Distribution of heavy metal elements in chromium (III), lead-doped cement pastes

Abstract

The distribution of heavy metal elements in cement pastes doped with nitrate salts of chromium (III) and lead were investigated by X-ray powder diffraction and field emission scanning electron microscope equipped with energy-dispersive X-ray spectroscopy analysis system in a backscattered mode. The results show that neither chromium nor lead was uniformly distributed in calcium silicate hydrate (C–S–H), calcium hydroxide (CH) and trisulfoaluminates/monosulfaluminate (AFt/AFm) phases, but they were most inclined to be immobilised in the AFt/AFm phase, and then in C–S–H. When the dosage (mass percentage of heavy metal elements in the cement) reached 1·0%, chromium was also immobilised by precipitation as chromium calcium hydroxide hydrate, whereas no obvious lead compound was detected in the lead-doped hydrated pastes. The chromium calcium hydroxide hydrate was more inclined to form in some pores of the pastes. The presence of chromium and lead resulted in the formation of more ettringite, and the content of ettringite in all doped hydrated pastes, especially the chromium-doped pastes, was much higher than in undoped pastes at the age of 3 d; this content increased as the dosage increased.