Modified Time of Setting Test for Fly Ash Paste and Fly Ash–Soil Mixtures

A survey of current literature has revealed that the testing protocol for determining the time of setting of fly ash–soil mixtures is not available. Although the current ASTM standard recommends using the Vicat needle method with a water-to-fly ash weight ratio of 0.35, it was originally proposed for testing the time of setting of cement. The current study showed that the Vicat needle method yielded poor test results for some common fly ashes. Moreover, the water-to-fly ash weight ratio of 0.35 may not be appropriate for some fly ashes because the chemical components are varied in different fly ashes. Therefore, a British fall cone was adopted to determine the time of setting of fly ash paste and fly ash–soil mixtures. It suggests that the best starting water-to-fly ash weight ratio should be at its liquid limit. As the water content increases, the initial and final times of setting of fly ash paste increase accordingly. For the fly ash–soil mixture, the initial and final times of setting decrease as the ash-to-soil weight ratio increases. Finally, a linear correlation was established based upon the water content, ash-to-soil weight ratio, and the ratio of calcium oxide to silicon dioxide present in the fly ash.