Aggregates – Lightweight Aggregates

Lightweight aggregates include a variety of mineral and rock materials used to provide bulk in concrete building units (block), light- weight structural concrete, and precast concrete units, as plaster aggregate, insulating fill, and other structural and/or insulation purposes. They are distinguished from other mineral aggregate materials by their lighter unit weight- 80 to 100 pounds per cubic foot (pcf) or less for loosely packed lightweight aggregates as compared to 125 pcf or more for crushed stone, sand, gravel, air-cooled slag, and other more common mineral aggregates. The various lightweight aggregate materials may be classified into four groups as follows: 1) Natural Lightweight Aggregate Materials-prepared by crushing and sizing natural rock materials such as pumice, scoria, tuff, breccia, and volcanic cinders. 2) Manufactured Structural Lightweight Aggregates-prepared by pyroprocessing clay, shale, or slate in rotary kilns or on traveling grate sintering machines. 3) Byproduct Lightweight Aggregates-prepared by crushing and sizing foamed and granulated slag, cinders, and coke breeze. With the exception of processed slag, this group is becoming less important as time passes. 4) Manufactured Insulating Ultralight- weight Aggregates-prepared by pyroprocessing ground vermiculite or perlite. This chapter will focus on the use of natural lightweight aggregates and manufactured insulating ultralightweight aggregates, and both the production and use of manufactured structural lightweight aggregates with some comments on byproduct materials. Detailed discussion of the production and use of both expanded and air-cooled slag is presented in a separate chapter in the utilization section of this volume, and there are separate commodity chapters on vermiculite, perlite, pumice, and volcanic cinders. Discussion of the commodities covered elsewhere in the book will be limited to observations on their utilization and relation to other lightweight aggregate materials. All materials considered as lightweight aggregates are derived from some initial natural raw material. While historical definitions have included such organic materials as straw and peat as lightweight aggregates, the discussion in this chapter will restrict itself to lightweight aggregates derived from mineral or rock materials. Thus while the actual lightweight material in use may be a manufactured product, it is derived from an ore or raw material that is obviously a mineral resource. In this context, then, the reserves of raw materials for lightweight aggregate use may include iron ore and flux stone (the components of blast furnace slag) as well as coal (the raw material from which cinders are derived) as well as those clay, shale, and slate deposits that display the characteristics of bloating or controlled expansion when subjected to elevated temperatures (1600 to 2400°F.).