The chemical reaction that occurs when cement is mixed with water is called the hydration of cement. This reaction is exothermic, meaning that heat is generated as a byproduct. The heat generated during this reaction is known as the heat of hydration. In the first 7 days after mixing, cement typically produces 89-90 calories per gram of heat, while in 28 days, it produces 90-100 calories per gram.
The process of cement hydration is not instantaneous, and it can take several years for the reaction to be completed fully. Cement consists of four main compounds, which are C3A, C4AF, C3S, and C2S, collectively known as Bogue’s compounds. These compounds participate in the chemical reaction during the hydration process. As the compounds hydrate, new products are formed, which ultimately lead to the setting and hardening of cement. This transformation results in the creation of a solid, durable mass.
Major Products of Hydration
1. Calcium Silicate Hydrate
When water is added to cement particles, a chemical reaction takes place between the cement particles and the water. This reaction involves the compounds C3S and C2S, which react with water to produce two important substances: calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2). The reaction is as follows: 2C3S + 6H → C3S2H3 + 3Ca(OH)2 and 2C2S + 4H → C3S2H3 + Ca(OH)2.
C-S-H, also known as Tobermorite gel, is the most important product of this chemical reaction. It is responsible for many of the desirable properties of concrete, such as its strength and durability. However, the term C-S-H is hyphenated because the ratio of CaO to SiO2 in its composition is not well-defined, and can vary from 50-60%.
Interestingly, it has been found that the hydration of C3S produces less C-S-H than Ca(OH)2 compared to the hydration of C2S. Additionally, the quality and density of C-S-H produced by C3S is inferior to that produced by C2S. Nevertheless, the production of C-S-H remains a crucial aspect of the chemical reaction between cement particles and water.
2. Calcium Hydroxide
Calcium hydroxide, or Ca(OH)2, is a byproduct of the hydration process of cement components C3S and C2S. It comprises 20 to 25% of the solidified volume of hydrated cement paste. While it plays a role in maintaining a pH value of 13 around reinforcement, which helps prevent corrosion, this is the only benefit of its presence in concrete.
On the other hand, the presence of Ca(OH)2 in concrete can cause a loss of durability due to three primary reasons. Firstly, Ca(OH)2 is highly soluble in water, and as a result, it can get leached out, leading to porosity in the concrete mass and resulting in lower strength and durability.
Secondly, Ca(OH)2 reacts with sulfates present in water or soil and forms CaSO4, which reacts with C3A to create higher volume products. These products can create cracks, leading to the deterioration of the concrete. This phenomenon is known as Sulphate Attack.
Finally, Ca(OH)2 reacts with CO2 present in the atmosphere, forming CaCO3. Initially, this reaction takes place on the surface of concrete, but it can penetrate into the mass over time. If the concrete is even slightly porous and reduces the pH value of the passive protective layer, it can make reinforcement susceptible to corrosion. This type of deterioration is known as carbonation of concrete.
While Ca(OH)2 can help protect reinforcement, its negative effects on concrete durability cannot be ignored.
3. Calcium Aluminate Hydrate
Cement contains two types of aluminates, namely C3A and C4AF, which undergo hydration and transform into Calcium Aluminate Hydrate. Among these hydrated compounds, C3AH6 and C3FH6 are considered to be relatively stable.
However, it is important to note that the presence of these hydration products does not contribute to the strength or any beneficial property of concrete. In fact, they can be detrimental to concrete, especially in situations where sulfate attack is likely to occur.
Therefore, despite being an inevitable byproduct of the hydration process, the formation of C3AH6 and C3FH6 should be minimized to prevent potential harm to the concrete. It is crucial to consider their presence in the design and construction of concrete structures, particularly in areas where sulfate exposure is expected.
4. Ettringite
Flash setting is a common issue that occurs during cement hydration due to the rapid reaction of C3A and C4AF. This can lead to the formation of cracks in concrete, which is undesirable. To address this problem, gypsum (CaSO4) is added during the manufacturing of cement. The addition of gypsum decreases the solubility of C3A and C4AF, thereby controlling the flash setting behavior. When gypsum is added, the sulfate reacts with C3A and C4AF, resulting in the formation of calcium aluminate sulfate, also known as ettringite. It is important to note that this compound is formed prior to the hardening of concrete, which means that it does not cause any negative effects on the material at this stage.
However, if ettringite is formed after the concrete has already hardened, it can have a detrimental impact on the material. This phenomenon is referred to as Delayed Ettringite Formation (DEF). DEF can lead to the development of microcracks in concrete, making it porous and less durable. Therefore, it is crucial to prevent the formation of ettringite after the concrete has already hardened to ensure its strength and durability.
FAQs on Hydration Products of Cement
Define hydration of cement?
When cement and water react, it results in the process known as hydration of cement. This chemical reaction is accompanied by the release of heat, making it exothermic in nature. The heat generated during this reaction is referred to as the heat of hydration.
Which are the four basic componets of cement?
Cement is composed of four main types of compounds which are essential in the chemical reaction process. These compounds are commonly referred to as Bogue’s compounds and are identified as C3A, C4AF, C3S, and C2S. Each of these compounds plays a critical role in the chemical reaction that occurs during the cement-making process.
The first compound, C3A, is an abbreviation for tricalcium aluminate. It reacts quickly with water, which can lead to the setting of cement. This compound is responsible for providing the initial strength to the cement structure.
The second compound, C4AF, is an abbreviation for tetracalcium aluminoferrite. It provides the cement with its color and helps in increasing its resistance to sulfate attack.
The third compound, C3S, is an abbreviation for tricalcium silicate. It is the most abundant compound present in cement and contributes to the overall strength and durability of the final product.
Finally, C2S, an abbreviation for dicalcium silicate, reacts at a slower rate compared to C3S. This compound contributes to the long-term strength of the cement structure.
In conclusion, these four compounds, C3A, C4AF, C3S, and C2S, play crucial roles in the chemical reaction process involved in the manufacture of cement. The correct proportions of these compounds must be maintained to ensure that the final product has the desired properties such as strength, durability, and resistance to various types of attacks.
What is ettringite in cement?
When gypsum is present, it can react with C3A and C4AF to create a compound known as calcium aluminate sulfate or ettringite. This process occurs when the sulfate ions within the gypsum combine with the calcium and aluminum ions from the C3A and C4AF. Ettringite is a well-known product of this chemical reaction and is commonly formed in the presence of these compounds. This reaction is significant in many industries and applications, such as in construction where it plays a crucial role in the formation and setting of concrete.