AbstractPulverised fuel ash (pfa) has been mixed with lime and water, pressed into cylinders, and cured in 100% r.h. at temperatures of 50°C, 75°C and 95°C for times of between 30 minutes and 28 days.
The expansion of the cylinders and their unconfined compressive strengths have been measured at different stages during the curing cycle and the lime consumption has been determined at each stage using thermogravimetric analysis. The raw materials and the reaction products formed during curing have been analysed by X-ray diffraction analysis (XRD), thermogravimetric analysis (TG and DTG), scanning and transmission electron microscopy (SEM and TEM) and energy dispersive X-ray analysis (EDAX). The two latter techniques have also been employed to follow the development of microstructure during curing. In addition mercury intrusion porosimetry (MIP) has been used to determine the corresponding changes in pore size distribution.
When moistened with water the pfa was found to contain 1.34% of gypsum. In the dry pfa this gypsum is present on the pfa particle surfaces as anhydrous calcium sulphate, and it was found possible to remove this surface layer by acid treatment. The presence of this gypsum has been shown to be a critical factor in controlling the reaction rate, influencing the reaction mechanism and hence affecting the strength development and expansion behaviour of the cured pfa-lime composites.
Removal of the calcium sulphate coating from the original pfa by acid treatment, before mixing with lime, substantially reduces the expansion during curing. Also, the strength development begins at earlier curing times, although the ultimate strength developed is substantially below that for the untreated pfa.
The addition of small amounts of gypsum to the pfa-lime mixes show that there is an optimum gypsum level required to give maximum strength and maximum expansion during curing.
The behaviour is explained in relation to the following observed sequence of events :
Surface ettringite formation on the pfa particles; initial formation of gel coating; ettringite decomposition and release of sulphate; hydrogarnet formation and colloidal membrane development; membrane bursting and precipitation of C-A-S-S -H gel.
Two factors are found to contribute to the expansion. One is osmotic swelling of the colloidal membrane and the second is the level of sulphate incorporated into the precipitating C-A-S- S-H gel. Excessive addition of gypsum to the system retards the reaction with lime and results in the formation of excessive amounts of globular colloidal material being formed at the expense of the fibrous and foil-like C-A-S- -H gel which is primarily responsible for strength development. Also formation of excessive amounts of globular colloidal material leads to dimensional instability and a large drying shrinkage. The work shows that the production of a strong and dimensionally stable product by this process, requires very precise control of the initial composition.
|Date of Award||Apr 1990|