Title: Microstructure of Concrete from Pavement Containing Alkali-Reactive Carbonate Aggregate: an SEM and Optical Microscope Study

Author(s): E. F. Duke, M. R. Hansen and D. P. Johnston

Publication: Symposium Paper

Volume: 170

Issue:

Appears on pages(s): 617-632

Keywords: Alkali carbonate reactions; aggregates; dolomite; microstructure.

DOI: 10.14359/6845

Date: 7/1/1997

Abstract:
Cores from five concrete pavement sites in eastern South Dakota, U.S.A., have been examined for microstructural and microchemical evidence of alkali-carbonate reactivity (ACR) using optical microscopy, scanning electron microscopy, and energy-dispersive X-ray microanalysis (EDX). Dolomitic aggregate used at each of the five sites shows evidence of reaction with the cement paste resulting in dedolomitization products and textures. In altered aggregate, dolomite (CaMg(CO3)2) is replaced by an irregular, fine-grained (l-10 um) intergrowth of crystals that have either Ca or Mg as the only peaks in their EDX spectra. These reaction products are interpreted to be calcite (CaC03) and brucite (Mg(OH)2), respectively, as would be expected from dedolomitization. Dedolomitization is most prevalent in fine aggregate, and is generally concentrated just inside the aggregate rim or along cracks penetrating into the particle. An unreacted rim of dolomite, 5-10 um thick, is present, however, even in the most altered particles. Unreacted dolomite and dedolomitized zones have nearly indistinguishable Ca/Mg, indicating that Ca and Mg are conserved in the reaction, and Si02+A1203 is typically < 1 % by mass, so it is unlikely that clay mineral impurities play a role in the deterioration of these aggregates.