Study of cement-fly ash paste exposed to sodium sulfate solutions with different concentrations at different temperatures

When concrete elements are partially exposed to sulfate environment, in the upper part of concrete elements above ground an aquiferous zone containing almost saturated and high pH value (> 12.5) sulfate pore solution will be formed. The concentration of sulfate solution is much higher than 5 %, as usually used in laboratory. It is necessary to study the performance of cement-fly ash paste in different high concentration sulfate solutions at different temperatures. In this paper, pure cement paste and cement - fly ash (25 % dosage) paste specimens were immersed in the 5 %, 15 %, 20 %, 30 %, 40 % and 50 % sodium solutions at 20°C, 30°C and 40°C respectively. After 1, 3 and 6 months immersion, the compressive strength of the specimens was measured. XRD and thermal analysis were employed to analyze the reactive products of the paste. The experimental results show that the reactive aluminum in fly ash is activated by high concentration sodium sulfate solution at different temperatures and more ettringite is generated than pure cement paste.

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