Diagnosis of alkali-aggregate reaction—polarizing microscopy and SEM-EDS analysis
ABSTRACT: Petrographic diagnosis of alkali-aggregate reaction in concrete was introduced. Combined use of polarizing microscopy and SEM-EDS analysis on polished thin section well identified the loci of alkali-silica reaction (ASR) of reactive silica minerals (cristobalite, tridymite) and interstitial glass in the volcanic aggregates in concrete. Slowly cooled volcanic rocks oversaturated with silica contained silica minerals, nearly comparable with the calculated normative quartz in CIPW norm. Interstitial glass in volcanic rocks becomes enriched in SiO2 with diminishing content by cooling of magma, which suggests that reactive rhyolitic glass can be formed in andesite and even basalt oversaturated with silica. Fly ash glass in ASR-affected concrete was also analyzed. SEM-EDS analysis revealed compositional trend lines of ASR gel, which reflects position of gel, evolutionary stage and environments of ASR in concrete.