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 ≈ 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.