Clinker

Cement clinkers are typically produced by firing limestone and other raw materials in a kiln specifically for the purpose of grinding them up to make Portland cement. Therefore, cement clinkers typically do not qualify as recycled materials. The term, “clinker” is also commonly used to describe an industrial waste product derived from burning fossil fuels. If repurposed for another use, this kind of clinker could qualify as pre-consumer recycled content. However, this type of clinker is not used to make cement. However, Portland cement manufacturers sometimes add to their clinkers a percentage of synthetic (FGD) gypsum, fly ash, or bottom ash from fossil fuel waste. Some also add foundry sand, a byproduct of metals casting. Check with your cement manufacturer. I have seen data from some manufacturers reporting 7 to 10% post-industrial recycled content due to these fillers.