Expanded Shale, Clay and Slate

Expanded shale, clay and slate aggregates are produced by the rotary kiln process, which was first developed in 1908 and subsequently patented as Haydite in 1914. Expanded shale aggregates played innovative roles in both World War I and World War II when ships and barges were constructed of lightweight concrete. Because of the 105 concrete ships produced during World War II, steel supplies could be used for other essential uses.

To produce expanded shale, clay or slate, raw material deposits are mined from the earth, usually from a location that is in close proximity to the kiln. These raw materials generally contain all of the necessary materials to enable them to bloat. They are sometimes crushed prior to being introduced into the cold, upper end of the rotary kiln. As the minerals make their nearly one-hour journey down through the slowly rotating kiln, they are fired at temperatures of approximately 2,000 degrees Fahrenheit (1110 degrees Celsius). At these temperatures, the minerals soften and begin to melt. Meanwhile, reactions to the heat from certain constituents produce gases, ultimately creating non-connecting cells or bubbles in the vitrified material sometimes described as puffed or bloated aggregate. The resulting material is cooled and then may be crushed and screened in order to control the gradation, which varies depending on the intended use.