INFLUENCE OF NaOH SOLUTION ON THE SYNTHESIS OF HOSPITAL WASTE FLY ASH GEOPOLYMER

In pursuing their aims of reducing health problems and eliminating potential risks to people's health, health-care services inevitably create waste that may itself be hazardous. In fact, medical and health-care wastes have sharply increased in recent decades due to the increased population, number, and size of health care facilities, as well as the use of disposable medical products. In Greece more than 14,000 tonnes of infectious hospital waste are produced yearly; a significant part of it is still mismanaged. Several types of treatment and disposal processes have been applied to medical waste. However, incineration has been identified as the best option for the disposal of medical waste in many areas. One of the inconveniences of incineration, is that generates solid residues, such as bottom and fly ash as well as off-gas cleaning residues with high levels of heavy metals, inorganic salts and other organic compounds. According to bibliography, fly ash possesses a high content of heavy metals, dioxins and furans. Therefore, attempts to stabilize the fly ash into a stable product, which is environmentally acceptable, has received considerable attention. In the last decades, geopolymerization has emerged as a promising process for the immobilization of several kinds of wastes. The present work focuses on the evaluation of the possibility to use geopolymerization technology to stabilize and solidify fly ash generated from incinerated medical waste. Medical waste fly ash (MWFA), sodium hydroxide, sodium silicate solution and metakaolin, produced by calcination at 700 degrees C for 4 h of kaolinite, were mixed. Four series of experiments were carried out as are shown in Table 1. NaOH solutions with concentrations of 1, 2, 4 and 6 M were used as an alkali activator, different for each sere. In all series three different percentages of MWFA, 20, 30 and 50 wt%, were applied. After a certain aging time of 7 and 28 days, the compressive strength of the geopolymer mortars, the leachability of heavy metals, as well as the mineralogical properties of the produced geopolymers were studied. This study demonstrates that medical waste incineration fly ash containing heavy metals, such as Zn and Pb, can be effectively stabilized/solidified using fly ash-based geopolymerization technology. Compressive strength tests performed on the geopolymer specimens after 7 and 28 days of solidification, revealed values higher than the regulatory limit of 0.414 MPa. Moreover, the synthesized geopolymers showed a significant increase in compressive strength, as the percentage of fly ash, with regard to metakaolinite, was increased from 20 to 50%. Also, by increasing the ratio Na2O/SiO2 in the geopolymer binders from 0.63 to 0.73, an important decrease in compressive strengths was observed. Geopolymers with strengths of 7 MPa were obtained when 2 M NaOH and 50% fly ash were used. With regard to the leachability of heavy metals, it can be concluded that geopolymerisation technology is able to immobilize all the heavy metals found in medical waste fly ash. The leachability of the produced binders reduced drastically through this process. Finally, a significant decrease in leachability of heavy metals was noted by increasing NaOH concentration from 1 M to 6 M.