Blood. Sacrifice. Dispose of like kitchen waste.
The Romans used blood in their cement too. These people are sick, pure evil. Could they dispose of people and the blood and mix it right into the concrete steps we walk on or in the very foundations our monuments? Think about that possibility when we hear about cement and concrete.
Back digging... getting dark down here...
SS1
Putting Bone in Bone china works; I doubt it does in large-scale cement.