The effect of Pb²⁺and Cd²⁺ addition to mechanical properties of fly ash geopolymer paste

Immobilization of heavy metal ions using geopolymer paste is a promising way to solve the problem with heavy metal waste from industries. This research focused on the immobilization of Pb²⁺ and Cd²⁺ which are common heavy metal waste with distinctive cation size. The cations were immobilized by geopolymerization process to form geopolymer paste. The paste was prepared by using fly ash from a power plant in Cilacap, Indonesia, with SiO₂/Al₂O₃ ratio = 3 and S/L = 1.6. According to XRF analysis, the ash is type C fly ash and additional Al(OH)₃ was needed to achieve the SiO₂/Al₂O₃ ratio. A solution of Cd²⁺ or Pb²⁺ from their respective nitrate salts was added to the geopolymer mixture to make an immobilized Cd²⁺ or Pb²⁺ in a fly ash geopolymer matrix. Compressive strength tests showed that the maximum strength was achieved when 3855 ppm of Cd²⁺ or 765 ppm of Pb²⁺ was added to the geopolymer mixture. The change of strength is explained to be caused by the filling of geopolymer cavities that were formed during the geopolymerization process. Due to the smaller size of Cd²⁺ than Pb²⁺, the strongest geopolymer paste was achieved at higher Cd²⁺ concentration than Pb²⁺. X-ray diffraction analyses results show that the addition of Cd²⁺ or Pb²⁺ shift the position on hump to larger 2θ.