Use of fly and bottom ashes from a thermoelectrical plant in the synthesis of geopolymers: Evaluation of reaction efficiency

Abstract

The use of by-products as raw materials in the manufacturing of industrial products has risen in the last years because of environmental considerations. One example is the use of coal ashes from thermal-electrical plants in the production of geopolymer -- a green cement made by mixing aluminosilicate with alkaline activator. In this study, fly and bottom ashes from a thermal-electrical unit were used as sources of aluminosilicate in the synthesis of geopolymers. A mixture of sodium hydroxide (10 mol/L) and sodium silicate (SiO2/Na2O ratio of 2.2) was used as the alkaline activator. The type (fly or bottom ash) and content of the ash were the variables in the synthesis. The ashes were characterized by X-ray fluorescence (XRF), X-ray diffraction (DRX), particle size distribution (PSD), specific surface area (BET), and thermal analysis (DTA/TGA). The ash-based geopolymer samples were measured to obtain their compressive strength after curing. The evolution of the geopolymerization process was also assessed based on final alkali concentration measurements. The results show that it is possible to obtain geopolymers using coal ashes as raw materials with high solid content. The compressive strength for the bottom ash geopolymer after 90 days of curing is 35 MPa. The low concentration of unreacted alkalis after curing (1.5× 10-3 – 3.5 × 10-3 M) corresponds to high efficiency of the geopolymerization reaction.

Keywords

• Coal ash
• Geopolymer
• Industrial by-product
• Geopolymerization reaction