What is Cement Hydration?
When cement, water, aggregate, and additives are mixed together, a significant heat increase occurs. This is due to the exothermic process in the reaction between cement and water (called hydration).
Measuring the concrete temperature over time enables you to know how far the concrete is in the hydration process (Concrete Maturity) and thereby also an estimated concrete strength. The hydration process is divided into five phases:
5 Phases of the hydration process
1. Phase: Initial mixing reaction
2. Phase: Dormancy
3. Phase: Strength acceleration
4. Phase: Speed reduction
5. Phase: Steady development
Phase 1: Initial Mixing Reaction
Initial after mixing the cement and water comes into contact with each other, a peak in temperature happens. The aluminate (C3A) reacts with H2O (Calcium and sulfate ions) to form ettringite (aluminate hydrate). The release of the energy from these reactions causes the initial peak.
Phase 2: Dormancy
A result of the reaction described in phase 1 is a surface coating of the cement particles. This coating keeps increases, but also slows down the reaction (hydration) as the access to H2O isn’t as good as when the concrete was mixed. The amount of hydrated concrete keeps increasing on a steady level while the surface of the concrete keeps fluid.
This is why this phase is used for transporting and pouring the concrete, as the concrete stays on a fluid level. The length of this period depends on each individual concrete mix and can, therefore, be optimized depending on the application like winter concreting, length of transport, etc.
This phase ends with an initial set of the concrete.
Phase 3: Strength Acceleration
A heat increase happens due to the reaction between calcium silicate (C3S and C2S) which creates the silicate hydrate CSH (heat increase also caused by other minor reactions). The creation of CSH also has a major impact on the concrete strength during this phase.
In the case of for example mass concrete application, it can be very important to monitor the internal temperature variances, as the concrete temperature during this phase can increase rapidly to reach internal temperatures like 70-80C (in some cases even higher). It is normally not recommended to exceed temperatures at around 70C.
Phase 4: Speed reduction
A maximum temperature has now been reached and the availability of free particles is now reduced and therefore slows down the temperature increase.
This phase often ends with the desired strength and the formwork around the concrete can now be removed. Monitoring of concrete maturity and temperature and therefore enable the user with the exact time where this is possible.
Phase 5: Steady Development / Post Formwork
The hydration process is now slowed down and will continue slowly to finish the remaining available cement and water particles. The formwork is now often removed and the concrete will now over time (can take a long time) finish the hydration process and reach final strengths (can take weeks or months).