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THE EFFECT OF RE-CALCINATION FLY ASH ON GEOPOLYMER PERFORMANCE

Geopolymer is one of the concrete technologies to utilize fly ash from coal combustion residue. However, the unburnt carbon in fly ash deteriorates the mechanical properties. To reduce the carbon, the calcination is required. This study presents the effect of calcination at 1000 o C on five samples of fly ash. All fly ashes were activated with alkali activator having a mass ratio of Na2SiO3 to NaOH was kept constant at 2. Compression test was conducted at 3, 7, 14, 21, 28, and 56 days to obtain the effect of calcination to the compressive strength. It was found that the Si/Al ratio significantly affected the compression strength. The calcination reduced the carbon content in fly ash. However, amorphous phase of fly ash dropped after calcination as all the compression test result were less than 1 MPa. the calcination hindered the geopolymerization as all of the setting time exceeded seven days causing no compressive strength at early age.