Lightweight aggregates for structural concrete
"In recent years interest in lightweight concrete has become widespread, not only on account of its light weight, which is desirable in floor slabs, walls and partitions, but also because of its high thermal and sound insulation properties. Lightweight concretes may be produced by the following methods: 1) Formation of air voids in cement paste. Such voids may be produced either by the addition of some substance to the cement causing it to foam, or by mixing a cement slurry with a preformed foam. 2) Formation of air voids in the concrete by omitting the fine aggregates. 3) The use of lightweight aggregates which owe their low density to the presence of enclosed voids in the aggregate. Combinations of the above methods may also be employed in producing lightweight concrete."--Page 584-1.