Question

In: Civil Engineering

The composition of "ordinary" Portland cement is 63.82% CaO, 20.56% SiO 2, 5.50%Al 2O 3, 0.76%K...

The composition of "ordinary" Portland cement is 63.82% CaO, 20.56% SiO 2, 5.50%Al 2O 3, 0.76%K 2O, 0.32% Na 2O, 0.95% other, and 0.52% insoluble. When mixed with water the resulting cement slurry is basic. Identify three bases of Portland cement and the compounds they form when mixed with water.

Solutions

Expert Solution

Compound

Formula

Shorthand form
Calcium oxide (lime)

Ca0

C
Silicon dioxide (silica)

SiO2

S
Aluminum oxide (alumina)

Al2O3

A
Potassium

K

K
Sodium dioxide

Na2O

N

Chemical composition of clinker

Compound

Formula

Shorthand form
Tricalcium aluminate Ca3Al2O6 C3A
Belite or dicalcium silicate Ca2SiO5 C2S
Alite or tricalcium silicate Ca3SiO4 C3S
Sodium oxide Na2O N
Potassium oxide K2O K
Gypsum CaSO4.2H2O CSH2

Bases of portland cement

Tricalcium silicate (CaO)3 · SiO2 C3S
Dicalcium silicate (CaO)2 · SiO2 C2S
Tricalcium aluminate (CaO)3 · Al2O3 C3A

Chemical reactions during hydration
When water is added to cement, the following series of reactions occur:

  • The tricalcium aluminate reacts with the gypsum in the presence of water to produce ettringite and heat:
    • Tricalcium aluminate + gypsum + water ® ettringite + heat
      C3A + 3CSH2 + 26H =C6AS3H32, D H = 207 cal/g
  • Ettringite consists of long crystals that are only stable in a solution with gypsum. The compound does not contribute to the strength of the cement glue.
  • The tricalcium silicate is hydrated to produce calcium silicate hydrates, lime and heat:
    • Tricalcium silicate + water = calcium silicate hydrate + lime + heat
      2C3S + 6H =C3S2H3 + 3CH,
  • The CSH has networked fiber structure which gives the initial strength to the cement glue.
  • Once all the gypsum is used up as per reaction (i), the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals:
    • Tricalcium aluminate + ettringite + water = monosulfate aluminate hydrate
      2C3A + 3 C6AS3H32 + 22H = 3C4ASH18,
  • The dicalcium silicate hydrates to form calcium silicate hydrates and heat:
    • Dicalcium silicates + water =calcium silicate hydrate + lime
      C2S + 4H = C3S2H3 + CH,

Ally Wells answered 1 year ago
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