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The retarding action of gypsum works by depositing on the tricalcium aluminate a temporary layer of

  • A.unreacted dicalcium silicate
  • B.calcium sulphoaluminate
  • C.calcium silicate hydrate
  • D.free calcium hydroxide

Answer: B

The sulphate reacts at the aluminate surface and the coating slows further access of water for long enough to place the concrete. The same compound formed later, from sulphate entering hardened concrete, is expansive and destructive.

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