Influence of CFBC Fly Ash Chemical Composition and Mechanical Activation on the Properties of Geopolymer

Circulating fluidized bed combustion (CFBC) fly ash has the potential as a precursor to making geopolymer concrete because of its rich silica and alumina content. However, there is a problem in utilizing CFBC fly ash caused by its chemical and physical properties that differ from the widely used pulverized coal combustion (PCC) fly ash. CFBC fly ash has a higher water requirement than PCC fly ash due to its angular particle shape, and higher sulfur and lime contained also caused a different reaction in the geopolymer system. Mechanical activation by milling the CFBC fly ash could decrease the water requirement in the mixture and make good quality CFBC fly sh-based geopolymer concrete. Three CFBC fly ash samples from different power plants in Indonesia with different chemical compositions were used in this research. The first had a low lime and no sulfur content, the second had high lime and no sulfur content, and the third had high lime and sulfur content. The milling process using a ball mill for two hours decreased the water requirement, as shown in the lower normal consistency of the fly ash. The reactivity also was increased, shown by the faster initial setting time. Besides its higher reactivity, lower water requirement increased the compressive strength of geopolymer mortar produced. This study also showed that the existence of calcium and sulfur content in the CFBC fly ash could cause unexpected results shown by the change of initial setting time, water requirement, and compressive strength.