Krasimira Georgieva and  Yordan Denev,  Producing and Mechanical Testing of Urea-Formaldehyde Resin Foams Reinforced by Waste Phosphogypsum.   journal   = {International Journal of Materials and Metallurgical Engineering}, [online]. World Academy of Science, Engineering and Technology.
    July 2019, vol. 152(8). 408 - 414
    [viewed 22 November 2019]. Available from: https://publications.waset.org/pdf/10010652.