This paper discusses the possible mechanismsby which organic compounds influence the setting properties of portland cement. From a consideration of the hydration mechanisms of tricalcium silicate and tricalcium aluminate, it is concluded that organic compounds influence the hydration of these compounds by their effect on the hydration products through complexing and nucleation processes, It is hypothesized that the hydration of tricalcium silicate is controlled by the nucleation and growth of calcium hydroxide. The poisoning of nuclei by soluble silica is responsible for the induction period. Adsorption of organic compounds onto clcium hydroxide nuclei, through a chelation process, inhibits crystal growth even more effectively and prolongs the induction period. Tricalcium aluminate is considered to hydrate rapidly because the protective hydrate layer is destroyed by conversion to the hydrate of triclcium aluminate. Organic retarders enter the interlayer region of the hexagonal hydrates to inhibit this conversion reaction and therefore maintain the integrity of the hydration barrier. A similar mechanism may hold in the presence of sulfate ions. In portland cement, adsorption of admixtures by the hydration products of tricalcium aluminate controls the supply of admixture to tricalcium silicate. This explains the enhanced retardation when the addition of admixtures is delayed after the first addition of water.

Media Info

  • Media Type: Print
  • Features: Figures; References; Tables;
  • Pagination: pp 1-9
  • Monograph Title: Concrete admixtures
  • Serial:

Subject/Index Terms

Filing Info

  • Accession Number: 00135022
  • Record Type: Publication
  • ISBN: 0309024749
  • Files: TRIS, TRB
  • Created Date: Jul 13 1976 12:00AM