IMPACT OF FLY ASH ON THE PROCESS OF CEMENT-BASED OLIDIFICATION OF HEAVY METAL CONTAMINATED BENTONITE

Cement-based Stabilization/Solidification (S/S) process is one of the best technologies available for the treatment of heavy metal contaminated soils. This method has been very popular among researchers in field application. Two mechanisms of solidification and stabilization in this method include the formation of cementation products (such as C-S-H) and achievement of alkaline condition for development of pozzolanic interactions. The pH variation range directly affects the heavy metal ion precipitation. On the other hand, environmental factors and soil-cement-contaminant interaction affect the cement hydration products and pH range of solidified contaminated soil. Therefore, this interaction process controls the immobilization of metal ions in solidified/stabilized sample. The main objective of this paper is to evaluate the effect of fly ash on the heavy metal retention in cement-based solidification/stabilization in a long-term process. In order to investigate the effect of fly ash on the optimum amount of required cement at different concentrations of contaminants, several mixtures of fly ash and cement were applied to solidified contaminated bentonite. In these series of experiments, 75-25 and 50-50 wt% mixtures of Portland cement and flay-ash (class F) were mixed and were added to contaminated bentonite samples. The bentonite samples were laboratory contaminated with Pb(NO3)2 in the concentration range of 5 to 100 cmol/kg-soil. The prepared samples were kept for 7 to 90 days. Different experiments which include TCLP, XRD, pH measurement, and solubility evaluation in different alkaline and acidic conditions were performed on samples. Furthermore, the trend of pozzolanic reactions of the samples in the short and long terms was evaluated by determining the setting time of solidified/stabilized samples. The results were analyzed based on the effect of pH variation upon concentration of released lead ions from samples. The results indicated that the use of 10% binder, despite common recommendations in geotechnical stabilization, would not be suitable for S/S of contaminated bentonite soil at all studied Pb concentrations. Moreover, the results indicated that in contaminated samples with 50 and 100 cmol/kg-soil lead nitrate, due to the high precipitation of lead hydroxide, a reduction in C-S-H formation and an increase in the setting time occurred. In addition, replacing cement with fly ash in samples reduced the pH and decreased the concentration of released Pb in TCLP test. According to the achieved results due to the precipitated Pb ions in stabilized/solidified contaminated bentonite, an appropriate quantity of the cement-fly ash binder is when the pH of the stabilized/solidified samples is in a safe zone that occurs in the pH range of 8 to 12. According to the results of this paper, the use of fly ash in the S/S process helps achieve this safe range of pHs.