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
- Tricalcium aluminate + gypsum + water ® ettringite +
heat
- 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,
- Tricalcium silicate + water = calcium silicate hydrate +
lime + heat
- 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,
- Tricalcium aluminate + ettringite + water = monosulfate
aluminate hydrate
- The dicalcium silicate hydrates to form calcium silicate
hydrates and heat:
- Dicalcium silicates + water =calcium silicate hydrate +
lime
C2S + 4H = C3S2H3 + CH,
- Dicalcium silicates + water =calcium silicate hydrate +
lime
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