The Effect of Composition of Calcined Clays and Fly Ash on Their Dissolution Behavior in Alkaline Medium and Compressive Strength of Mortars

Abstract

There is a wide scope for use of calcined clays in cement as kaolinitic clays are available in huge quantities around the world. Similarly, fly ashes are also available in substantial quantities, though the quality of fly ash is still a major concern. One is used in large quantities, and the other has the potential and the resources to be utilized. The difference in their physical and chemical properties and their behavior in alkaline medium are studied in this work. One class F fly ash and one calcined clay with 60% kaolinite are selected for this study. The materials are characterized using X-ray fluorescence (XRF), X-ray diffraction (XRD) and scanning electron microscopy (SEM) to find chemical composition, mineral composition and morphology. The difference in their solubility behavior in alkaline medium was determined using ICP-MS to find the dissolution of silica, alumina and calcium ions in NaOH solution at 20 °C. Though the hydration behavior of calcined clays and fly ash in cement systems are very distinct, their solubility behavior was found to be similar. Calcined clays exhibited higher dissolution of Si and Al ions in the solution compared to fly ash. The effect of dissolution of Si and Al on the compressive strength of mortars is also studied. Strength activity index of mortars increases quickly starting from day 1 for calcined clay and reaches a maximum at 7 days, while for fly ash it increases steadily and reaches a maximum at 28 days.