Effect of Pb^II and Cr^VI ions on the hydration of slag alkaline cement and the immobilization of these heavy metal ions

The effect of the curing condition of slag alkaline cement (SAC) on the immobilization of Pb(II) and Cr(VI) ion is discussed. The SAC had properties of rapid and high strength development. The slag hydration products consisted mainly of the gel-like C-S-H phase and silica-alumina gel. The pastes had a high number of very fine pores with a radius of less than 5 nm. This was obtained by autoclaving at a low temperature of 105°C for a very short time (0.5 day). When SAC was autoclaved at temperatures higher than 105°C, the pore radius became wider. When curing at temperatures lower than 105°C, it took longer to obtain the same hydration ratio. Also, the addition of heavy metal ions (Pb(NO₃)₂ or CrO₃ solution) did not affect the hydration of SAC as much. The amount of leached Pb^II ions was 1.3 × 10^-6 mmol/cm² at 28 days immersion for SAC-NSP pastes (2 mass% Pb(NO₃)₂ added as Pb^II ions with sodium silicate solution as an activator) autoclaved at 105°C for 0.5 day. The amount of leached Cr^VI ions was 6.5 × 10^-5 mmol/cm² at 28 days immersion for SAC-NSC pastes (2 mass% CrO₃ added as Cr^VI ions with sodium silicate solution as an activator) autoclaved at 105°C for 0.5 day. The apparent diffusion coefficient of the Cr^VI ion had a close relation to the micropore volume. The apparent diffusion coefficient for the Cr^VI ion in the SAC pastes was about 7.2 × 10^-13 cm²/s. SAC pastes was evaluated as an available material for solidifying waste materials.