Photograph 1. Montage of Specialist Aggregates - (L to R) Hematite, Crushed Barytes, Crushed Pumice, Scoria and Crushed Ilmenite.
Specialist aggregates are required for use in certain demanding applications; notably where lightweight members are required in constructions or where heavy, high density materials are required.
Light weight aggregate
Light weight aggregates (LWA) are required for situations where concrete is required that imposes low loads on structural elements of constructions. The aggregates therefore need to have low density and weight, achieved by using porous materials, but adequate strength. A variety of materials can be pelletised and heated causing them to harden and expand. These include clay, shale and slate, sludge (e.g. from sewerage works), fine grained sediments dredged from the sea or rivers, and pulverised fuel ash see industrial by-products. Other materials that can be used are natural volcanic materials that are full of gas cavities including pumice and scoriae ("natural cinders"). The volcanic materials do not occur in the United Kingdom but can be imported, particularly from Italy 534, 535.
High density aggregate
High density aggregates are required for manufacture of concrete that is capable of, for instance, attenuating radiation and thus be suitable for shielding nuclear installations and medical facilities such as radio-therapy treatment rooms. Aggregates are most critical and important component of high density concrete and are principally crushed minerals containing relatively heavy elements, in combination with various additives but can also include scrap steel or certain ceramics. The relevant minerals include crushed hematite and magnetite (both iron oxides), limonite (iron hydroxide) ilmenite (iron titanium oxide) and barite (barytes – barium sulphate). Other additives can include serpentinite (hydrous calcium magnesium silicate) and borax (sodium borate) 533, 536.