عنوان مقاله [English]
Cement based Stabilization/Solidification (S/S) process, is one of the best technologies available for the treatment of heavy metal contaminated soils. Two mechanisms of stabilization and solidification 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 influence of fly ash on the heavy metal retention in cement based solidification/stabilization in a long-term process. In order to achieve this objective, several mixtures of fly ash and cement were used 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 at 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, setting time experiment, and solubility evaluation at different alkaline and acidic conditions were performed on samples. The results were analyzed based on the effect of pH variation upon concentration of released lead ions from samples. The results indicate that the use of 10% binder, despite common recommendations in geotechnical stabilization, is not suitable for S/S of contaminated bentonite soil at all studied Pb concentrations. Moreover, the results indicate 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 occur. In addition, replacing cement with fly ash in samples reduces the pH and decreases the concentration of released Pb in TCLP test. According to the achieved results due to the precipitated Pb ions in stabilized/solidified contaminated bentonite, the 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 between 8 to 12. According to the results of this paper, the use of fly ash in S/S process helps to achieve this safe range of pH.