Understanding the sulfate attack of Portland cement–based materials exposed to applied electric fields: Mineralogical alteration and migration behavior of ionic species

Abstract

The magnesium and sodium sulfate attacks on Portland cement paste in the presence of applied electric fields were studied, and the mineralogical alterations were investigated by both experiments and thermodynamic modeling. When an electric current flows out of the cement paste, the electric migration of ions induced sulfate ingress and decalcification. Compared with the specimen exposed to Na2SO4, that exposed to MgSO4 for 28 d proceeded to a later degradation stage, which is characterized by the decomposition of ettringite, portlandite, and AFm phases, and the formation of CaSO4. Thermodynamic modeling indicates a neutralization process induced by the electric migration of OH−, which is potentially responsible for the decomposition of ettringite. When an electric current flows into the cement paste, the Mg2+ and Na+ showed different migration behavior. Mg2+ was incorporated to form brucite and M-S-H–like products in a shallow area (~100 μm) on the surface of the specimen, whilst a part of the Na+ could be bonded to form Na-rich silica gel with the other part penetrating through the specimen. By coupling the pore solution chemistry obtained from thermodynamic modeling with the Nernst-Planck equation, the migration behaviors of the ionic species (SO42-, Mg2+, and Na+) were analyzed.