Grandma''s Concrete Recipe | HowStuffWorks
Supplementary cementitious materials (SCM) such as pozzolans (such as powerplant ash) and slag (a byproduct of iron smelting) are sometimes added to the cement mixture as well. Pozzolans are materials that exhibit cementlike properties when combined with lime. When it comes to ensuring proper proportions, making concrete is a lot like baking.