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Pozzolans, Heat of Hydration and Maturity: When Is Concrete Ready?
When can the formwork of a slab be struck? The answer depends not on the calendar but on the concrete's temperature history: in cold weather reaching the same strength can take twice as long. Pozzolans change the picture once more: they reduce heat and improve durability, but slow early strength.
1. Heat: a by-product of hydration
The reaction of cement with water is exothermic: heat is released during hydration. In a thin slab the heat dissipates quickly. But in mass concrete such as a thick foundation or a dam, the core heats up while the surface cools. The temperature difference between core and surface creates tension at the surface and can cause cracking while the concrete is still young and weak. In mass concrete the key design question is less strength than how to limit this temperature difference.
2. What pozzolans do
Pozzolans (fly ash, silica fume, natural pozzolans) do not bind on their own. They react with calcium hydroxide, a by-product of cement hydration, to form additional binding products (C-S-H). This has three consequences:
- Less heat, more slowly: the pozzolanic reaction is slower than cement hydration and spreads the heat over time — a major advantage for mass concrete.
- A denser structure: the additional binding products fill capillary pores; the chloride diffusion coefficient falls markedly (see MLZ-02).
- Slower early strength: a significant part of the strength is gained later, which directly affects striking and loading schedules.
3. Maturity: temperature × time
Hydration speeds up with temperature. The maturity method captures this in a single quantity. In the Nurse–Saul approach (ASTM C1074) the equivalent age at 20 °C is:
Concrete cured at 5 °C therefore matures at half the rate it would at 20 °C: (5 + 10) / (20 + 10) = 0.5. At 30 °C it matures 1.33 times faster.
4. How strength develops
EN 1992-1-1 gives the development of strength with time at 20 °C as (Eq. 3.2):
s depends on how fast the cement gains strength: rapid (class R) 0.20, normal (N) 0.25, slow (S) 0.38. With a normal cement, concrete reaches 70% of its 28-day strength at an equivalent age of 4.8 days; with a slow cement, at 7.5 days.
5. The cost of cold weather
Combining the maturity and strength relations, the time to reach 70% strength varies with curing temperature as follows:
| Curing temperature | Equivalent-age factor | Time to 70% strength, s = 0.25 (days) | Time to 70% strength, s = 0.38 (days) |
|---|---|---|---|
| 5 °C | 0.50 | 9.5 | 14.9 |
| 20 °C | 1.00 | 4.8 | 7.5 |
| 30 °C | 1.33 | 3.6 | 5.6 |
At 5 °C the time grows to 9.5 days with a normal cement and 14.9 days with a slow one. Striking formwork in winter "after a week, as in summer" may mean loading concrete before it has the required strength. The maturity chart in the gallery shows this calculation interactively.
6. Uncertainty and limits
- The parameters are representative. T0 and s are typical values; ASTM C1074 requires the maturity method to be calibrated in the laboratory for the specific mix before use on site.
- The temperature is the concrete's own. What enters the calculation is the concrete temperature, not the air temperature; in thick members it can be well above ambient because of the heat of hydration.
- Freezing is a separate risk. These relations assume the concrete does not freeze at early age; early freezing can leave permanent damage that no maturity can make up for.
All values in this article are checked by tests/v582-consolidation-maturity.test.js.
References
- ASTM C1074 — Standard Practice for Estimating Concrete Strength by the Maturity Method.
- EN 1992-1-1 — Clause 3.1.2(6), Eq. (3.2): βcc(t) and the s coefficients.
- Mehta, P. K. and Monteiro, P. J. M. (2014). Concrete: Microstructure, Properties, and Materials, 4th ed., Chapters 6 and 13 (mineral admixtures, mass concrete). McGraw-Hill.
The numbers in this article were computed with closed-form relations and are checked by the stated test.