Cost consciousness and a desire to utilize new designs has stimulated builder's interest in lightweight structural concrete. Thirty percent of the production of expanded clay, shale, and slate, and around 5 percent of the expanded slag aggregates was used in structural cast in place concrete. There are several processes currently used for manufacturing expanded slag aggregates. In the Kinney-Osborne process, the slag first passes through a diaphragm and then is treated with either air, steam or water. In the Caldwell machine, water is mixed with the slag in a cylindrical casing and radial blades fling the expanded product through an opening in the casing to a steel target a few feet away. The non-mechanical means of producing expanded slag is called the jet or pit system. Here molten slag falls in a stream in front of a nozzle which emits steam, air or water. Expansion occurs as the slag falls. The properties of lightweight concrete show rather wide variations in respect to the relationship of cement content, strength and weight. For example various lightweight aggregates, even those produced by similar processes, require a considerable range in cement contents to produce concrete having equivalent strengths. Thus there is some logic in looking to the manufacturer of a particular material for recommendations concerning trial mix proportions. Other characteristics are: the modulus of elasticity ranges for one and a half to two-third the values for concretes containing gravel, stone, or air-entrained, they have excellent thermal insulating properties, but the drying shrinkage tends to run somewhat higher. Satisfactory results with clay, shale, slate and slag lightweight aggregates require adequate mixing time, and a longer cycle than with conventional concrete. The complete recommended procedure is: (1) charge the mixer with 65 to 75 percent of the total water and all of the pre-wet aggregate. (2) Mix for about 1 minute. (3) Add cement, any admixture, and the balance of the mixing water. (4) Mix the whole batch for 60 seconds for 1 cubic yard mixers plus 15 seconds for each cubic yard.