Application of microwave hydrothermal synthesis for the solidification of copper: Effect of heavy metal content and microwave time

The synthesis type and heavy metal content significantly affect the performance of microwave hydrothermal synthesis. This synthesis approach has lower energy consumption and shorter reaction time for consolidating the heavy metals in the tobermorite lattice. The goal of this study was to investigate the influence of microwave reaction time and copper (Cu) content on the preparation of Cu-substituted tobermorite (Cu-tobermorite). Fumed silica, calcium hydroxide and copper chloride dihydrate were used to prepare the Cu-tobermorite under a liquid-solid ratio of 30 mL/g, reaction temperature of 220 celcius, and (Ca+Cu)/Si ratio of 0.83. The effect of reaction time (2 h and 4 h) and Cu content (Cu/Ca ratios are 0, 0.05, 0.1, 0.15 and 0.2) on the microstructure and morphology of the Cu-tobermorite were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and thermogravimetry (TG), and Fourier-transform infrared (FTIR) spectroscopy. Moreover, the solidification effi-ciency was assessed by inductively coupled plasma-optical emission spectrometry (ICP-OES). The results showed that the main products of microwave synthesis were Cu-tobermorite and tobermorite, while the conventional hydrothermal synthesis products were Cu-tobermorite and gyrolite. Prolonging the elongation of the reaction time did not change the type of hydration product. Both microwave and traditional hydrothermal synthesis of tobermorite solidified Cu with a solidification rate of almost 100%. However, the microwave method provided a 1/7th shorter reaction time and less than 10% of the energy composition (< 10%