Evaluation of Micro and Nano-Structure of Process of Cement Hydration in Solidification and Stabilization of Zn Heavy Metal Pollutants at the Presence of Soft Clay

Solidification and stabilization of heavy metal contaminants is recognized as the technology to prevent transfer of contaminants to the lower layers of soil and groundwater. A noticeable increase in distribution of heavy metal contaminants in recent years highlights the importance of effective methods for engineering disposal of industrial wastes. The most important challenge ahead of this endeavor is perhaps the determination of right framework and mechanism of action. Precise mechanism of mobility of contaminants can be grasped by gaining accurate and comprehensive understanding about system behavior and evaluating it from the nano- and micro-structure perspectives. Nano- and micro-sized clay particles can be used effectively as adsorbents of many contaminants (e.g. heavy metal ions and organic compounds) in sewage and wastewater. Moreover, as clay soils have high cation exchange capacity (CEC), they provide appropriate conditions for cation exchange and create considerable capacity to retain heavy metal contaminants. In spite of conducting extensive studies on stabilizing contaminants by the use of cement, inadequate attentions have been paid to microstructure study of interaction process of clay particles, heavy metal ions, and cement, specifically in cement hydration process in different time intervals. Based on this, the present research aims to study the interaction process of clay particles, heavy metal contaminants, and cement over time from the perspective of microstructure. This include the investigation of the effect of presence of heavy metal on cement hydration process and formation of nano-structure calcium silicate hydrate (C-S-H).
Material and In this study, the behavioral tests were conducted on natural clay soil collected from the Qazvin Plain, Iran. The purpose of this selection was to determine geotechnical-environmental properties and contaminant adsorption-retention capability of samples of natural clay with average specific surface area and CEC and the effects of natural clay on the solidification and stabilization process. The majority of experiments of this study were conducted based on ASTM standards and geotechnical-environmental test guidelines of McGill University (Canada). Density and pH of clay samples were determined in accordance with ASTM, D854 and ASTM, D4972 standards. Soil carbon content was determined by titration. Specific surface area (SSA) of the soil was measured using EGME solution. The cation exchange capacity (CEC) of the soil was determined using 0.1 M barium chloride solution. Meanwhile, different concentrations of heavy metal contaminant (zinc) and different percentages of Portland cement were added to natural clay. The interaction process was analyzed experimentally by examining pH changes and evaluating microstructure study (XRD).
Result and According to laboratory results obtained in this study, the high specific surface area of C-S-H nanostructure improves the adsorption characteristics and leads to better filling of pores. It also improves the retention capability by decreasing the mobility of heavy metal contaminants via encapsulation of their ions (solidification). The results show that formation of C-S-H nanostructure improves absorption features due to high specific surface area and decreases mobility of the heavy metal ions through their encapsulation (solidification). In addition, the presence of the heavy mental contaminant (zinc) reduces formation of C-S-H nanostructure so that the presence of 25 cmol/kg-soil of heavy metal ion (zinc) decreases peak intensity of C-S-H nanostructure about 160 CpS.