The microstructure of four types of manufactured lightweight aggregate was studied using scanning electron microscopy and the results were used to provide insight into the dimensional stability of concretes made from these aggregates. Dimensional stability was determined according to the Test Method for Length Change of Hardened Hydraulic-Cement Mortar and Concrete (ASTM C 157) as modified by the procedures covered in the Specification for Lightweight Aggregates for Concrete Masonry Units (ASTM C 331).

Aggregate types studied were rotary kiln produced expanded shale, sintered fly ash, palletized cold bonded fly ash and expanded glass. Scanning electron microscopy revealed the nature of the aggregate pore structure and the extent to which the vesicular structure, typical of most lightweight aggregates, is interconnected. When used in concrete, the three aggregates produced at high temperature met the ASTM C 331 shrinkage requirements while the one made by cold-bonding did not. After 100 days of drying the aggregates were immersed in a lime saturated water for an additional 251 days, followed by air drying during which length measurements were taken periodically. The microstructure was shown to have a pronounced effect on the volume stability of the aggregate.