Characterization of pore solutions expressed from high-calcium fly-ash–water pastes

Abstract

A pore solution study was undertaken to provide additional understanding of the hydration behavior of high-calcium fly ash in fly-ash–water pastes. Three sources of fly ash were selected from a previous study on the basis of their hydration behavior, particularly their ability to form ettringite. Pastes were made from each fly ash and water, and pore solutions were expressed at varying times and were analyzed for the following ions: SO 4 2−, Ca 2+, Al 3+, Mg +, K +, Na +, Si 4+, Fe 3+, and OH −. The mineralogical compositions of the paste solids after expression of pore solutions were examined by X-ray diffraction analysis. The principal hydrated phases formed in the fly ash pastes were typical of those seen previously in hydrated high-calcium Class C fly ash, namely, the calcium aluminosilicate hydrate minerals, ettringite and monosulfate, and the calcium aluminosilicate hydrate mineral, strätlingite. Variations in the chemical composition of the pore solutions helped to explain differences in the relative amounts of ettringite, monosulfate and strätlingite formed in the pastes. Two of the fly ash samples had very similar bulk chemical compositions but contained slightly different amounts of soluble crystalline sulfur-bearing minerals. The fly ash that had the smaller amount of soluble sulfur-bearing minerals formed less ettringite in its paste and more monosulfate indicating that sulfur was more limited in this fly ash. The fly ash paste that formed the least ettringite and the most strätlingite had the lowest solution concentration of sulfur and the highest of aluminum. Strätlingite formation continued to increase with curing in all three fly-ash–water pastes suggesting that dissolution of the fly glass during pozzolanic reactions was releasing calcium, silicon, and aluminum ions. Sulfate concentrations in pore solutions from all three of the fly ash pastes rose significantly between 7 and 90 days, but data on pore solutions only extended to 90 days. At that time, the sulfate concentration in the pore solutions of one fly ash paste had decreased; concentrations in the other two fly ash pastes had not. However, it is likely that this increase in sulfate concentrations was a short-term phenomenon. Fly ash pastes studied previously have not been observed to form significant amounts of ettringite or any other crystalline sulfur-containing phase, after 28 days. The likely source of the sulfur between 7 and 90 days is the fly ash glass.