An assessment of the effects of pozzolanic activity on the behaviour of fly ash

Abstract

This research used the ideas and methods, employed in previous studies of residual and weakly bonded soils, to help understand the shear strength and yielding characteristics of pozzolanically reacted fly ash. Samples of lagoon fly ash were supplemented by manufactured samples of fly ash/lime mortar. These were necessary to address the problem of wide variations and lack of observed bonding found in early tests on lagoon fly ash. The manufactured samples proved to be stronger and stiffer with a greater consistence between specimens, although their internal structure proved to be clumpy and the de-bonding method failed to remove all the bonding. The two forms of fly ash showed that the pozzolanic bonding was affected by confining pressure and axial strains. Under undrained conditions the build up of pore water pressures was shown to reduce the influence of pozzolanic bonding in stress space. The critical state line derived for the lagoon fly ash was linear to the extent of using conditions and also adequately described those of the mortar fly ash. As no one method for analysing the yielding characteristics could be found, a number of methods were cross-referenced in this research. Both forms of fly ash showed multiple yields that could be resolved to match the classic First and Second yield model. The loci of yields identified demonstrated the isotropic and anistropic nature of the mortar and lagoon fly ashes respectively. Despite the variations seen in the lagoon fly ash samples and the difference with the mortar samples the results point to common critical state parameters for unbonded fly ash and limiting condition for pozzolanic bonding within stress space.