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

A pore solution study was undertaken to provide additional understanding of the hydration behavior of high-calcium fly ash in fly-ashwater pastes. Thr ee sources of fly ash were selected from a previous study on the basis of t heir hydration behavior, particularly their ability to form ettringite. Pas tes were made from each fly ash and water, and pore solutions were expresse d at varying times and were analyzed for the following ions: SO42-, Ca2+, A l3+, Mg+, K+, Na+, Si4+, Fe3+, and OH-. The mineralogical compositions of t he paste solids after expression of pore solutions were examined by X-ray d iffraction 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, stratlingite. Variations in th e chemical composition of the pore solutions helped to explain differences in the relative amounts of ettringite, monosulfate and stratlingite 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 indicati ng that sulfur was more limited in this fly ash. The fly ash paste that for med the least ettringite and the most stratlingite had the lowest solution concentration of sulfur and the highest of aluminum. Stratlingite formation continued to increase with curing in all three fly-ash-water pastes sugges ting that dissolution of the fly glass during pozzolanic reactions was rele asing calcium, silicon, and aluminum ions. Sulfate concentrations in pore solutions from all three of the fly ash past es rose significantly between 7 and 90 days, but data on pore solutions onl y 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 t wo Ay ash pastes had not. However, it is likely that this increase in sulfa te concentrations was a short-term phenomenon. Fly ash pastes studied previ ously have not been observed to form significant amounts of ettringite or a ny other crystalline sulfur-containing phase, after 28 days. The likely sou rce of the sulfur between 7 and 90 days is the fly ash glass. (C) 2001 Else vier Science Ltd. All rights reserved.