Lightweight aggregate(sometimes is called as low density aggregate) is a generic name of a group of aggregates for structural concrete, mortar, or plaster that weighs less than usual rock aggregates including natural sand, gravel, and crushed stones. At an earlier stage, lightweight aggregates were mainly from natural origin, mostly volcanic pumice, tuff, etc. The production of lightweight aggregate begins with mining or quarrying the raw material and with the development of mining machinery and technology, they can be manufactured from the natural raw materials like expanded clay, shale, slate(expanded about twice the original volume of the raw material) as well as from the by-products such as fly ash, blast furnace slag, etc. However, the natural pumice, vermiculite, and perlite can also be used as substitutes. The expanded raw material(excluding pumice) has properties similar to natural aggregate, but is less dense and therefore yields a lighter concrete product.
Lightweight aggregates can be divided into two categories as following: 1. Those occurring naturally and are ready to use only with mechanical treatment like crushing and sieving. 2. Those produced by thermal treatment from either naturally occurring materials or from industrial byproducts, waste materials, etc. The industrial byproducts are pulverized fly ash, blast furnace slag, industrial waste, sludge, etc. These are produced either by expansion or agglomeration. The heat treatment is carried out in different types of industrial furnaces, such as rotary kilns. Generally speaking, the natural lightweight aggregate is less preferred over artificial aggregate. It is often used in the production of lightweight concrete products such as concrete block, structural concrete, and pavement and is possible to make LWAC of 80Mpa compressive strength too.