Stone

Stone is the oldest structural material and still widely used for masonry walls, foundations, retaining walls, pavements, flooring, cladding and as crushed aggregate in concrete and roads. Its properties come from its geological origin. Igneous rocks such as granite and basalt formed from cooled magma and are hard, dense and durable. Sedimentary rocks such as sandstone and limestone formed from deposited layers; they are easier to work but vary widely in strength and porosity. Metamorphic rocks such as marble, slate and quartzite formed when other rocks were changed by heat and pressure.

A good building stone is strong in compression, has low water absorption, resists weathering and frost, has a uniform texture free of cracks and soft patches, and dresses well. Tests include compressive strength, water absorption, specific gravity, abrasion (Los Angeles test for aggregate), impact and crushing values, and durability tests such as soundness with sodium or magnesium sulphate. For masonry, stones should be laid on their natural bed, the plane along which sedimentary layers formed, so that load acts across the layers rather than along them.

Stone masonry is classed as rubble (random or coursed, with irregular stones) or ashlar (finely dressed blocks with thin joints). Quarrying uses drilling and controlled blasting or wire sawing; dressing gives the required shape. Weathering of stone in polluted air, salt crystallisation in coastal areas and moisture rising from the ground are the main causes of decay, controlled by selection, damp-proof courses and good detailing.

Crushed stone aggregate is by far the largest use of stone today. Its shape, grading, strength, cleanliness and absence of reactive silica directly affect the concrete or bituminous mix it goes into, so aggregate testing is a routine part of quality control on every road and building project.