Heavyweight aggregates include magnetite, with a specific gravity S of 4.3; barite, S = 4.2; limonite, S= 3.8; ferrophosphorus, S=6.3; and steel shot or punchings, 7.6. Such heavyweight aggregates may be used instead of gravel or crushed stone to produce a dense concrete; for example, for shielding of nuclear reactors as specified in ASTM C637.
Lightweight Aggregates. These can be divided into two categories: structural and nonstructural. The structural lightweight aggregates are defined by ASTM C330 and C331. They are either manufactured (expanded clay, shale, or slate, or blastfurnace slag) or natural (scoria and pumice). These aggregates produce concretes generally in the strength range of 3000 to 4000 psi; higher strengths are attainable and are discussed in Art. 4.17. The air-dry unit weight of normal strength lightweight concrete (less than 5000 psi) ranges from 100 to 115 pcf. High-performance lightweight concrete has unit weights in the range of 120 pcf.
The common nonstructural lightweight aggregates (ASTM C332) are vermiculite and perlite, although scoria and pumice can also be used. These materials are used in insulating concretes for soundproofing and nonstructural floor toppings.
Lightweight aggregates produce concrete with low thermal conductivities, which equate to good fire protection. When concrete is exposed to extreme heat, the moisture present within the concrete rapidly changes from a liquid to steam having a volume of up to 15 times larger. The large number and large sizes of pores within lightweight aggregates create pressure-relief regions.
