Sulfate attack on cement paste with volcanic ash : durability analysis

Abstract

Sulfate attack is responsible for deterioration of infrastructure and often occurs in sulfate rich soil or brackish environment. The aim of this study is to investigate the effect of volcanic ash cements when exposed to different forms of sulfate attack, specifically to sodium and magnesium sulfates. Pozzolanic volcanic ash can be a viable partial substitute for Portland Cements to develop cement paste compositions for superior sulfate resistance with potential for durability and sustainable solutions. Pumiceous volcanic ash was used in preparing Roman hydraulic pozzolan concrete that was used to build the Bay of Naples. This study reports the microstructural and mechanical characterization of cement paste with volcanic ash when exposed to accelerated sulfate attack via electrokinetics. The test specimens were exposed to sodium and magnesium sulfate solutions for a period of 30 days. The effect of gradual decomposition of calcium-silicate-hydrate (C-S-H) gel was examined using Raman spectroscopy and Magic Angle Nuclear Magnetic Resonance (MAS NMR), while the mechanical properties were determined using nanoindentation and compression tests. Exposure to magnesium sulfate solution led to formation of magnesium-silicate-hydrate (M-S-H) along with crystallization of gypsum and brucite, while exposure to sodium sulfates led to the formation of thenardite and mirabilite. An optimum mix combination of 10-30% partial substitution of volcanic ash was determined for optimal sulfate resistance and compressive strength. This mix combination was determined by considering the resulting mechanical, micro and pore structure characteristics of the hardened cement pastes.