Comparative study on the characteristics of coal fly ash and biomass ash geopolymers

Abstract

Geopolymers are a new class of materials that can be synthesized using natural minerals, and waste materials. Among these substrates, the use of fl y ash is desirable as it involves the conversion of a copious waste material into a useful product. The aim of the research was geopolymers synthesis from coal fl y ash and biomass ash. Concentrated sodium hydroxide and sodium silicate solutions were used as activators in geopolymerisation reaction. The results show that both coal fl y ash and biomass ash can be utilized as source materials for the production of geopolymers. The surface morphology and chemical composition analysis were examined for the obtained geopolymers and ashes from coal and biomass combustion by SEM-EDS methods. It was found almost total disappearance of spherical forms of grains and reduction the porosity of structure for geopolymer based on fl y ash from coal combustion. While the structure of the geopolymer based ash from biomass combustion is more porous. The UV-VIS-NIR spectra were performed on the coal fl y ash, biomass ash and geopolymers. They showed that the obtained geopolymers possess optical and photocatalytic properties. The similarity of the geopolymer network and the zeolite framework in relation to ion exchange and accommodation of metal ions open questions on possibilities for the application of geopolymer materials as amorphous analogues of zeolite. The FT-IR spectra analyses were used on the geopolymers before and after metals sorption. It was found that geopolymer based on ash from biomass combustion has better sorption properties compared to geopolymer based on ash from coal combustion. Comparative study on the characteristics of coal fl y ash and biomass ash geopolymers 127 geopolymers, respectively: bases, e.g. sodium or potassium hydroxide (Romisuhani 2015), sodium or potassium water glass or acids, however phosphoric acid is most often used for the activation of metakaolin. Acid-activated geopolymers constitute a relatively new group of materials with higher porosity, which suggests the possibility of using this group of geopolymers as adsorbents in the process of water purifi cation. In contrast, alkali-activated geopolymers have been studied in many scientifi c papers due to their higher mechanical properties comparable to Portland cement (Zivica et al. 2014). It should be emphasized that the specifi c properties of a geopolymer are mostly determined by the aluminosilicate matrix, e.g. the structure of disiloxosialate is resistant to fi re and corrosion, which results from the greater number of siloxane bonds compared to sialate (Khale and Chaudhary 2007). The aim of the presented research was to demonstrate the synthesis of a geopolymer on the basis of coal fl y ash and biomass ash and the indication of the possibility of their application. Concentrated NaOH and Na2SiO3 solutions were used as activators in the geopolymerization reaction. Surface morphology research and the analysis of chemical composition of the obtained geopolymers were performed using the SEM-EDS method. As part of the research, a comparison of the sorption and catalytic properties of geopolymers on the basis of coal fl y ash and biomass ash was also made. For this purpose an experiment of sorption of the selected heavy metals (Cu, Mn, Pb and Zn) was performed and spectroscopic studies of the molecular structure of the obtained geopolymers before and after the sorption experiment using the FT-IR method were done. Also, spectra in the UV-VIS-NIR range were recorded. In addition, specifi c surface area measurements were made for the obtained geopolymers (BET isotherm and Langmuir isotherm) and the analysis of these isotherms for porous solids (t-Plot method and BJH method) was carried out.