Probably one of the most important developments in the history of lightweight concrete was the introduction of a lightweight aggregate, Haydite, around 1920. This excellent structural aggregate consisted of clays, shales, and slates which were bloated at high temperatures in rotary kilns very similar to those used in the manufacture of portland cement. Today, these expanded shales and clays are marketed under many brand names. They have given construction workers high grade aggregates ranging in weight from 40 to 60 pounds per cubic foot. When properly used, they produce dry concretes weighting form 90 to 100 pounds per cubic foot, as compared with 140 to 150 pounds per cubic foot for concretes made with most natural aggregates. These expanded materials make it possible to reduce weight without sacrificing strength. This is accomplished by using only slightly richer mixes than would be required to obtain comparable strengths with sand and gravel or crushed stone aggregates. Lightweight concretes are also work well for jobs in which the insulating and acoustical properties of the materials are important. They also have excellent fireproofing characteristics and for this reason they are used on many jobs as covering for structural steel beams and columns.