<Li> Used wooden railroad ties (railway sleepers) </Li> <Li> Spent cell liner from the aluminium smelting industry (also called spent pot liner) </Li> <P> Portland cement manufacture also has the potential to benefit from using industrial byproducts from the waste stream . These include in particular: </P> <Ul> <Li> Slag </Li> <Li> Fly ash (from power plants) </Li> <Li> Silica fume (from steel mills) </Li> <Li> Synthetic gypsum (from desulfurisation) </Li> </Ul>

The three main raw materials for portland cement are