Beneficiated Fly Ash: Hydration, Microstructure, and Strength Development in Portland Cement Systems

A study of the effects of beneficiation on the performance and properties of fly ash from Lingan Generating Station, Nova Scotia, has been conducted in this paper. A pilot scale high-efficiency air classifier was used successfully to separate the fly ash into product grades with nominal sizes of -45µm, -10µm, and -45/ + 10µm. Overall recovery of the -45µm fraction from the raw ash was 66 percent. Beneficiation was found to generally improve the quality of the fly ash by increasing both the glass content and the proportion of spherical particles. The raw fly ash and the process fractions were all acceptable pozzolans complying with the requirements of CSA Standard A23.5-M1982. Beneficiated ashes showed improved pozzolanic activity, reduced water demand, and enhanced ability to reduce alkali-aggregate reactivity. In ASTM C 109 mortars, approximately twice the quantity of the -45aem fraction compared to the raw ash could be substituted for portland cement without loss of strength at 28d. This indicates that considerable cement savings would be realized from beneficiation of the ash. The -10aem fraction was an even more effective pozzolan with improved strength development and higher ultimate strength (110 percent of control). The hydration of control and portland cement-fly ash pastes (85:15 and 70:30) was investigated at w/c = 0.5 and curing ages from 2 hr to 28 days. In the early stages of hydration, fly ash substitution increased both the rate and extent of ettringite precipitation. At 28 days, the major ions in pore solutions were Na+, K+, and OH- with concentrations similar to the control. Mechanisms for the apparent net depletion of Na+ and K+ with increasing curing age are discussed in terms of a general theory of the chemistry of pore fluids. In general, the ash-containing systems showed less dissolved total alkali than the control. The pore structures of the portland cement-fly ash pastes were investigated by mercury intrusion porosimetry and their permeability to water at 500 psi. While permeability was not uniquely related to the pore structure parameters, it is clear that the fly ash particles in the pastes serve to close the pore structures in a way that generally restricts water intrusion. Whether this is attributable to reduced permeability of the matrix material (possibly as a result of improved nucleation and phase growth) or increased tortuosity remains to be established.