Expansion mechanisms and cementation in cured PFA-lime composites Part 1: Development of mechanical strength and expansion

Pulverized fuel ash (PFA) was mixed with lime and water, pressed into cylinders, and cured in 100% relative humidity at temperatures of 50, 75 and 95°C for times of 30 min-28 days. The expansion of the cylinders and their unconfined compressive strengths were measured at various stages of the curing cycle; lime consumption was determined at each stage using thermogravimetric analysis. The raw materials and the reaction products formed during curing were analysed using X-ray diffraction analysis, thermogravimetric analysis scanning and transmission electron microscopy and energy dispersive X-ray analysis. The last two techniques were also employed to follow the development of microstructure during curing. When moistened with water, the PFA wasfound to contain 1·34wt% of gypsum. In the dry P FA this gypsum was present on the particle surfaces, mainly as anhydrous calcium sulphate, and it was possible to remove this surface layer by acid treatment. The presence of this surface layer has been shown to be a critical factor in controlling the reaction rate, irifluencing the reaction mechanism and hence affecting the strength development and expansion behaviour of the cured PFA-lime composites.