External Magnesium Sulfate Attack on Hydrated Portland-Limestone Cements: Effect of the Concurrent Presence of Sodium Chloride in the Corrosive Environment and Metakaolin Admixture in the Binder

Abstract

Magnesium sulfate attack on Portland-limestone cement pastes at low temperature was investigated considering the concurrent presence of sodium chloride in the corrosive solution. The common deterioration mechanism, involving leaching of portlandite and C─(A─)S─H phase and formation of crystalline phases and amorphous cross-linked aluminosilicate structures, was mitigated in the pastes made from ordinary or Portland-limestone cements, by increasing amounts of dissolved sodium chloride. Conversely, it was intensified in the blend with metakaolin by the additional decomposition of C─A─H phases with the formation of amorphous aluminate hydrate. The reduced cross-linking of the aluminosilicate structures is attributed to the structural association of sodium in the low-Ca/Si chains. C─A─H phase was consumed also in the heavily deteriorated cement paste with the highest limestone content. The mitigating effect of sodium chloride is proposed to be due to the reduced acidity of the corrosive environment and to the protective effect exerted by the sodium ions absorbed at the surface of aluminosilicate structures.

Keywords: Portland-limestone cement, Thaumasite sulfate attack, chlorides, Metakaolin, Solid-state NMR spectroscopy