Effect of Dry Condition and Temperature History on Crystal Structure of Carbonate Ettringite

Abstract

Sulfate attack is known as one of the deterioration phenomena of concrete structure. Especially, expansion due to delayed ettringite formation (DEF) has been serious problem nowadays. According to previous researches, continuous water supply and temperature history above 70°C are needed for DEF expansion. Moreover, recent research indicated possibility that CO₃^2- promoted DEF expansion. Therefore, the purpose of this study is to discuss that behavior of water molecules and structural changes of carbonate ettringite which is substituted SO₄^2- for CO₃^2- in a possible relationship to the mechanism of DEF expansion in terms of dehydration and rehydration of ettringite.From the experimental result, carbonate ettringite was amorphous on XRD by thermal dehydration and then amorphous calcium carbonate was produced. After rehydration, sample was recrystallized and calcite, isolated from ettringite structure, was produced. In addition, from ²⁷Al NMR spectrum and XRD result, recrystallized carbonate ettringite was coexistent calcite and amorphous carbonate ettringite. Furthermore, in water adsorption and desorption isotherm, amount of adsorption of carbonate ettirngite was much larger than normal ettringite and dehydrated carbonate ettringite had significant hysteresis. Therefore, there are possibility that structure in nanometer scale of carbonate ettringite changed by heating, and thermal dehydrated carbonate ettringite can hold a large volume of water in crystal structure.