Micro-structural evaluation of the impact of curing method on the process of cement based stabilization/solidification of Pb ion-contaminated bentonite

Cement-based stabilization/solidification is a commonly used method to prevent the transportation of heavy metal ions in soils. The main objective of this paper is to investigate the controlling mechanisms in cement-based stabilization/solidification of Pb ion-contaminated bentonite under two different curing conditions: closed and open systems. In an open system, the stabilized/solidified sample continues to have access to free water, while in a closed system, the stabilized/solidified sample is prevented from accessing any external water after initial mixing. To achieve this objective, a series of geo-environmental experiments, including pH, solubility measurements, TCLP, and XRD, were performed. In the first step, the bentonite sample was contaminated with 100 cmol/kg-soil of lead nitrate. After achieving equilibrium, the contaminated sample was stabilized/solidified with 15% cement. The results indicate that when 15% cement is applied to the contaminated bentonite, the pH ranges from 10.5 to 11.5, which is a safe domain for lead precipitation. In other words, the minimum required percentage of cement for stabilization/solidification is the quantity in which the pH of the system is in the necessary range for heavy metal precipitation. This quantity is generally a function of the type and concentration of the heavy metal contaminant. According to the experimental results of this research, the method of curing does not have a noticeable impact on the stabilization process of stabilized/solidified contaminated bentonite. However, the XRD results show that more pozzolanic components have formed in the closed system. Therefore, the achievement of EPA criteria for TCLP experiments in cured samples in the closed system is attributed to the more significant progress in pozzolanic interaction and more formation of C-S-H and C-A-S-H components at 28 days for cured samples under closed conditions.