Alkaline modified oil shale fly ash: Optimal synthesis conditions and preliminary tests on CO2 adsorption

► Alkaline hydrothermal treatment of oil shale fly ash is presented. ► Quartz in fly ash is converted mainly into tobermorites (75 wt%). ► Optimal conditions were found to be 5 M NaOH solution, temp. 130 °C. ► Drastic increase in BETN2 specific area and BJHN2 pore volumes was observed. ► Product has ca. 100 times higher CO2 adsorption capacity compared to fly ash.