Optimisation of Syntheses Conditions and Investigation of Durability and Transport Properties of High Temperature Performance Geopolymer Coating for Substrates

Abstract High temperature performance geopolymer coatings are widely used as passive fire protection systems, designed to delay structural failure under high temperatures. This study explores the development and performance of a high temperature resistance geopolymer coating focusing on its mechanical strength, durability, and transport properties. The high temperature performance was synthesized using metakaolin and silica fume as aluminosilicate precursors. Activation was achieved with a 7M sodium hydroxide (NaOH) solution and a potassium silicate solution with a molar ratio of 1.7. The precursor-to-activator ratio and NaOH-to-potassium ratio were kept constant at 2.8 and 0.48, respectively, across all formulations. Mixes with varying metakaolin-to-silica fume ratios (100:0, 90:10, 70:30, 50:50) were tested. The MK70SF30 mix demonstrated the highest compressive strength (25.2 MPa) but the lowest flexural strength (4.4 MPa). The optimized high temperature resistance geopolymer showed a strong stability at high temperature exposure, maintaining its structural integrity without cracking or disintegration up to 1050 °C. Thermal conductivity increased from 0.5 W/m•K at 20 °C to 1.3 W/m•K at 800 °C, while specific heat capacity rose from 368 J/kg•K to 925 J/kg•K. Above 800 °C, both properties decreased sharply. Thermal diffusivity declined from 5.99x10 −7 m 2 /s at 20 °C to 4.43x10 −7 m 2 /s at 500 °C, then increased by 28% up to 800°C. The material exhibited negative linear thermal expansion between 200°C and 900 °C, and a positive expansion of 2.6%at higher temperatures. Gas permeability tests of this material revealed an intrinsic permeability of 1.59 × 10 −17 ; m 2 and a Klinkenberg coefficient of 0.445 MPa and water permeability coefficient of saturated synthetic high performance geopolymer ranged from 2.42×10 −9 to 8.49×10 −9 m/s at constant confining pressure of 3 MPa. These findings underscore this material strong potential for high-temperature applications, offering robust resistance along with customisable mechanical and transport properties.