1. School of Chemical and Metallurgical Engineering, DSI-NRF Centre of Excellence in Strong Materials, University of the Witwatersrand, Johannesburg, Private Bag 3, Johannesburg 2050, South Africa
2. African Academy of Sciences, Nairobi P.O. Box 24916-00502, Kenya
3. Department of Mining, Materials and Petroleum Engineering, Jomo Kenyatta University of Agricultural Technology, Nairobi P.O. Box 62000-00200, Kenya