Chapter 3 Cement

§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

3.2.2 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

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.

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.

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

Hydration Mechanism 4CaO·Al2O3·Fe2O3+H2O 3CaO·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.

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.

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

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.

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.3.2.2 Degree of hydration

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

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.

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

Process of Setting and Hardening

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