Afriani, F., J, E., Zaitun and Tiandho, Y. 2020. Improvement of Hardness of Hydroxyapatite by the Addition of Silica from Tin Tailings. Journal of Engineering and Scientific Research. 2, 2 (Dec. 2020), 85–89. DOI:https://doi.org/10.23960/jesr.v2i2.48.