Question

Discuss the process of setting of cement.

Short answer

The setting of cement involves 4 main stages: Wetting process where water dissociates ions, initial stages of chemical reactions to set the paste and acquire initial hardness, later reactions involving silicates and aluminates to provide additional strength and finally, a slow reaction called the 'Calceolate Reaction' which increases the volume of cement.

Step-by-step solution

Wetting Process

When cement comes in contact with water, it starts getting wetted. Water molecules dissociate \(Ca^{2+}\) and \(OH^-\) ions from the cement mixture into their separate ions. This will lead to the beginning of the setting process of cement.

Initial Stage

In the early stage of wetting, chemical reactions occur which cause the paste to set and acquire initial hardness. These reactions involve the mixture of dicalcium and tricalcium silicates. Each react with water to form insoluble calcium hydroxide and two types of calcium silicate crystals, these are responsible for early strength.

Later Stage

Later, major reactions occur between silicates and aluminates with water to form calcium silicate and calcium aluminate hydrates which provide additional strength. These reactions continue for a significant amount of time before the cement eventually hardens.

Calceolate Reaction

Finally, a slow reaction of calcium aluminate with the gypsum (added at the time of grinding the clinker) to form calcium sulphoaluminate takes place. This is responsible for the increase in the volume of cement and is termed as 'Calceolate Reaction'.

Key concepts

Wetting Process

The wetting process is the crucial first step in the setting of cement. When cement interacts with water, a dynamic transformation begins as water molecules start to dissociate ions from the cement. Specifically, calcium ions ( Ca^{2+} ) and hydroxide ions ( OH^− ) are separated. This ionic separation initiates the setting process of the cement.

• Water plays a key role in initiating the process, acting as a catalyst to switch the cement from a dry state to a reactive paste.
• The wet cement mixture starts to acquire initial rigidity, as chemical changes begin within.

Understanding this early process is vital because it sets the stage for more intricate chemical reactions that follow, leading to cement hardening.

Calcium Silicate Hydrates

Calcium silicate hydrates form during the initial and later stages of the cement setting process. Once calcium silicates in the cement react with water, they produce calcium silicate hydrates, which are a key component of the hardened cement matrix.

• The early formation of these hydrates is one of the main reasons cement begins to set and gain strength shortly after mixing with water.
• These hydrates create a gel-like structure that fills voids and contributes to the solidification and hardness of the cement paste.

The ongoing formation of calcium silicate hydrates provides continued strength and durability to the hardened cement over time.

Calcium Aluminate Hydrates

Calcium aluminate hydrates form alongside calcium silicate hydrates during the cement setting process, contributing further to the strength and stability of the material. When aluminates in the cement react with water, they produce these special hydrates.

• These reactions are crucial during the later stages of cement setting as they add to the structural integrity and continued hardening of the cement.
• With time, they improve the density and durability of the cementitious material.

The presence of calcium aluminate hydrates is essential for achieving the full strength potential of the cement, especially under different environmental conditions.

Calceolate Reaction

The calceolate reaction is a slow and final process in the setting and hardening of cement. In this reaction, calcium aluminate in the cement continues to react slowly with gypsum, which is added during the grinding of clinker.

• This reaction results in the formation of calcium sulphoaluminate, a compound that contributes to an increase in cement volume.
• The calceolate reaction ensures that the cement expands slightly, which helps in filling voids and improving the toughness of the resulting concrete structure.

By understanding the calceolate reaction, one can better grasp how cement achieves its final set and strength, enhancing the quality and durability of construction elements.