1. Chapter 3
Cement

2. §3.2 Portland Cement Production and Composition of Portland Cement
Hydration, Setting and Hardening of Portland Cement
Technical Properties of Portland Cement
Corrosion and Prevention of Hardened Cement
Characteristics and Application of Portland Cement

3. 3.2.2 Hydration, Setting and Hardening of Portland Cement
Mechanism
Ways to avoid flash set
Mechanism of modifying setting time by gypsum
Not much gypsum
Hydrate formed by clinker hydration

4. Hydration Mechanism
When mixed with water, Portland cement hydrates and forms heat
3CaO·SiO2+H2O CaO·2SiO2·3H2O+Ca(OH)2
Triealcium silicate hydrates rapidly and forms earlier strength and earlier heat of hydration.
Tricalcium silicate is formed in a kiln when the charge contains an abundance of lime, and hence the greater amount of CaO in the reaction.

5. Hydration Mechanism 2CaO·SiO2+H2O 3CaO·2SiO2·3H2O+Ca(OH)2
Dicalcium silicate hydrates slowly and is responsible for strength increase in concrete at later ages. The heat of hydration is liberated slowly.
More dicalcium silicate is formed when the raw materials contain less lime.

6. 3CaO·Al2O3+H2O 3CaO·Al2O3·6H2O
Hydration Mechanism
Tricalcium aluminate hydrates rapidly and liberates a large amount of heat.
If this reaction were not controlled, concrete would set so rapidly (flash set) that it could not be used in modern construction.
3CaO·Al2O3+H2O 3CaO·Al2O3·6H2O

7. Hydration Mechanism
4CaO·Al2O3·Fe2O3+H2O CaO·Al2O3·6H2O+CaO·Fe2O3·H2O
The addition of iron or iron ore to the raw materials lowers the percentage of tricalcium aluminate. The iron reacts with the tricalcium aluminate, forming the tetra calcium aluminoferrite.
This compound does not add significantly to the properties of concrete, but does cause the charge in the cement kiln to liquefy at lower temperature (lowers the point of fusion), thus reducing the energy required in cement manufacture.

8. Ways to avoid flash set
A small amount of gypsum (CaSO4·2H2O) intergrounding with the cement clinker retards the hydration reaction of tricalcium aluminate so that the calcium silicates can set firstly.

9. Mechanism of Modifying Setting Time by Gypsum
The protecting layer is formed by wrapping up the clinker by Calcium sulfoaluminate hydrate needle crystal, hinders the entry of water, slows down hydration and stops flash setting. Thus the gypsum can modify the setting time.
3CaO·Al2O3·6H2O+ H2O+CaSO4·2H2O
3CaO·Al2O3·3CaSO4·31H2O
Ettringite

10. Not Too Much of Gypsum Hardened cement corrosion
After hydration, tricalcium aluminate will react principally with sodium and magnesium sulfate, found in soil and water, which causes the concrete to disintegrate
Moderate sulfate-resisting cement limits the percentage of tricalcium aluminate to 8 percent.
This compound is limited to 5 percent in sulfate-resisting cement.

11. Hydrate formed by clinker hydration
In full hydration hydrate produced by
clinker hydration is as following:
Calcium silicate hydrate gel(70%)
Calcium hydroxide (20%)
Calcium aluminate hydrate
Calcium ferrite hydrate
Ettringite
Fig Degree of hydration

12. Hydration, Setting and Hardening
of Portland Cement
Setting and Hardening
Definition
Process of Setting and Hardening
Factors Influencing Setting and Hardening of Portland Cement

13. Definition Setting Process Hardening Process
The process of losing plasticity and becoming dense of the paste, which is formed by mixing the cement and water.
Hardening Process
The process of forming hardening cement paste by the paste, which loses plasticity and its strength increases with time.

14. Process of Setting and Hardening
Initial period
Initial dissolving and hydration; mobile and plastic lasting for 5-10min
Incubation period
The performance of gel layer surrounding cement is excellent; particle growth; lasts for 1h
The cement particle expands , bonds and hydrates after the cracking gel layer; Lasts for 6h; Porous spatial net-coagulation structure;Losing plasticity
Setting period
Fillings of gel to capillaries; lasting for 6h~several years.
Hardening period

15. Process of Setting and Hardening

16. Factors Influencing Setting and Hardening of Portland Cement
Factors such as: fineness and types of cement, blend of gypsum,
age, temperature and humidity which influence setting and hardening of Portland cement.
Fig3.2.3 Factors influencing Setting and Hardening of Portland Cement