K. S. Hui and  K. N. Hui and  Seong Kon Lee,  A Novel and Green Approach to Produce Nano- Porous Materials Zeolite A and MCM-41 from Coal Fly Ash and their Applications in Environmental Protection.   journal   = {International Journal of Industrial and Manufacturing Engineering}, [online]. World Academy of Science, Engineering and Technology.
    May 2009, vol. 29(5). 489 - 499
    [viewed 20 April 2021]. Available from: https://publications.waset.org/pdf/4103.