This is because such waste often contains a mixture of limestone and gypsum which can attack concrete. Quarry waste is a good source of hardcore material, although care should be taken not to include waste from gypsum mines. They should also be resistant to deterioration, chemically inert, should not be absorbent and should not be affected by the presence of water (for example, colliery shale expands when moist). This removes gaps or voids which could otherwise threaten the supportive properties of the hardcore layer. Materials need to be sufficiently hard as well as being capable of being compressed (rammed to form a compact base) before the upper layer is added.
The hardcore then helps evenly spread imposed loads. The term ' hardcore' refers to the mass of solid materials used as a make up, formation material to raise levels, fill irregularities in excavations and create a firm and level working base onto which heavy load-bearing surfaces such as stone or concrete can be laid.
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