A SIMPLE TREATMENT TO DISTINGUISH ALKALI-SILICA GEL FROM DELAYED ETTRINGITE FORMATIONS IN CONCRETE

Alkali-silica gels or formations of late-developing ettringite (delaye d ettringite formation (DEF) that fill sockets around aggregate partic les may sometimes be observed by an optical microscope on unpolished o r polished sawn concrete surfaces only as a 'gel' or as dark circles a round the aggregate particles. They may thus be indistinguishable from each other. It has been found that the preparation of an unpolished o r polished sawn concrete surface by carbon deposition (for SEM work) a llows alkali-silica gel reaction and DEF to be distinguished by the na ked eye. However, X-ray elemental analysis is recommended to confirm t he visual observation.