Crushed rock aggregates
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| Higher quality aggregates are required for demanding applications, such as in road pavements and in concrete. This is commonly derived from hard, dense and cemented sedimentary rock (most limestones and certain sandstones) and the tougher, crystalline, igneous rocks. |
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A variety of rocks are, when crushed, suitable for use as aggregates. Their technical suitability for different aggregate applications depends on their physical characteristics, such as crushing strength, porosity and resistance to impact, abrasion and polishing. Lower quality aggregates may be acceptable for other applications, such as constructional fill. Higher quality aggregates are required for demanding applications, such as in road pavements and in concrete. This is commonly derived from hard, dense and cemented sedimentary rock (most limestones and certain sandstones) and the tougher, crystalline, igneous rocks.
Limestone
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| Limestones are commonly thickly bedded, consistent deposits, which are structurally simple and can be quarried extensively and economically. |
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Limestones are sedimentary rocks composed mainly of calcium carbonate (CaCO3). With an increase in magnesium carbonate (MgCO3) content they grade into dolomite. Most limestones and dolomites are hard and durable and useful for aggregate. They are common rock types and consequently are widely extracted for aggregate materials. Chalk is a form of fine-grained limestone but is soft and porous and generally unsuitable for aggregate use.
Igneous rock quarry.
In Great Britain, limestone (including dolomite) provides 54% of the crushed rock aggregate produced. Limestones of Carboniferous age are the major source of limestone aggregate and they represent the largest resources of good-quality aggregate in Britain. These limestones are commonly thickly bedded, consistent deposits, which are structurally simple and can be quarried extensively and economically. They typically produce aggregates that are strong and durable, with low water absorption suitable for roadstone (sub-base and road-base layers) and concreting aggregate.
Sandstone quarry.
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Limestone quarry.
Igneous and metamorphic rocks
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| Igneous rocks produce strong aggregates with a degree of skid resistance and are hence suitable for many road surfacing applications |
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Igneous rocks are particularly important in areas where other aggregate materials such as crushed limestone or natural gravels are not widespread. Roadstone is the largest consumer of igneous rock in Britain. Igneous rocks tend to produce strong aggregates with a degree of skid resistance and are hence suitable for many road surfacing applications as well as for use in the lower parts of the road pavement.
Sandstones
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| The suitability of a sandstone for aggregate use mainly depends on its strength, porosity and durability. These qualities are related to mode of formation and geological history. |
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Sandstones are sedimentary rocks consisting of sand-sized particles composed predominantly of quartz, but with variable amounts of feldspar and rock fragments set in a fine-grained matrix or mineral cement. Compositional differences, both of the sand grains and the matrix, give rise to different rock names under the general heading of 'sandstone.'
The suitability of a sandstone for aggregate use mainly depends on its strength, porosity and durability. These qualities are related to mode of formation and geological history. Thus the mineralogical composition, grain size, degree of grain sorting, nature and degree of cementation, degree of compaction and weathering state are fundamental rock properties which directly affect the end-use performance of the sandstone and its economic potential. Individual sandstone units also vary in thickness and lateral extent.
Many types of sandstone are too porous and weak to be used other than as sources of constructional fill. In general, older more indurated sandstones (subjected to tectonic compression), exhibit higher strengths and are suitable for more demanding aggregate uses, such as in concrete and particularly as roadstone. Some sandstones also have a high polishing and abrasion resistance and are particularly valued for road surfacing where they provide resistance to skidding.
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