Concrete blocks madenfrom a mixture of cement and slag, a by-product of blast furnaces, have been found to stop bullets that would shatter ordinary cement. As it comes from the furnace, the slag is whipped up with cold water, making a bubbly cellular mass which, after being crushed, is combined with cement, making a block less than three-fourths the weight of ordinary concrete. In tests of the blocks, a standard army rifle, using the new armor-piercing bullets, penetrated only five inches. A Krag rifle, firing ammunition loaded with 180 grains of powder, pierced to three or four inches, while a submachine gun kicked up a lot of dust but did no damage. It is said that for civil defense the material could be used instead of sand bags, as the blocks are cheaper and would not rot from exposure.