A few hours after the concrete components are mixed, concrete components form a solid structure that loses its plastic properties. The reason for this is the chemical reaction between cement and water known as hydration. The cement paste which was initially soft plastic, becomes less plastic as time progresses and solidifies and hardens. The chemical reactions between the water and cement in the cement paste continue as long as the temperature and humidity conditions are appropriate, and strength continues to rise.
What Oxides Create Portland Cement?
Portland cement is produced by firing limestone and clay at high temperatures and grinding the clinker formed as a result of this process together with gypsum.
Figure 1. Cement Production
Before talking about hydration processes, it is necessary to know the main oxides that constitute the main components of cement, which play a leading role in these processes.
The oxides in Portland cement are CaO obtained from the limestone in the raw material mix, while SiO2 and Al2O3 come from the clay. The raw material also contains some Fe2O3. These four oxides are the primary oxides that make up the main cement components. The oxides found in Portland cement and the approximate amounts are provided in Table 1.
Table 1. The Oxides in Portland Cement and Amounts
All of the oxides in the table except for SO3 are what constitute the cement clinker. The SO3 found in the cement comes from the gypsum used to produce it.
The MgO and alkali elements in the cement have no beneficial function. These are oxides found in the raw materials that are used. If there are too many of these oxides, they have harmful effects as they cause expansion in the concrete.
What are the Primary Components of Portland Cement?
The four main component groups formed in clinker as a result of heating (Table 2) directly affect the hydration reaction process. The chemical properties and effects of these four main component groups are different.
Table 2. Main Components of Cement
Silicates (C3S and C2S) are formed as a result of the combination of SiO2 and CaO oxides. Silicates make up approximately 80% of clinker.
C3S: Constitutes approximately 55-65% of cement by volume. It affects early strength.
C2S: Constitutes approximately 15-20% of cement by volume. It affects late strength.
Aluminates account for approximately 20% of cement. Alumina combines with CaO to form C3A and with iron oxide to form C4AF.
C3A: This is the most important phase affecting setting time. During the reaction, a high amount of heat is released.
C4AF: It affects the color of clinker and consequently of the cement.
As soon as cement and water come together, water reacts separately with each cement component. The reaction rate of each main component, the heat released during the reaction and the contribution of the product formed as a result of the reaction to the strength of the cement paste are different.
The increased strength achieved as a result of the reaction between water and the primary components in cement is provided in Figure 2. The final strength that cement paste achieves is formed as a result of the hydration of calcium silicate components like C3S and C2S. As can be seen in the figure, the contribution of C2S to the strength of cement paste is relatively low in the first few days but is greater in the later stages. The contribution of C3A to the strength of cement paste is low both early and later on.
Figure 2. How the Primary Components in Cement Paste Affect Strength
Hydration is a reaction that produces heat (exothermic). Each of the primary components have a different hydration heat (Table 3).
Table 3. Properties of the Primary Components of Cement
How much hydration heat will be released by a cement depends on the amount of a particular component in the cement paste and how long hydration continues. The average and peak hydration heat values for Portland cement are approximately 100 cal/g and 120 cal/g.
How Does Hydration Take Place in Portland Cement?
Figure 3. Main Components of Cement
When either C3S or C2S are combined with water, the type of hydration products that result are the same, namely C3S2H3 (calcium-silica-hydrate) and CH (calcium hydroxide), but the amounts are different.
The hydration product that emerges as calcium-silica-hydrate is referred to only as C-S-H. The strength gained by the cement paste depends on the amount of C-S-H gels that forms as a result of the hydration of the main constituents C3S and C2S. All of the C-S-H gels are not created at the same time. As hydration continues, the formation of C-S-H gels also continues, so that the cement paste continues to get stronger.
The reaction between water and C3S:
2C3S + 6H → C3S2H3 + 3CH
The reaction between water and C2S:
2C2S + 4H → C3S2H3 + CH
In order to prevent the cement paste from setting suddenly, some gypsum (3-6%) is added to the clinker during the manufacturing process and the two substances are milled together. The sulfate from the gypsum combines with C3A and precipitates on the C3A molecules again, preventing further reaction.
The sulfate content is optimized very precisely by the manufacturer. Sulfate that penetrates concrete from the outside causes expansion and cracks by forming ettringite crystals. Therefore, the C3A content of cement that is resistant to sulfate is kept beneath a certain level (less than 5%).
Image 1. Çimsa Sulfate-Resistant Cement
What are the Hydration Products of Portland Cement?
- Calcium Silicate Hydrate (C-S-H):
Calcium silicate hydrate (C-S-H) comprises approximately 60% of the hydration products. It is the produce that gives cement its binding properties and causes it to gain strength. It is impermeable and creates the concrete matrix by binding to the aggregate.
- Calcium Hydroxide (CH):
CH (calcium hydroxide) is a product that has no effect on strength, but provides the basic structure in concrete. Due to its weak structure and adherence-weakening effect, more than necessary is an undesirable product. CH usually accumulates in voids and the interface between the cement paste and the aggregate. It constitutes approximately 20% of the hydration products. It dissolves in water and is one of the reasons that concrete is porous. The CH content of concretes made with mineral admixtures is much lower.
Figure 4. The CH crystals and C-S-H gels in cement paste.
- Ettringite The formation of ettringite affects the setting of concrete.
Figure 5. Ettringite formed in cement paste
- Monosulfate: Has a negative effect on durability.
Why You Should Pay Attention to Cement Hydration?
As soon as cement and water are combined to make concrete, there is a different reaction between water and each component. As a result of hydration, different hydration products are formed by each primary component. The cement paste hardens (sets) as a result of these reactions and gains strength. The properties of cement paste vary and are determined by the hydration products that form as a result of chemical reactions.