What is cement hydration? How it works and its importance

Cement gains its adhesive strength only when it is hydrated. Here’s a guide to the cement hydration process.

Concrete has a crucial role in civil engineering. Concrete is the final mixed output of cement, water, and sand. But when cement and water come into touch, an exothermic chemical reaction takes place. Huge heat is produced when the reaction takes place. This heat is known as the heat of hydration. Though cement hydration is a very fast process at the initial stage, with time, the whole process is slowed down.

 

What is cement hydration? How it works and its importance

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See also: How is cement density measured?

 

Cement hydration: Various phases of the process

A total of five phases are there in the cement hydration process.

Phase 1: Initial mixing reaction

In the very first step, cement, water, and sand are mixed. In this stage, when water and cement come into contact, a temperature rise happens. The aluminate present in cement reacts with water and ettringite is formed in this reaction. The discharge of the energy from these reactions causes the initial peak.

 

Phase 2: Dormancy

The result of the reaction in Phase 1 is basically a surface coating of all cement particles. As this coating increases, the reaction slows down gradually because it does not have as good access to H2O as when the concrete is mixed. While the concrete surface remains liquid, the amount of hydrated concrete increases steadily.

This phase is therefore used for the transportation and pouring of concrete as it remains at the liquid level. The length of this period depends on the individual concrete formulation and can be optimised according to the application, such as winter concrete placement, transportation time, etc. This phase ends with the initial setting of concrete.

 

Phase 3: Strength acceleration

The temperature rise is caused by the reaction of calcium silicates (C3S and C2S) to form the silicate hydrate (temperature rise is also caused by other minor reactions). At this stage, CSH formation also has a substantial effect on the strength of concrete.

For bulk concrete applications, it is very vital to observe internal temperature changes. This is because the temperature of the concrete rises quickly during this stage and can reach inner temperatures of 70-80°C. Typically, it is not suggested to surpass temperatures around 70°C.

 

Phase 4: Speed reduction

The highest temperature is reached and the availability of free particles is decreased, so the temperature rise is slowed.

Often this stage ends with the required strength and the formwork around the concrete can be released.

 

Phase 5: Steady development

The hydration process slows down and slowly resumes to finish off the remaining cement and water particles. The formwork is often removed and the concrete, over time, completes the hydration process. When the reaction is completely ended, it reaches its ultimate strength. But it may take weeks or months.

 

What is cement hydration? How it works and its importance

Source: Pinterest

 

Cement hydration: Products of the process

In the cement hydration process, a few products come out. Those are as follows.

  • calcium silicate hydrates
  • calcium hydroxide
  • calcium aluminate hydrates

 

Cement hydration: Water requirements

At least 38% water is required. Among this, 23% works as bound water, while 15% is gel water.

 

Cement hydration: Sulphate attack

Sulphate attack is usually only a problem when the bottom hole static temperature (BHST) at which ettringite is present is below about 60 °C (140 °F). Some formation waters include high concentrations of sulphates. These sulphates attack cement and as a result of this, cement crumbles with time.

Modifying the cement powder makes it more resistant to sulphate attack. Replacing aluminate with ferrite reduces the amount of ettringite formed during hydration and also reduces the amount of free lime. Adding pozzolanic materials such as fly ash also reduces sulphate attack as it reacts with CH in cement to make it less reactive.

 

What is cement hydration? How it works and its importance

Source: Pinterest

 

Changes seen in cement particles after the process

At the macro-phase level of the mix, concrete can be understood as 2-phase material.

  • Paste phase
  • Aggregate phase

The paste structure of cement concrete is the most crucial one because it gains a few vital properties like strength, permeability, drying shrinkage, durability, creep, volume change properties, etc.

 

Cement hydration: Importance

The main function of cement in concrete is to act as a binder, thus binding aggregates. But as a matter of fact, anhydrous cement (non-hydrated cement) does not bond with fine or coarse aggregates. Cement gains its adhesive strength only when it is hydrated. Hydration of cement is therefore of utmost importance in concrete technology.

 

FAQs

What are the stages of cement hydration?

The steps of cement hydration are pre-induction, dormant(induction) period, acceleration, deceleration, and steady state.

What are the four major crystalline phases of the cement hydration process?

The four phases of the crystalline stage are Tricalcium silicate, Dicalcium silicate, Tricalcium aluminate, and Tetracalcium aluminoferrite.

What is sulphate attack in the cement hydration process?

A sulphate attack takes place when the Bottom Hole Statics Temperatures (BHSTs) are under 60°C. In this situation, sulphates attack the cement, and the cement will crumble with time.

What are the products of cement hydration?

The products of the cement hydration process are calcium silicate hydrates, calcium hydroxide, and calcium aluminate hydrates.

Got any questions or point of view on our article? We would love to hear from you.

Write to our Editor-in-Chief Jhumur Ghosh at jhumur.ghosh1@housing.com

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