In-Situ Stabilization of Sedimented Ash Deposits by Chemical Columns

Abstract

Coal-based thermal power plants produce a massive amount of fly ash from the combustion of coal. It requires vast stretches of valuable lands for disposal. Acquiring open lands for disposal of ash in developing countries like India is tough, where the land to population ratio is decreasing day-by-day. Hence, novel disposal methods including recycling have been an attractive alternative solution. Among the various alternatives available for disposal, the wet method of disposing fly ash into ash ponds, lagoons or ash dykes has been widely practiced by most of the thermal power plants. These sedimented ash deposits possess very low density, high compressibility, and poor bearing capacity that are very much susceptible to liquefaction during an earthquake. Further, continuous leaching of toxic and heavy metals from these ash ponds contaminates the surface and groundwater bodies. The best possible way is to use this by-product as an alternative construction material. Although coal ash is put to use in a variety of ways, only the geotechnical applications provide the outlets for its bulk utilization. Besides, an alternative approach of effective use of the abandoned ash pond sites is its stabilization.

Considering the aforementioned issues, the present research work examines the efficiency of sodium hydroxide (NaOH) column in stabilizing the sedimented ash deposits. Additionally, a comparison study between NaOH and calcium hydroxide [Ca(OH)2] columns was made to understand their effectiveness in improving the engineering performances of the sedimented ash deposits through a series of laboratory model tests. Natural sedimentation process was simulated in the laboratory environment to prepare the model ash beds of 1.05 m diameter and 1.20 m height. For preparing a sedimented ash bed, approximately 1 ton of dry fly ash was blended with 72% of water for 10 minutes in a conventional concrete mixture. These ash beds were treated with different configured (full column, ½ column, and 1/4th column) chemical columns of NaOH and Ca(OH)2 of either 9 M or 18 M concentration for 60 and 120 days. The effects of chemical treatment on the engineering characteristics of sedimented ash beds were studied by determining and comparing various geotechnical parameters such as in-situ dry unit weight, moisture content, bearing resistance, compressibility characteristics, collapse potential, pore structure, and hydraulic conductivity at different locations and time intervals. These properties are again correlated with the developed hydration products and microstructural changes. Studies on leachate characteristics were also carried out to address the environmental issues concerning to the presence of different toxic elements. Leachate samples were collected from five different depths and four different radial distances of the treated and un-treated ash beds and were analysed for elements such as sodium (Na), calcium (Ca), magnesium (Mg), iron (Fe), nickel (Ni), lead (Pb), zinc (Zn), copper (Cu), chromium (Cr), arsenic (As), and mercury (Hg). The effects of stabilization on the contamination potential of the leachate were studied through a parameter termed “Contamination Potential Ratio”, which helps to quantify the combined effects of hydraulic conductivity and metal concentration in the leachate.

Test results related to various geo-engineering and geo-environmental characteristics of ash deposits and their variation with stabilization period, the concentration of the chemical, column configuration, and sampling positions are determined. It is observed that the ash beds treated with chemical columns show changes in in-situ moisture content and dry unit weight which signify the extent of chemical migration towards the surrounding area. A wide range of bearing resistance is observed based on the column configuration, the concentration of the chemical, type of chemical, curing period, and the sampling locations. Contours of bearing resistance indicate that each stabilized zone can be idealized as a bell-shaped one with a larger extent of spreading towards the bottom of the columns. Under similar conditions, the Ca(OH)2 column stabilized ash bed exhibits a poor bearing resistance than beds stabilized by NaOH columns. Based on the magnitude of bearing resistance, the entire ash bed is divided into five different stabilized zones such as main influence zone, effective influence zone, moderately influence zone, slightly improved zone, and less-improved zone. The strength contours formed around the Ca(OH)2 column falls under slightly and less improved zones whereas the major volume of the ash bed treated with the NaOH column falls under the main, effective, and moderately influence zone. The magnitude of strength diminishes rapidly with an increase in radial distance. Further, the reduction in void ratio under an imposed consolidation pressure is found to be least for specimens collected nearest to column peripheral zones and is the maximum at the farther location. Similarly, the ash specimen collected from farther location displays a higher degree of collapse under inundation loading than the specimens collected from column peripheral zones. The pore size distribution of the stabilized zones gets changed owing to the formation and migration of reaction compounds. The capillary pores are either clogged, sub-divided or gradually converted to gel pores that reduce the hydraulic conductivity, compressibility and collapse behaviour of the ash bed. Diffraction analysis shows the occurrence of a series of Na and Ca-based compounds in the stabilized zones. These compounds are primarily responsible for modifying the geotechnical properties of the sedimented ash bed. The solubility of metals present in fly ash is pH sensitive. A higher pH favours the formation of more amounts of hydration and geopolymeric products. The encapsulation of metal ions by the reaction matrix reduces their concentration in the leachate sample. The declining trend of the CPR value with curing period is attributed to the reduction of the hydraulic conductivity of the ash bed and encapsulation of metal ions in the reaction matrix.
Stabilization of sedimented ash deposits by chemical columns is found to be a promising technique in the laboratory environment. This technique improves the strength, reduces the hydraulic conductivity, compressibility, collapsibility along with attenuating the migration of leachable elements from the sedimented ash deposits.