Relative importance of deleterious reactions in concrete: formation of AAR products and secondary ettringi te

Aggregates, varying in alkali reactivity, were used for making mortar bars and concrete prisms, employing the same Portland cement. Measurement of length changes of the specimens was carried out under various storage conditions and for different lengths of time. Scanning electron microscopy of the specimens and energy-dispersive X-ray analysis of the products formed in them has shown that alkali-aggregate reaction (AAR) products develop first and much faster than ‘secondary’ ettringite (mat-form, filling cracks) associated with some deteriorated steam-cured concrete elements. The expansion and cracking due to AAR was more rapid and extensive at higher temperature (80°C) than at lower temperature (40°C). Mat-forms of ettringite developed only after prolonged storage of the specimens in a fogroom, and the amount was larger in the specimens initially treated at 80°C, mainly because more AAR expansion and cracking occurred at the higher temperature, but also because the higher initial temperature caused larger formation of secondary ettringite. It is concluded that AAR products were primarily responsible for the expansion and cracking that occurred in the specimens containing reactive aggregates. The formation of ettringite was not involved in this deterioration, but it may have had a secondary role at later ages. The deterioration of some steam-cured concrete railway sleepers may be explained based on these results.