Immobilization of metal-containing waste in alkali-activated lime-RHA cementitious matrices

This research investigated the immobilization potential of alkali-activated lime-rice husk ash (RHA) for synthetic Cr(OH)(3), Fe(OH)(3), Zn(OH)(2) and zinc cyanide plating sludge. The binder consists of hydrated lime and RHA at a weight ratio of 45:55. Waterglass (Na2SiO3) with SiO2/Na2O approximate to 3 and anhydrous sodium carbonate (Na2CO3) were used as alkali activator between 0 and 8 wt.% of the binder. Results showed that Zn (OH)(2) addition causes a considerable strength development in control and sodium silicate-activated samples but only after 14 days. Similar observations were found for the sample loaded with 10 wt.% plating sludge but this only occurred after 28 days. A possible explanation for these phenomena is that the initial formation of calcium zincate, which has a set retarding effect, inhibits early strength development. At later ages, calcium zincate dissolves and Zn is taken up in the formation of C-S-Z-H solid solutions leading to strength development. These phenomena were not observed from the sodium carbonate-activated lime-RHA matrices. In these, it is believed that zinc/calcium carbonates readily form inhibiting calcium zincate and C-S-Z-H formation. Despite this, carbonate-containing mixes with up to 30 wt.% plating sludge gave a 14-day strength and Cr concentration in TCLP leachate that meet the regulatory limit for landfilling. (C) 2007 Elsevier Ltd. All rights reserved.