Comparative studies of three methods for activating rejected fly ash

Pulverised fuel ash is a by-product of burning coal during the generation of electricity. In many places, the collected pulverised fuel ash has to be processed through a classifying stage before it can be used as a construction material. The rejected portion of the classifying process has remained unused due to its high carbon content and large particle size (> 45 μm). However, the rejected ash may be used in certain areas, such as in solidification and stabilisation processes of hazardous waste, and materials for road base or sub-base construction, which require relatively lower strength and reactivity. This paper presents the results of a study on the pozzolanic properties of cementitious materials prepared by blending rejected fly ash (rFA) with calcium hydroxide. The effects of adding chemical activators, elevating curing temperature, and grinding of the ash particles on the activation of rFA were compared by means of compressive strength, differential scanning calorimetry-thermogravimetric analysis, X-ray diffraction analysis and mercury intrusion porosimetry tests. The results show that the addition of Na₂SO₄ can significantly accelerate the strength development both at 28 and 90 days. K₂SO₄ is only effective at 28 days. The addition of flue gas desulphurisation sludge (a low-grade gypsum) and grinding can only increase the strength at 90 days. Pretreatment by steam curing decreases the 90 day strength of the samples. The compressive strength results correlate nicely with the results of the porosity and the thermal analysis tests. Adding chemical activators seems to be an effective and economical method to accelerate the hydration of rFA.