Abstract
Chemical properties of smectites in concrete aggregates and its relation to the evaluation method of aggregate have been studied. Commonly many kind of volcanic rocks are utilized for concrete aggregate. Andesites collected from Kagawa and Saga Prefecture were investigated by polarizing microscope, electron microscope, XRD, DTA, EPMA and chemical analysis. It has been clear that plagioclase phenocrysts in andesites were often altered to smectite which was identified as beidellite by Greene-Kelly test. Alkali reactivity of aggregates was evaluated based on the correlation between dissolved silica (Sc) and reduction in alkalinity (Rc) estimated by conventional chemical method (ASTM C 289). Interlayer calcium and magmesium in smectite were completely exchanged for sodium in the sodium hydroxyde solution during this chemical test. Rc increases linearly in course of reaction time from 12 to 48 hours. We can obtain Rc0 value by extrapolating the temporal change curve of Rc to reaction time “0 hour”. It was found that Rc0 value approximates to total amount of exchangeable calcium and magnesium in samples. The result reveals that Rc0 value does not participate in alkali silica reactions, but is caused by cation exchange reactions of smectite, and that Rc0 in addition to Sc and Rc is important index for the evaluation of aggregates.
