Stability and decomposition mechanism of ettringite in presence of ammonium sulfate solution

The formation and transformation characteristics of ettringite play an extremely important effect on the accelerated hardening and expansion of concrete as well as its long term durability. In alkaline condition, ettringite can be existed stably; but when the alkalinity exceeds a certain value, ettringite can be decomposed into calcium monosulfoaluminate (AFm). In this paper, the stability and its decomposition mechanism of ettringite in alkaline (NH₄)₂SO₄ solution were then investigated. The results show that in alkaline environment, decomposition of ettringite can be occurred and main produces were gypsum and Al(OH)₃ as long as the concentration of (NH₄)₂SO₄ reached to a certain extent. Based on experiment data, a kind of NH₄⁺ involved decomposition mechanism of ettringite had been proposed by method of quantum mechanics combined with frontier molecular orbital theory and thermodynamic theory.