SULFURIC ACID ATTACK ON HARDENED PASTE OF GEOPOLYMER CEMENTS
Part I: Mechanism of corrosion at relatively high concentrations
ALI ALLAHVERDI, FRANTIŠEK ŠKVÁRA
Mechanism of corrosion of hardened paste of geopolymer cements at relatively high concentrations of sulfuric acid (pH » 1) consists of two subsequent steps. The first step starts by an ion exchange reaction between the charge compensating cations of the framework, i.e. sodium and calcium, and H+ or H3O+ ions from the solution along with an electrophilic attack by acid protons on polymeric Si-O-Al bonds. The electrophilic attack of acid protons results in the ejection of tetrahedral aluminum from the aluminosilicate framework. In the second step, the exchanged calcium ions diffusing toward the acid solution react with counter-diffusing sulfate anions resulting in the formation and deposition of gypsum crystals inside corroding layer. Deposition of gypsum crystals inside corroding matrix provides a protective effect inhibiting the total process of deterioration.