Heat of hydration of cement

Heat of hydration test of Cement – Stages and Procedure

The heat of hydration of cement is the heat liberated from the reaction between cement and water. Due to the exothermic nature of this reaction, cement hydration releases a significant amount of heat. The hydration of cement is a gradual process. Hydration reaction is faster in the beginning and continues indefinitely at a slower rate.

Cement is a binding material. It contains ingredients such as lime, Silica, Alumina, and others. Water is added to cement to make it adhesive, allowing it to be used in construction. This article is about the heat of hydration of cement.

Categories of Cement test

Cement is the oldest and most used binding material and an integral ingredient used in the construction sector. The quality, serviceability, and stability of a structure are directly related to the quality of cement used in it. Hence it is necessary to analyse the quality of cement before using it for work. They are as follows.

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Field tests on Cement

Simple field tests are performed to assess cement quality. However, these tests do not require any sophisticated equipment or professional skills, and the results are obtained quickly. By performing these simple tests and analysing the results, we will be able to determine the quality of the cement and decide whether to accept or reject it.

These are first-look tests and the quality of cement is ensured by its smoothness to touch, the colour of cement, etc.

  • Checking manufacturing date
  • Visual checking of lumps
  • Feel test
  • Heat of cement
  • colour
  • Water float test
  • Setting test

These basic tests give an approximate characteristic of cement. These are easy and quick but not accurate, however, help in concluding the acceptance of cement for works. For more details about field tests, you can go through our earlier article Field tests of Cement – A significant step towards quality

Article about field test for cement

Laboratory tests for Cement

The laboratory tests define the physical and chemical properties of cement. So it is not possible to check all the cement properties at the site. The main laboratory tests conducted on cement are as follows.

Lab tests require time. But it provides accurate results. Let us discuss the heat of hydration test in this article

For more details about laboratory tests please refer to the following article

Article about laboratory tests

What is the heat of hydration?

Hydration process
Hydration process

Cement is comprised of chemical compounds that assist in the setting and hardening processes. The chemical compounds Alite (C3S), Belite (C2S), Celite (C3A), and Felite (C4AF) are referred to as Bogue compounds. During the clinkering process, these inactive compounds are formed. When these compounds are exposed to water, they initiate a chemical reaction known as hydration. Hydration is an exothermic reaction. That is, it generates sufficient heat. Additionally, hydration releases 89–90 cal/gm of heat in 7 days and 90–100 cal/gm in 28 days. Moreover, the physical properties of cement are affected by the rate of hydration. Generally, low strength and structural deterioration are caused by improper hydration processes.

The following are the chemical compounds that are responsible for hydration

  • Tricalcium silicate, C3S
  • Dicalcium silicate, C2S
  • Tricalcium aluminate, C3A
  • Tetracalcium aluminoferrite, C4AF
  • Gypsum, CSH2
Chemical reaction during hydration process
Chemical reaction during the hydration process

During the hydration process, hydration products like Tobermorite gel (C-S-H gel), calcium hydroxide and other small compounds are formed. Generally, these substances deposit on the outer periphery of the nucleus of hydrated cement grains. Moreover, hydration is a slow reaction that occurs for 2 to 5 hours. Correspondingly, the initial period of the hydration process is the Induction or dormant period. Consequently, the hydration process stops after the reaction between the entire cement particle. The tobermorite gel occupies 50 to 60 % of the volume of hydrated cement. 

STAGES OF HYDRATION
STAGES OF HYDRATION

Apparatus for the heat of hydration test

The heat of hydration test is conducted at a standard room temperature of 27 degrees Celsius. The apparatus for this test is

  1. Calorimeter
  2. Mortar and pestle 
  3. Glass/Plastic vials
  4. Stopwatch or timer
  5. Sieve
  6. Muffle furnace
  7. Weighing balance

The calorimeter determines the heat of the paste. Basically, a glass vial has a dimension of 80×20 mm. The muffle furnace maintains the temperature. Before performing the test, the calorimeter temperature is to be made uniform. Accordingly, the heat of hydration should be determined at the 7th and 28 days.

Procedure for the heat of hydration test

  • To find the heat of hydration, firstly we have to measure the heat released from the unhydrated cement using a calorimeter.
  • Then we have to measure the heat of hydration from hydrated cement
  • For that take 60 grams of cement and add 24 ml of water. 
  • Then fill this mixture in three glasses and seal them with wax to avoid the entry of air. 
  • The standard temperature should be 27 degrees Celsius. 
  • Then measure the heat of the solution using a calorimeter. 

The heat of hydration = Heat released from hydrated cement – Heat released from unhydrated cement

For low-heat cement, The heat of hydration for 7 days should not be greater than 66 cal/g and for 28 days should not be greater than 75 cal/gm.