Microstructures of soft clay stabilized with coal ash and cement

Abstract

As the growth of industries increase electricity demand, the amount of coal ash and waste produced by power plants has significantly increased. Malaysia alone produces approximately 11 million tons of coal annually. For every 2.9 million metric tons of coal that is burned, 1.2 million metric tons of coal ash is produced. Therefore, there is a pressing need to reutilise coal ash waste in a sustainable matter for the good of the environment. The purpose of this study was to identify the macrostructure and microstructure behaviour of marine clay that had been stabilised with coal ash and a minimal amount of ordinary Portland cement (OPC) for use as a subgrade layer in road construction. Very few studies have been conducted on contamination analysis of coal ash as well as mixtures of coal ash and marine clay. In fact, this present study is the first to investigate the performance of marine clay that has been treated with coal ash and OPC with a pre-assessment of load repetition behaviour. The physical properties of all the materials used in this study were examined where necessary. The treated marine clay samples were subjected to an unconfined compressive strength (UCS), California bearing ratio (CBR) as well as a mini tracker tests. Microstructural tests; such as X-ray diffraction (XRD) analysis and energy dispersive X-ray (EDX) spectrometry using a field emission scanning electron microscope (FESEM); as well as toxicity characteristic leaching procedure (TCLP) were also performed. The results of the physical property tests indicated that the use of stabilisers; such as coal ash and OPC; improved the properties of marine clay. The results of the UCS test concluded that the 15% 50BA:50FA sample was the most effective stabiliser as it satisfy the minimum requirements of the Standard Specification for Roadworks for stabilised subgrade layers. During microstructural analysis, XRD analysis showed that the mineral composition intensity of untreated marine clay decreased as the curing duration increased. A new mineral; calcium aluminium silicate hydroxide hydrate (CASH, Ca5Si5Al(OH)O17.5) was also found to form. The FESEM results showed the surface particles of treated marine clay seemed denser with fewer voids in the structure. In line with the findings of the XRD and FESEM tests, the EDX results confirmed that cementitious product formed was CASH. During the performance testing for subgrade evaluation, the CBR of treated marine clay increased unlike untreated marine clay in both soaked and unsoaked conditions. Of the treated marine clay samples, only the 15% 50BA:50FA sample at Day 7 had a CBR of 12.8%, which was above the minimum 12% required by the Standard Specification for Road Works in Malaysia. The findings of the mini tracker test indicated an increase in the number of cycles when both soaked and unsoaked conditions as well as untreated soil samples were compared. During the contamination analysis, the elemental concentrations of all three samples (fly ash, bottom ash, and treated marine clay) were found to satisfy the regulatory limits outlined by the US Department of Environmental (DOE). However, only the levels of cadmium, selenium, and silver detected in all three samples were within the regulatory limits of the National Standard for Drinking Water Quality. Therefore, marine clay stabilised with coal ash and OPC not only increases in strength but performs better. Moreover, treated marine clay is also safe for the environment.