Stabilized phosphogypsum: class C fly ash: Portland type II cement composites for potential marine application

Phosphogypsum (PG, CaSO4.H2O), a byproduct of phosphoric acid manufacturing , contains low levels of Ra-226. PG can be stabilized with portland type II cement and class C fly ash for use in marine environments, thus eliminatin g the airborne vector of transmission for radon gas. An augmented simplex c entroid design with pseud components was used to select 10 PG:class C fly a sh:portland type II cement compositions. The 43 cm(3) blocks were fabricate d and subjected to a 1.5-yr field submergence test and a 28-d saltwater dyn amic leaching study. All field composites survived with no signs of degrada tion. Dynamic leaching resulted in effective calcium diffusion coefficients ranging from 0.21 to 7.5 x 10(-14) m(2) s(-1). Effective diffusion depths, calculated for t = 1 and 30 yr, ranged from 0.4 to 2.2 mm and from 2.0 to 11.9 mm, respectively. Scanning electron microscopy and wavelength dispersi ve microprobe and X-ray diffraction analyses of the leached composites iden tified a 40-60-mum calcite layer that was absent in the control composites. This suggests that a reaction between the composites and the saltwater res ults in the precipitation of calcite onto the block surface, encapsulating the composites and protecting them from saltwater attack and dissolution.