Geopolymer coating was prepared from metakaolin-based aluminosilicate for the formulation of spray. The effects of Si/Al ratios on the properties of the slurry were investigated using X-ray diffraction measurements (XRD), Fourier-transform infrared spectroscopy (FTIR), viscometry, and scanning electron microscopes (SEM). The results showed the geopolymer slurry with Si/Al molar ratios of 1:5, 1:6, and 1:7 had viscosity appropriate for spraying. All the geopolymer specimens were amorphous phase. The FTIR pattern showed the Si-O-Si bonds from geopolymerization in all geopolymer specimens. The spray slurry with a Si/Al molar ratio of 1:7 was found to be suitable for coating. The thickness of the coating layer (3-4 layers) was around 60-80 micrometers.