A CASE STUDY OF ALKALI-SILICA REACTION IN CONCRETE (INVOLVING VERY REACTIVE SILICEOUS GLASS) USING PETROGRAPHIC TECHNIQUES

A CASE STUDY OF ALKALI-SILICA REACTION IN CONCRETE (INVOLVING VERY REACTIVE SILICEOUS GLASS) USING PETROGRAPHIC TECHNIQUES KEN SPRING, DIRECTOR AND SENIOR PETROLOGIST HEATHER SPRING AND THOMAS SPRING Geochempet Services, Unit 5/14 Redcliffe Gardens Drive, Clontarf, Queensland Phone: +617 32840020 Website: www.geochemept.com Email: [email protected] ABSTRACT The forms of potentially reactive silica most likely to result in a deleterious degree of alkali- silica reactivity (ASR) in concrete are finely microcrystalline quartz and moderately to heavily- strained quartz. Other forms of potentially more reactive silica of significance comprise opal, chalcedony, tridymite, cristobalite and siliceous glass. Determination of the abundance and form of the potentially reactive free silica is best achieved by petrographic examination, based mainly on microscopic analysis of the concrete. Free silica refers to silicon dioxide in a disordered state (such as glass) or within various crystalline minerals (of which quartz is only one example). It is different to combined silica which refers to silicon dioxide which is chemically combined with other elements or oxides to create chemically more complex silicate minerals (of which feldspar will serve as a common example). The total silica content of an aggregate or sand which can be determined by bulk chemical analysis (such as X-ray fluorescence - XRF) is not an effective guide to its potential for deleterious alkali-silica reactivity: it reports the total of combined silica and free silica. Determination of the abundance and form of the free silica is best achieved by petrographic examination, but supplemented where necessary by X-ray diffraction (XRD) or electron microscopy methods such as SEM/EDS. The most reactive forms of silica are richly siliceous opal, chalcedony and glass. The substances are essentially disordered, non-crystalline to cryptocrystalline (individual crystals not resolvable by optical microscopy) and/or unstable minerals. Natural glass in rocks carries variable amounts of other oxides in addition to silica. The glass is generally produced by quenching of volcanic igneous or tuffaceous rocks. Although all types of glass in rocks carry silica in a non-combined or "free" condition, it seems that only those compositions which would have been capable of crystallizing free silica minerals (if not quenched) give rise to deleterious amounts of silica in glass for reaction with alkalis in concrete. Thus, glass of acid and intermediate igneous compositions is usually deleterious and amounts of richly siliceous glass as low as several percent can be quite deleterious within aggregate or sand. A case study of concrete which contained acid and/or intermediate lavas and tuffs of Tertiary or younger age is presented. Very ASR reactive rock types (broken fragments of pumice and glassy acid and intermediate volcanic rocks) were identified in the petrographic examination of the supplied samples including liberated high temperature quartz phenocrysts (tridymite and

A CASE STUDY OF ALKALI-SILICA REACTION IN CONCRETE (INVOLVING VERY REACTIVE SILICEOUS GLASS) USING PETROGRAPHIC TECHNIQUES

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