Most processes of steel manufacture necessarily generate a substantial proportion of slag. Often much of this is landfilled, and at many plants it will be the main item (by tonnage) that is dumped. This rock-like material can be used as an aggregate, but the main obstacle to widespread use is concern over the volume stability. The main cause of volume instability is hydration of free lime, although hydration of free magnesia may also contribute in some cases. There are three key steps to effective use of this abundant secondary aggregated First, some pretreatment of the slag is usually necessary (such as weathering). Second, a test is necessary that reliably predicts the behaviour in use within a reasonable time. This is commonly an expansion test. Third, calibration of the test is necessary so that test results can provide a useful distinction between material which is suitable and that which is not; this is normally achieved by linking laboratory tests to road trials. Subject to these three steps, steelmaking slag can make a valuable contribution to the need for secondary aggregates.
