Ettringite and C---S---H Portland cement phases for waste ion immobilization: A review

The formation, structure and chemistry of the ettringite and C---S---H phases of Portland cement have been reviewed as they relate to waste ion immobilization. The purpose of this review was to investigate the use of Portland cement as a host for priority metallic pollutants (Cr, Pb, Ba, Se, Zn, Ag, Hg, As and Cd) as identified by the Environmental Protection Agency and as a host for radioactive waste ions (various isotopes of Am, C, Cs, I, Np, Pu, Ra, Sr, Tc, Th, Sn, U) as identified in 40 CFR 191. Ettringite acts as host to a number of these ions in both the columnar and channel sections of the crystal structure. Substitutions have been made at the calcium, aluminum, hydroxide and sulfate sites. C---S---H also hosts a number of the waste species in both ionic and salt form. Immobilization mechanisms for C---S---H include sorption, phase mixing and substitution. The following ions have not apparently been reported as specifically immobilized by one of these phases: Ag, Am, Np, Pu, Ra, Tc, Th and Sn; however, some of these ions are immobilized by Portland cement.