Hydration of Portland Cement

Portland cement is the most commonly used type of cement in the construction industry, and its chemical reaction with water, known as hydration, is a complex process that generates heat. The heat of hydration refers to the heat released during the chemical reaction between cement and water, which is a crucial factor in the production of concrete. Understanding the heat of hydration and the resulting hydration products is essential for designing durable and efficient concrete structures.

Heat of Hydration

The heat of hydration of Portland cement is the heat released during the exothermic chemical reaction between cement and water. This reaction is highly exothermic and can generate significant amounts of heat, depending on the type and composition of the cement. During hydration, the water molecules react with the cement compounds to form calcium silicate hydrate (C-S-H) and calcium hydroxide (Ca(OH)2). The reaction can be expressed as follows:

C3S + 6H → C-S-H + Ca(OH)2 + 31.2 kcal/mol

C2S + 4.5H → C-S-H + Ca(OH)2 + 22.4 kcal/mol

 Where C3S and C2S are the primary compounds in Portland cement, and H is water.

The heat of hydration is a critical parameter in the design of concrete structures as it influences the setting time, strength gain, and durability of the concrete. The heat generated during hydration can cause thermal cracking in the concrete, which can significantly impact its long-term performance.

Hydration Products

The hydration of Portland cement results in the formation of several compounds, including calcium silicate hydrate (C-S-H), calcium hydroxide (Ca(OH)2), and various hydrated phases of aluminate and ferrite compounds.

C-S-H is the primary product of Portland cement hydration and is responsible for most of the strength gain in concrete. It forms a gel-like structure that fills the voids between the cement particles, creating a dense and cohesive matrix. The composition and structure of C-S-H can vary depending on the type of cement and the curing conditions. It is generally considered to be a hydrated form of calcium silicates with the general formula (CaO)x(SiO2)y(H2O)z.

Calcium hydroxide (Ca(OH)2) is another significant product of cement hydration and is formed by the reaction of calcium oxide (CaO) with water. It is a weak compound and contributes little to the strength of the concrete. However, it is essential in the formation of other hydrated phases and can provide some protection against sulfate attack.

The hydration of aluminate and ferrite compounds in Portland cement results in the formation of various hydrated phases, including ettringite, monosulfate, and calcium aluminate hydrates. These hydrated phases can contribute to the strength and durability of the concrete, but their formation is highly dependent on the curing conditions and the presence of other chemicals, such as sulfates.

Conclusion

In summary, the heat of hydration of Portland cement is a crucial factor in the design of concrete structures. The exothermic reaction between cement and water can generate significant amounts of heat, which can impact the setting time, strength gain, and durability of the concrete. The hydration of Portland cement results in the formation of several compounds, including calcium silicate hydrate (C-S-H), calcium hydroxide (Ca(OH)2), and various hydrated phases of aluminate and ferrite compounds. Understanding the hydration process and the resulting products is essential for producing durable and efficient concrete structures.