**4. Conclusions and summary**

The XRD spectra showed that the coal sample mainly composed of siliceous mineral (such as quartz and kaolinite) and the non-siliceous mineral (such as potassium selenium chloride) and little quantities of pyrite. The results of the XRD analysis of samples of the drilled weathered dry disposed fly ash aged 2 week, 1 year and 20-year-old showed quartz and mullite as main crystalline mineral phases. Other minor mineral phases identified are; hematite, calcite, lime, anorthite, mica and enstatite.

The IR spectrum revealed the presence of quartz, kaolinite, potassium selenium chloride and pyrite in coal sample. SEM image of coal ash reveals spherical shaped and aggregate that contains varying sizes and quantity of particles. Conversely, the coal sample consists of irregular shaped pyrite crystal coated with kaolinite. TEM images of coal ash sample show nearly spherical shaped haematite structure and cluster texture agglomeration of ultrafine particles. Conversely, the TEM images of coal sample show irregular shaped Fe-rich particles (i.e. pyrite) encrusted with Al, Si-rich particles (i.e. kaolinite).

The main trend in the major oxides indicates that the coal used in the present study is a higher-ash coals which is enriched in elements associated with probable detrital minerals. The 1-year-old ash core samples were both sialic and ferrocalsialic in chemical composition (i.e. essentially Fe, Ca, Al and Si). Although, the 2 week and 20-year-old dry disposed ash core samples were sialic in chemical composition (i.e. essentially dominated by Al and Si).

Major elements such as Fe, Ca, Mg, Mn and Ti were as expected in coal samples but significantly enriched in the coal ash. Although, P, Na and K are slightly enriched in the coal ash samples used in this study.

Trace elements such as U, Cr, Th, V, Ni, Cu, Zn, Rb, Sr, Mo and Sn are slightly enriched in the coal ash. This slight enrichment of these trace elements in the coal ash is attributed to the combustion process. Nevertheless, trace elements (such as Hf, Ta, Pb, Cr, Zr and Nb) showed significant enrichment in the coal ash. On the contrary, W, As, Cs and Ba are considerably enriched in the coal samples used in the present study.

Rare earth elements (REEs) such as La, Ce, Pr, Nd, Sm, Eu and Gd are slightly enriched in the coal ash. On the contrary, Lu, Y, Dy, Tb, Yb, Tm, Er and Ho are considerably enriched in the coal ash used in the present study.

The proximate analysis revealed that the moisture content, ash content and volatile organic matter of pulverised coal used in this study is relatively low in values compare to the American coal, Polish coal, Lafia-Obi and Chikila coals. The ultimate analysis showed that the fixed carbon of coal sample used in this study is relatively higher than fixed carbons obtained from Chikila coal. On the contrary, it is comparatively lower than the fixed carbons in the Lafia-Obi, American and Polish coals

In conclusion, factors such as the nature of combustion process, type of coal and chemical interaction of fly ash with the ingressed CO2 and percolating rain water would ultimately determine the mineralogy and chemical composition of coal combustion products.
