The truth about the world's most argued flyover

When we think of cement, we often picture it as a gray, powdery substance that hardens into a solid structure when mixed with water. While this basic understanding is correct, there's a complex and fascinating chemical process happening beneath the surface. In this article, we will explore the chemical reaction between cement and water in detail.

Cement hydration | source

The Composition of Cement

Cement, in its most common form, is Portland cement, which is a fine powder composed mainly of four primary minerals:

• Tricalcium silicate (C₃S)
• Dicalcium silicate (C₂S)
• Tricalcium aluminate (C₃A)
• Tetracalcium aluminoferrite (C₄AF)

The Chemical Reaction

When water is added to cement, a series of complex chemical reactions occur. The primary reactions involve the hydration of the minerals in cement, particularly C₃S and C₂S. The key reactions can be represented as follows:

• The tricalcium aluminate reacts with the gypsum in the presence of water to produce ettringite and heat:
Tricalcium aluminate + gypsum + water ® ettringite + heat
C₃A + 3CSH₂ + 26H ® C₆AS₃H₃₂, D H = 207 cal/g
Ettringite consists of long crystals that are only stable in a solution with gypsum. The compound does not contribute to the strength of the cement glue.
 
• The tricalcium silicate (alite) is hydrated to produce calcium silicate hydrates, lime and heat:
Tricalcium silicate + water ® calcium silicate hydrate + lime + heat
2C₃S + 6H ® C₃S₂H₃ + 3CH, D H = 120 cal/g
The CSH has a short-networked fiber structure which contributes greatly to the initial strength of the cement glue.
 
• Once all the gypsum is used up as per reaction (i), the ettringite becomes unstable and reacts with any remaining tricalcium aluminate to form monosulfate aluminate hydrate crystals:
Tricalcium aluminate + ettringite + water ® monosulfate aluminate hydrate
2C₃A + 3 C₆AS₃H₃₂ + 22H ® 3C₄ASH₁₈,
The monosulfate crystals are only stable in a sulfate deficient solution. In the presence of sulfates, the crystals resort back into ettringite, whose crystals are two-and-a-half times the size of the monosulfate. It is this increase in size that causes cracking when cement is subjected to sulfate attack.
 
• The belite (dicalcium silicate) also hydrates to form calcium silicate hydrates and heat:
Dicalcium silicates + water ® calcium silicate hydrate + lime
C₂S + 4H ® C₃S₂H₃ + CH, D H = 62 cal/g
Like in reaction (ii), the calcium silicate hydrates contribute to the strength of the cement paste. This reaction generates less heat and proceeds at a slower rate, meaning that the contribution of C₂S to the strength of the cement paste will be slow initially. This compound is however responsible for the long-term strength of portland cement concrete.
 
• The ferrite undergoes two progressive reactions with the gypsum:
• in the first of the reactions, the ettringite reacts with the gypsum and water to form ettringite, lime and alumina hydroxides, i.e.
• Ferrite + gypsum + water ® ettringite + ferric aluminum hydroxide + lime
• C₄AF + 3CSH₂ + 3H ® C₆(A,F)S₃H₃₂ + (A,F)H₃ + CH

• the ferrite further reacts with the ettringite formed above to produce garnets, i.e.

• Ferrite + ettringite + lime + water ® garnets
• C₄AF + C₆(A,F)S₃H₃₂ + 2CH +23H ® 3C₄(A,F)SH₁₈ + (A,F)H₃

These reactions result in the formation of calcium silicate hydrate (C-S-H) gel and calcium hydroxide (Ca(OH)₂), both of which contribute to the hardening and strength of the concrete. The C-S-H gel acts as a binder that holds the concrete particles together, while calcium hydroxide contributes to the alkalinity of the mixture.

The Setting and Hardening Process

As the chemical reactions progress, the mixture gradually transforms from a liquid slurry into a solid material. This process is known as "setting" and "hardening." The setting time depends on various factors, including the type of cement, temperature, and water-cement ratio.

Importance of Water-Cement Ratio

The water-cement ratio is a critical factor in the quality and durability of the concrete. A high water-cement ratio can weaken the concrete and reduce its strength. Conversely, a low water-cement ratio can lead to incomplete hydration and poor workability. Achieving the right balance is essential for producing strong and durable concrete structures.

Conclusion

The chemical reaction between cement and water is a complex and crucial process in the creation of concrete. Understanding the chemistry behind this reaction helps engineers and builders design and construct resilient structures. By optimizing the water-cement ratio and ensuring proper curing, we can harness the full potential of this remarkable chemical transformation to create buildings and infrastructure that stand the test of time.