Preparation of metakaolin based geopolymer coatings on metal substrates as thermal barriers

Metakaolin based geopolymer coatings on both stainless and mild steel substrates have been prepared. Adhesion of the geopolymer coatings strongly depended on composition. After curing at 70 °C geopolymer with Si:Al = 1 crystallized to zeolites LTA and A, while for compositions of Si:Al = 2 and 2.5 the coating was amorphous. For compositions with Si:Al = 2.5 the adhesive strength was > 3.5 MPa while for Si:Al = 1 and 2 only weak adhesion to the metal substrates was achieved. Heating these geopolymer compositions to 1000 °C resulted in formation of sodium aluminosilicate or nepheline. Thermal expansion measurements of the Si:Al = 2.5 composition revealed expansion of up to 6% at 800 °C, while for Si:Al = 1 and Si:Al = 2 compositions up to 4% shrinkage was observed.