieces (cubical pieces are better than flat, elongated ones), cleanliness, grading and the distance from the quarry. In the trade, crushed stone is named by screen size, either by number or by a range in millimetres.

The fine dust from crushing and screening plants irritates the airways, and dust from quartz-bearing rocks (granite, sandstone, quartzite) also carries a risk of silicosis. Dust masks should be worn and dust kept down with water sprays. Blasting in quarries and the collapse of steep stockpiles are also dangerous; children should not play in quarries or on stone heaps.