Chemical characterisation of the steel–geopolymeric gel interface

Geopolymers could be used as non-combustible adhesives for steel, so that it is important to understand the interaction of stainless steel and pure iron with geopolymeric gel. Different artificial gel forming solutions with varied concentrations of alkali and Si/Al ratio were used to investigate the effect of the Al–Si precursors on the interactions between the gel and substrates. The gelation process and the interactions at the gel/substrate interface were studied by Fourier transform infrared spectroscopy-specular reflectance (FTIR), X-ray photoelectron spectroscopy (XPS) and Auger spectroscopy (AES). It was found that the different gel forming solutions formed different structures, which reacted to yield aluminosilicate gels of different composition and structure. Artificial gel forming solutions with high Si/Al ratios formed structures in which the Si/Al ratio decreased with increasing polymerisation, whereas at low Si/Al ratios, more ordered structures with almost a constant Si/Al ratio were formed. The aluminosilicate gel growth was more rapid on an iron than a stainless steel substrate. At the interface, there was a transition zone in which the Si/Al ratio differed from the bulk gel. At the gel/metal interface, the Si/Al ratio was less than 1 for both iron and stainless steel substrates. FTIR and XPS results indicated that the chemical interactions between gel polymer and iron-based substrates were likely attributed to the formation of the Al–O–Fe bond. The gel has reacted with the oxide layer, forming the transition zone.