Waelz slag is described in a brief entry on Eurekalert, Trash to treasure: Turning steel-mill waste into bricks, as a waste dust that steel mills produce.
The dust often is converted into a rock-like material known as Waelz slag, which is usually disposed of in landfills. The slag contains iron, calcium, silicon oxide and other minor oxides as manganese, lead or zinc oxide. Scientists have been searching for practical and safe uses for Waelz slag. In earlier research, scientists showed that Waelz slag had potential as an ingredient in bricks, roof tiles and other ceramic products.
Tentatively, it sounds like Waeltz slag is distinct from normal slags … that the dust from the steel mill is subject to a secondary slagging process, simply to make it easier to handle, and perhaps less subject to leaching.
It is unclear whether this dust-material arises during smelting, as part of the ‘milling’ of steel, or…