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Influence of pozzolana on C4AF hydratio n and the effects of chloride and sulfate io ns on the hydrates formed

Keywords: Aluminoferrite , Pozzolana , Chloride , Sulfate

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Abstract:

This study investigated the influence of natural pozzolana additive on the hydration of C4AF (aluminoferrite) and the effects of chloride and sulfate ions on the hydrates formed. In the samples, 25% (by weight) of the C4AF was replaced with pozzolana. The mixture was then hardened for 28 days in water, soaked in a saturated NaCl solution for 3 months, and then soaked in a 5% Na2SO4 solution for 3 months at 20°C. It is estimated that under normal conditions, pozzolana additive accelerates the formation of CO32-–AFm (monocarboaluminate) and gibbsite, however, impede the formation of cubic aluminum hydrates. Also, part of the amorphous SiO2 penetrates into the structure of hydrates of C4AF and initiates the formation of hydrated alumino-silicate (gismondine). Monocarboaluminate affected by NaCl becomes unstable and takes part in reactions producing Ca2Al(OH)6Cl·2H2O (hydrocalumite-M). After samples were transferred from a saturated NaCl solution to a 5% Na2SO4 solution, hydrocalumite-M was the source of aluminates for the formation of ettringite. In samples with pozzolana additive, the hydrated alumino-silicate and gibbsite compounds that were formed remained stable in an environment containing chloride and sulfate ions and retarded the corrosion reaction of C4AF hydrates.

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