4.4
Strength and Heat of Hydration
The reaction of cement with water to produce hydration products releases heat, known as the heat of hydration.
Initially, heat generation is rapid due to the hydration of aluminates and silicates in cement. A sudden drop follows as hydration products coat silicates, then a gradual increase as remaining silicates react after dissolution of the coating, followed by a decrease and eventual stabilization as hydration slows.
The compounds in the cement contribute to its heat of hydration; C3A contributes the most, and C2S contributes the least.
For Portland cement, about fifty percent of the total heat is liberated between one and three days, about seventy-five percent in seven days, and nearly ninety percent in six months.
The C-S-H formed during the hydration of silicates in cement contributes to its strength.
Considering the variation in the strength of hydrated cement with time, hydration of C3S contributes more to the strength development during the first four weeks, while the hydration of C2S contributes to the strength gained thereafter.
However, by about one year, hydration of both C3S and C2S equally contribute to the gain in strength.
In contrast, the hydration of C3A and C4AF contribute little to the strength.
The hydration of cement is an exothermic reaction in which heat is generated as cement hydrates. This heat of hydration is critical to cement's strength development. The rate at which this heat is generated affects the temperature rise, with a majority of the heat being released early in the hydration process, half within the first three days, and about 75% within the first week.
The heat of hydration for each cement compound is significant; for instance, tricalcium aluminate (C3A) and tricalcium silicate (C3S) contribute most to this process. Table 1 summarizes the heat of hydration for cement compounds like C3S, C2S, C2A, and C4A. Adjusting the proportions of these compounds can manage the rate and total amount of heat released during cement hydration.
Table 1. Heat of Hydration for Cement Compounds
| Compound | Heat of hydration (J/kg) |
| C3A | 867 |
| C3S | 502 |
| C4AF | 419 |
| C2S | 260 |
The strength development of hydrated cement is primarily influenced by the compounds tricalcium silicate (C3S) and dicalcium silicate (C3S). C3S contributes significantly to the strength during the initial four weeks after hydration, while C2S plays a more crucial role in the long-term strength gains, especially after the first year. After one year, both compounds, when considered on a mass-for-mass basis, contribute equally to the cement's overall strength.
The reaction of cement with water to produce hydration products releases heat, known as the heat of hydration.
Initially, heat generation is rapid due to the hydration of aluminates and silicates in cement. A sudden drop follows as hydration products coat silicates, then a gradual increase as remaining silicates react after dissolution of the coating, followed by a decrease and eventual stabilization as hydration slows.
The compounds in the cement contribute to its heat of hydration; C3A contributes the most, and C2S contributes the least.
For Portland cement, about fifty percent of the total heat is liberated between one and three days, about seventy-five percent in seven days, and nearly ninety percent in six months.
The C-S-H formed during the hydration of silicates in cement contributes to its strength.
Considering the variation in the strength of hydrated cement with time, hydration of C3S contributes more to the strength development during the first four weeks, while the hydration of C2S contributes to the strength gained thereafter.
However, by about one year, hydration of both C3S and C2S equally contribute to the gain in strength.
In contrast, the hydration of C3A and C4AF contribute little to the strength.
From Chapter 4:
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