The heat of hydration is the heat liberated from the reaction between cement and water. Cement is a binding material. It consists of ingredients such as lime, Silica, Alumina etc. Water is added to cement to make it adhesive so that we can use it in construction. This article is about the heat of hydration of cement.
Cement test – Categories
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 works. They are as follows.
- Laboratory test
- Field tests for cement
Also read : Cement tests – Laboratory and Field Tests
Also Read : Best cement of India -2021
Field tests on Cement
The cement quality is by simple field tests. However, these tests do not require any sophisticated types of equipment and professional skills and get the results very quickly. By conducting these simple tests and analysing the results we will get an idea about the cement quality so that, we can immediately decide on accepting or rejecting 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
- 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
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 is as follows.
- Fineness Test
- Compressive Strength Test
- Consistency Test on cement
- Setting time
- Soundness Test
- Tensile strength Test
- Heat of hydration
Lab tests require time. But it provides accurate results.
What is heat of hydration?
Cement consists of chemical compounds which help in the setting and hardening process. The chemical compounds Alite (C3S), Belite (C2S), Celite (C3A) and Felite (C4AF) are collectively called Bogue compounds. These inactive compounds are formed during the clinkering process. These compounds on addition of water initiates a chemical reaction with water known as hydration process.Hydration is an exothermic reaction. That is, it emits a sufficient amount of heat. Also, hydration liberates 89-90cal/gm of heat in 7 days and 90-100cal/gm in 28 days. The physical properties of cement depend on the rate of hydration. Improper hydration process results in low strength and deterioration of structures. Chemical compounds responsible for hydration are as follows.
Also Read : Physical properties of cement
Also Read : Water to cement ratio
- Tricalcium silicate, C3S
- Dicalcium silicate, C2S
- Tricalcium aluminate, C3A
- Tetracalcium aluminoferrite, C4AF
- Gypsum, CSH2
During the hydration process, hydration products like Tobermorite gel (C-S-H gel), calcium hydroxide and other small compounds are formed. 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. The initial period of the hydration process is the Induction or dormant period. 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.
Apparatus for 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
- Mortar and pestle
- Glass/Plastic vials
- Stopwatch or timer
- Muffle furnace
- Weighing balance
The calorimeter determines the heat of the paste. Glass vial has a dimension of 80×20 mm. Muffle furnace maintains the temperature. Before performing the test, the calorimeter temperature is to be made uniform. The heat of hydration should be determined at the 7th and 28 days.
Procedure for 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.
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.