I don't know that slag from copper smelting and processing has any real practical uses. It is simply an industrial waste. Historically, slag has been used as fill when developing sites, and as road base and, somewhat infamously, as railroad ballast (the base for the railroad). Slag is a very important source of soil contamination in some areas (such as the city where I live). Slag is associated with leachable (potentially mobile) heavy metal contamination and contamination by complex organic molecules (polycyclic aromatic hydrocarbons or PAHs). I do know that some slag wastes and foundery sands are used in the preparation of paving aggregate (road tar), but the materials have to meet relatively strict leach characteristics, at least where I live. For the most part these days, slag is treated as a waste and either impounded with mine tailings or placed into secure landfills. At least, that is my experience where I live.
In some remote areas where no suitable rock for road aggregate is available, copper slag has been crushed and used in bitumen and in railway ballast. In old mines where the mineral recovery was poor and the slag is rich in copper, attempts have been made to reprocess the slag to extract the copper and any other useful elements.
