This article reports the investigation on the effect of ladle furnace slag replacement on the flexural strength of thin fly ash-based geopolymer. The thin fly ash/slag geopolymers were prepared with the replacement of various percentages of ladle furnace slag (0%, 10%, 20%, 30% and 40%) into fly ash geopolymers with dimension of 160 mm × 40 mm × 10 mm. The thin geopolymerwas synthesised using 12M sodium hydroxide (NaOH) solution with solid-to-liquid (S/L) ratio of 2.5 and Na2SiO3 /NaOH ratio of 4.0. The curing temperature and time of samples were 60°C and 6 hours respectively. The mechanical properties of thin geopolymers was revealed using flexural test after 28 days. Several characterisation tools have been used including Scanning Electron Microscope (SEM) and X-Ray Diffraction (XRD) to correlate the flexural properties with the microstructure and phases of fly ash/slag geopolymers. Results obtained reported that a positive effect on flexural strength was observed with the increasing amount of slag. The thin fly ash geopolymers replaced with 40% of ladle furnace slag showed the highest flexural strength of 7.8 MPa. The rich CaO content in ladle furnace slag boosted the C-S-H gels formation which increased the flexural strength of thin geopolymers.
