Ceramics-Silikáty 50 (1) 1-4 (2006)

SULFURIC ACID ATTACK ON HARDENED PASTE OF GEOPOLYMER CEMENTS
Part 2. Corrosion mechanism at mild and relatively low concentrations

A. Allahverdi, F. Škvára

At mild concentrations of sulfuric acid (pH » 2), the first step of the total corrosion process, i.e. the ion exchange reaction between the charge compensating cations of the framework (Na⁺ and Ca²⁺) and H⁺ or H₃O⁺ ions from the solution along with an electrophilic attack by acid protons on polymeric Si-O-Al bonds resulting in the ejection of tetrahedral aluminium from the aluminosilicate framework, continues until it results in the formation of shrinkage cracks. When shrinkage cracks become wide enough, sulfate anions diffuse into the cracks, and react with the counter-diffusing calcium ions, resulting in the formation and deposition of gypsum crystals. At relatively low concentrations of sulfuric acid (pH » 3) and for limited periods of exposure time (» 90 days), the corrosion mechanism is exactly the same as that of pH 3 nitric acid, i.e. simply leaching of charge compensating cations and ejection of tetrahedral aluminum with no gypsum deposition.

Keywords: Geopolymer Cement, Sulfuric Acid Attack, Corrosion