The Structural Properties of Magnetite Concrete

Magnetite concrete for the Engineering Test Reactor (ETR) biological shield was made from a combination of crushed magnetite aggregate and a low-alkali Type I portland cement. Some combinations of cement and aggregate used in concrete are potentially self-destructive through internal volume changes caused by alkali-aggregate reaction or by thermal growth. Tests proved that this particular combination was non-reactive and with out deleterious thermal growth. The elongated shape and high density of the crushed magnetite particles tended to cause harshness and segregation in mixing the concrete, but by paying very careful attention to the proportion of fines and the composition of the cement paste in the mix, workable concrete mixes were designed having densities ranging from 225 to 239 psf. It proved important to prevent the cement paste from becoming too fluid, as this caused excessive bleeding and segregation.