Stabilization of Heavy Metals in Fly Ash and Its Effect on Strength

This study investigated the effect of Ethylenediaminetetraacetic acid (EDTA) and Triple Super Phosphate (TSP) as stabilization agents to immobilize various heavy metals to enhance the use of fly ash without any environmental concern. The effect for the application of stabilization agents on the strength on fly ash is also studied. First, the various dosages of TSP (0–8%) or EDTA (0–2%) were applied to Class C and F fly ash to identify the appropriate dosages of these two stabilization agents using Toxicity Characteristic Leaching Procedure (TCLP) tests. Thereafter, the two fly ashes were mixed with the determined dosages (i.e., 6% TSP and 1% EDTA) to assess the impact on time-depended leaching and the strength of the hardened fly ash. The column leaching tests disclosed that TSP immobilized well for all the heavy metals except As and EDTA also immobilized quite well for all the heavy metals except Hg and Pb for Class C fly Ash. The combined usage of EDTA and TSP showed mutually supplementary effects: EDTA responded well to some metals and TSP responded well to others. In addition, combination of the two agents further enhanced the stabilization of Cd, Cr, Zn, and As. For Class C fly ash, adding either EDTA or TSP decreased the compressive strength of 28 days cured specimens by 38%. The combination of both TSP and EDTA decreased the strength by 36%. For Class F fly ash, EDTA did not noticeably affect the compressive strength of the cured specimen, but TSP increased the strength significantly.