The Low Concentration of Sodium Hydroxide Influence on the Compressive Strength of Fly Ash/Natural Kaolin-Based Geopolymer

Fly ash/natural kaolin-based geopolymer with a low concentration of NaOH has been studied. The fly ash was taken from coal combustion waste of Asam-asam power plant, meanwhile the kaolin clay was taken from Tatakan, South Kalimantan, Indonesia. The kaolin powder was calcined at 750 °C for 3 hours to form pozzolanic metakaolin (Al₂O_3.2SiO₂). The silicon content from the X-ray fluorescence (XRF) test of the fly ash and the natural kaolin was respectively 44.58% and 72.98%. The aluminium weight percentage contained in the natural kaolin was roughly three times higher than that in the fly ash. Both starting materials together with the alkali solution (NaOH/Na₂SiO₃) were wisely mixed to produce geopolymers with Si/Al ratio of 4. The molarities of NaOH were controlled from 2M to 5M. The mixture was then heated at a curing temperature of 60 °C for 12 hours and dried at setting time of 14 to 28 days. The finest compressive strength, i.e. 91.6 MPa, was shown by the sample with 5M NaOH and 28 days setting time. This sample had a density of 2.04 g/ml and porosity of 5.88%. To the best of our knowledge, there is a prospective feature of using fly ash/natural kaolin-based geopolymer prepared by the low NaOH molarity in infrastructural industries.