Rapid compressive strength achievement of coal bottom ash based geopolymer by employing microwave energy

H. Kim, S. Hong
University of Seoul,

Keywords: geopolymer, coal bottom ash, rapid compressive strength achievement, microwave


Geopolymer has been introduced as a green alternative to Portland cement to reduce CO2 from cement industry. Unlike coal fly ash, coal bottom ash has not been used widely as a source material for geopolymer because of its irregular large particle size with higher impurity components, even though it has rich alumina and silica contents for geopolymer synthesis. Hence, first we have prepared ground bottom ash whose mean particle size is 44.57 μm and 90 % of ground bottom ash is below 84.83 μm as a source material. To achieve high compressive strength within a short time, we have newly proposed the following procedure to make geopolymer specimen: (1) mix ground bottom ash with an adequate ratio of alkali activator (14 M NaOH solution) to reduce the effect of impurity segregation from the geopolymer matrix, and then mold it in 5 cm cube, (2) heat the demolded specimens in 75 ˚C drying oven for 12, 24 and 36 hours after sealing with plastic bag, and then (3) irradiate microwave under 700 W power for several minutes. It has been found the values of compressive strength were reached to above 40 MPa in couple-of-minute microwaved-specimens which were already cured in the oven for 36 hours. On the contrary, over-irradiation of microwave causes its strength to be deteriorated. Our experiment shows that within only two days, high compressive strength is achieved from our procedure employing microwave energy; otherwise it would not arrive in the coal bottom ash based geopolymer.