Proceedings of International Structural Engineering and Construction, 9(1), 2022 State-of-the-art Materials and Techniques in Structural Engineering and Construction Edited by Holschemacher, K., Quapp, U., Singh, A., and Yazdani, S. Copyright © 2022 ISEC Press ISSN: 2644-108X www.doi.org/10.14455/ISEC.2022.9(1).AAC-10 AAC-10-1 THE EFFECT OF RE-CALCINATION FLY ASH ON GEOPOLYMER PERFORMANCE M SHAHIB AL BARI 1 , JANUARTI JAYA EKAPUTRI 1 , RASGIANTI 2 , BERNARDED OKA ANGGARANI 2 , and PRASETYO ADI WIBOWO 2 1 Dept of Civil Engineering, Institut Teknologi Sepuluh Nopember, Surabaya, Indonesia 2 PT PLN (Persero) Research Institute, South Jakarta, Indonesia 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. Keywords: Cementitious materials, Alkali activated materials, Si/Al, Soluble silica. 1 INTRODUCTION Fly ash is one of coal combustion residues that has been used in many sectors, mainly in construction and materials. However, only low loss of ignition (LOI) fly ash is accepted by cement producers following ASTM C618-19. LOI is the sum of burnable organic components, which contains mainly carbon. However, the LOI does not provide the form and properties of carbon which remained mystery on its effect on concrete (Freeman et al. 1997). Carbon has three effects on concrete: discoloration, lowering air entrainment behavior, and corrosion resistance (Freeman et al. 1997, Ha et al. 2005). High carbon of fly ash tends to have black color which is not preferable. Meanwhile, higher carbon content adsorbs more water and chemical content, such as superplasticizer and air entrainment agent (AEA) which decreases the workability and reduce freeze-thaw resistance (Freeman et al. 1997, Pedersen et al. 2008). The corrosion rate of rebar also increases when the LOI in fly ash is more than 6% (Ha et al. 2005). It is important to understand the effect of LOI and its removal using sintering method. LOI affects not only on Portland concrete, but also on geopolymer. High LOI in fly ash lowers compressive strength of geopolymer (Nurwidayati et al. 2016). To reduce LOI content, Temuujin and Riessen (2009) burned fly ash at 500 o C and 800 o C. It was found that the geopolymer performance was reduced which was caused by change of amorphous phase and crystallization on the surface of particles. This paper discusses the effect of calcination at 1000 o C on five high-carbon fly ashes from circulating fludizied bed combustion (CFBC) boilers which their steam temperature is under 850 o C. Due to the low combustion temperature, CFBC fly ash has low reactivity and irregular shape