What is known as "blue concrete"?
During the curing process, concrete containing a higher proportion of cement containing granulated blast furnace slag may turn blue. Fine types of cement, especially the bluish shimmering "Portland cement", are called "blaine" in English, which resembles the German word "blau".
Since "blue concrete" is produced with a high cement content, it has great strength. However, this is counterbalanced by increased brittleness.
It was used especially before and during the Second World War for the construction of bunkers.