Estimate of Potential Ettringite Formation in Concrete
| I. Physical Properties of Cementitious Materials Relevant to Ettringite Formation. | |||
|---|---|---|---|
| Phase | Molar Volume (cm3/mole) |
Molecular Weight (g/mole) |
Specific Gravity (g/cm3) |
| Ettringite | 735.0 | 1249.5 | 1.70 |
| C3A | 89.1 | 269.9 | 3.03 |
| C4AF | 128.0 | 477.4 | 3.73 |
| Gypsum | 74.2 | 172.1 | 2.32 |
To determine the potential volume fraction of ettringite in concrete, we find:
Assuming each mole of C3A or C4AF forms one mole of ettringite we can write:
Examples using median values of Iowa concrete cements
| US 20: | C3A= 0.0864, | C4AF 0.0541, | Air = 7 % |
| Cement content calculated at 0.2906 Mg cement/m 3 concrete | |||
| Substituting into equation 4, the potential ettringite volume fraction of 0.0926 m3 | |||
| ettringite / m3 concrete is greater than the 0.07 (7%) air void content | |||
| US 218: | C3A = 0.066, | C 4AF = 0.0541, | Air Content = 6.5 % |
| Cement Content = 0.3362 Mg cement / m 3 concrete | |||
| Potential ettringite volume fraction of 0.0957 m
3 ettringite / m3
concrete is greater than the .0065 (6.5%) air void content. | |||
The potential ettringite volumes exceed that of the air void system. This suggests that the entire entrained air void system could potentially be filled by ettringite (if sufficient sulfate is available) and so, the 2 % maximum volume filling observed in the cores does not necessarily indicate an external sulfate source.