Hydration is a series of irreversible chemical reactions between cement and water. During hydration, cement-water paste sets and hardens. Hydration begins as soon as cement comes into contact with water. The cement particles slowly, partially dissolve, and the various dissolved components start to react at various rates.
During the reactions, heat is generated, and new compounds called hydration products are produced. The new compounds form a solid and cause the plastic cement paste to harden, bond to the aggregate in the concrete mixture, and become strong and dense.
The measurable heat generated by hydration does not reach zero for days because the reactions continue over time, slowing as time progresses. The reactions can continue for years, as long as the concrete contains water and unreacted cement, resulting in continued development of strength and other desirable characteristics such as low permeability.
Hydration stages was dealt with in extensive studies and continues to be a subject of interest.
A general understanding of hydration reactions can help prevent or correct concrete problems and ensure that the concrete mix performs as designed.
In our article, we will talk about 5 different hydration stages.
- Mixing
- Dormancy
- Hardening
- Cooling
- Densification
Figure 1. Stages of Hydration
Table 1. Concrete characteristics, and implications for workers, during stages of hydration
1. Mixing
During hydration, the reactions between silicates and water produce the primary compounds that make concrete strong and durable. However, silicates dissolve very slowly and do not have an effect on strength in the Mixing Stage.
When mixed with water, C3A and gypsum dissolve instantly. Within minutes of being mixed with water, solid compounds form from these materials, generating significant heat and beginning the process of hardening, or setting.
Unchecked, these reactions would cause irreversible, flash set or stiffening of the concrete. To control C3A reactions, the addition of gypsum is critical.
The fast-dissolving gypsum reacts with the dissolved C3A and water to create a substance that coats the cement grains (Figure 2). This coating is often referred to as a “gel,”. The gel coating slows the aluminate reactions almost as soon as they start, reducing the amount of heat generated and the potential for flash set.
Within minutes of mixing cement and water, the aluminates start to dissolve and react, with the following results:
• Aluminate reacts with water and sulfate, forming a gel-like material (C-A-S-H). This reaction releases heat.
• The C-A-S-H gel builds up around the grains, limiting water’s access to the grains and thus controlling the rate of aluminate reaction.
If insufficient sulfate is in solution for the amount of aluminate* (from cement and fly ash), uncontrolled aluminate hydration may cause rapid, permanent stiffening or flash set. This is characterized by a temperature rise.
Excess sulfate in solution results in gypsum crystals being deposited out, prematurely stiffening the system, resulting in (temporary) false set. The gypsum eventually dissolves as the mixture is mixed, which is why false set is temporary. There is no effect on other hydration Stages.
The amount of sulfate in solution is controlled by the amount/form of sulfate in the cement. Gypsum dissolves slowly (increasing the risk of flash set). Plaster dissolves rapidly (increasing the risk of false set). Cement manufacturers normally balance these materials.
Figure 2: Gel formation around cement particles
2. Dormancy
The reaction of aluminates in the dormant period are generally controlled between 2 to 4 hours. The reason for that is the gel-like layer. C-A-S-H (calcium-alumino-trisulpho-hydrate) gel, i.e., ettringite, controls aluminate reaction.
During this time, the concrete is plastic and does not generate heat. The dormant stage gives the construction crew time to transport, place, and finish the concrete while it is workable.
Ready concrete produced with Portland cement has approximately 2 hours of workability. The dormant period may be extended with chemical additives.
Image 1: Transporting the Concrete
Image 2: Placing the Concrete
Figure-1 As we see in hydration stages chart, heat release rate remains almost constant in this period. However, this does not mean that no reaction is taking place. On the contrary, cement components continue to dissolve. Silicates start to dissolve at a slow rate. As Figure-3 shows, mixture water starts to get saturated with calcium (Ca+) and hydroxyl (OH-) ions. When the water has an overabundance of calcium ions, known as supersaturation, the hardening stage begins.
Figure 3: Ions Dissolving in the Mixture Water
3. Hardening
When the water becomes supersaturated with dissolved calcium ions, hydration products begin forming, heat is generated, and the mixture begins stiffening. This is the beginning of the hardening stage (Figure 1).
The mixture water gets supersaturated with the dissolved Ca+ ions. New hydration products, in other words C-S-H (Calcium-silica-hydrate) and CH (calcium hydroxide) start to form and heat output increases as a result.
This is the start of hardening, i.e., setting. With the initial setting, vibrator application and surface finishing operations cannot be conducted on concrete anymore. Curing operations such as watering the concrete surface, applying curing additives, covering the surface with membrane should be initiated at this period.
Workers should apply curing compound as soon as possible after finishing to control water evaporation from the concrete surface. If the water is lost, the hydration reaction will not reach completion, reducing the strength and durability of the final concrete product. Likewise, if the heat from the hydration reactions is lost, the hydration reaction will be slowed or will cease. This requirement of maintaining water and heat within the concrete (i.e., curing) is absolutely critical to achieving high-quality concrete.
Image 3. Watering Concrete Surface, Curing Operations
The amount of newly formed products continuously increases in acceleration period. In parallel to this, heat generation also increases. These products bond with each other, cluster around aggregates and enfold them. Concrete hardens and starts to become solid as a result.
The dominant main component is C3S (tricalcium silicate) in this stage. Ca+ ions formed as a result of dissolution of C3S become supersaturated with the solution. As a result of reaction, fiber-like C-S-H (Calcium-silica-hydrate) and crystalline CH (calcium hydroxide) are formed. C-S-H (Calcium-silica-hydrate) creates a net with other compounds, and this enables solidifying and hardening of concrete (Figure-4).
Figure 4: C-S-H and CH formation
Increase in heat generation and hardening of cement paste are indicators of acceleration of hydration. Reaction occurs faster in cement with high levels of fineness. Heat output starts to decrease with the slowing down of C3S (tricalcium silicate) reaction. At the completion of setting, hydration heat approaches its peak point. The hardening stage range may vary depending on the environmental impacts. However, it is usually between 3-5 hours.
In this period, aluminate and sulphate reaction continues, acicular ettringite crystals continue to be formed.
4. Cooling
In the Cooling Stage, C3S (tricalcium silicate) reaction starts to slow down and heat output which reached its peak starts to decrease (Figure 1). The reason for this is that formed C-S-H (Calcium-silica-hydrate) and CH (calcium hydroxide) products prevent the contact between water and undissolved cement particles. Reactions occurring in this period are called topochemical reactions.
In this period, concrete’s strength gain increases as C-S-H (Calcium-silica-hydrate) and CH (calcium hydroxide) formation increases. Concrete has gained solidness but has not yet gained enough strength, in addition it is permeable. However, it can carry light loads. Stresses due to shrinkage should not exceed the gained strength. This would cause cracks. If cutting joint is not done in time, random cracks would form.
Image 4. Concrete Cutting Joint
5. Densification
In the Densification Stage, reactions slow down and heat output decreases significantly. This period goes on for a very long time if unhydrated cement particles and water are present. In other words, strength, and durability of concrete increase in a time period that can take years.
In the Densification Stage, reaction continues as long as C3S and water are present. As the volume of formed products increases, concrete continues to gain strength and concrete permeability decreases. As seen in Figure 5, the concrete increasingly gains a harder and more durable structure. As a result of decrease in concrete’s permeability, water and salt dissolved in water cannot enter into the concrete as easily. Negative impacts due to environmental impacts such as freeze-thaw decrease over time.
C2S enters into a reaction much slower compared to C3S. The majority of C3S would enter into a reaction in a few days. And C2S reaction becomes even more important in this period.
Figure 5. Change in cement paste during hydration stages
What Should You Know About Hydration?
In this article about hydration, we addressed;
- Chemical reactions of aluminates, sulphates, and silicates,
- Products of these reactions,
- Importance of the development of density of cement paste during hydration,
- Development of concrete strength during hydration,
- Construction applications of various hydration stages to obtain strong, durable concrete.
You can check out What Is Hydration? How Does Cement Hydration Occur? for more information.
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