This forms the bulk of lightweight concrete on site and can be used as a structural material. The density depends on the type of grading of the aggregates used, the moisture content and the mix proportions. The fines can be natural sand or crushed lightweight coarse aggregate. The most commonly used aggregate is LYTAG, a sintered fly ash, also known as pulverised fuel ash (pfa).

The cellular nature of lightweight aggregates makes them inherently insulating (thermal conductivity coefficient of say 0.09 to 0.16 W/mK compared to 1.15 to 1.44 W/mK for normal concrete). They are also stable at elevated temperatures (except the organic types, e.g. word particles and plastics) and thus have good fire resistance properties.