Immobilization of metal-containing waste in alkali-activated lime–RHA cementitious matrices Suwimol Asavapisit a, ⁎ , Donald E. Macphee b a School of Energy and Materials, Bangkok, Thailand b Department of Chemistry, University of Aberdeen, Meston Walk, Old Aberdeen AB24 3UE, Scotland, UK Received 12 April 2006; accepted 12 February 2007 Abstract 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 (Na 2 SiO 3 ) with SiO 2 / Na 2 O ≈ 3 and anhydrous sodium carbonate (Na 2 CO 3 ) 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. © 2007 Elsevier Ltd. All rights reserved. Keywords: Rice husk ash; Hydrated lime; Plating sludge; Alkali activators; Strength; Leaching 1. Introduction Rice husk, an agricultural residue, is generated in Thailand in large quantities each year. The husk is partly used as bulking agents for composting of animal manure or as biomass fuel to replace fossil fuel in generating heat for drying rice or electricity [1,2]. The remaining husk is burned at the rice fields, causing environmental problems. If rice husk is burned under controlled conditions, the resultant ash (RHA) can be used as a supple- mentary cementing material due to its high reactive silicon dioxide content. Several studies have been conducted on the use of rice husk ash as a mineral additive to improve the performance of concrete [3–8]. Its application in hazardous waste treatment is relatively new and is under investigation. Previous study found that the interfering effects caused by both synthesized metal hydroxides (Zn(OH) 2 , Cr(OH) 3 and Fe (OH) 3 ) and plating sludges, derived from electroplating processes and containing these heavy metal hydroxides, on cement hydration was reduced when OPC was substituted by 10 wt.% reactive RHA during the solidification process [9]. This paper investigates the use of RHA, in this case, activated with hydrated lime rather than OPC, as a matrix for the immobili- zation of metals in zinc cyanide plating sludge. The plating sludge was generated from the wastewater treatment plant of a zinc-cyanide plating industry. Cyanide in the plating wastewater was first removed by chemical oxidation with ozone. Sodium hydroxide was then added to adjust pH of the remaining wastewater to around 7.5 to transform the soluble metals into metal hydroxides. These metal hydroxides were separated from wastewater by filtration through sand drying beds. The sludge was then oven dried in the laboratory and is subsequently referred to as the plating sludge (PS). The cementitious pro- perties, as well as the immobilization performance of the binder Cement and Concrete Research 37 (2007) 776 – 780 ⁎ Corresponding author. E-mail addresses: suwimol.asa@kmutt.ac.th (S. Asavapisit), d.e.macphee@abdn.ac.uk (D.E. Macphee). 0008-8846/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2007.02.003