Introduction

Cement, in its dry form, does not have any binding ability. However, when water is added, a chemical reaction called hydration takes place. This reaction is responsible for giving cement its strength and adhesive properties, allowing it to bond materials together. Since this process is exothermic, it releases heat.

In small quantities, this heat is not a major concern, but in large concrete structures such as dams or thick slabs, excessive heat can lead to thermal cracking. These cracks weaken the concrete, reducing its durability and mechanical strength. This is why it is important to measure the heat of hydration and control it during construction.

Principle

The heat released during hydration can be measured using a calorimeter. In this experiment, a known mass of cement is mixed with water inside an insulated container. The temperature change is recorded, and from this, the heat of hydration is calculated in joules per gram (J/g) of cement. To improve accuracy, solutions of hydrochloric acid and anhydrous calcium chloride are used in the calorimeter to absorb the heat effectively.

Why is This Important?

Prevents cracks – In mass concrete structures, excessive heat can cause cracks, reducing durability.
Ensures quality control – Helps confirm that the cement meets required standards.
Optimizes strength development – The rate of heat release gives insight into how quickly the cement will gain strength.

What Affects the Heat of Hydration?

The rate at which cement hydrates and releases heat depends on several factors:
Fineness of cement – Finer cement particles react faster, releasing more heat.
Water-cement ratio – More water slows down hydration and reduces heat.
Temperature of hydration – Higher temperatures speed up the reaction.
Cement composition – Different compounds in cement contribute differently to heat generation.

Chemical Reactions Involved

Cement is made up of different chemical compounds, and each reacts with water differently:
1. Tricalcium silicate (C₃S) reacts quickly and contributes to early strength.
2. Dicalcium silicate (C₂S) reacts slowly and helps in long-term strength.
3. Tricalcium aluminate (C₃A) reacts very fast, releasing a lot of heat. Gypsum is added to control this reaction.
4. Tetracalcium aluminoferrite (C₄AF) contributes to strength but generates less heat.