Gypsum in Cement

Gypsum (CaSO₄·2H₂O) is a naturally occurring mineral composed of calcium sulphate dihydrate. It plays a vital and specific role in the manufacture of Portland cement, where it is used as an additive to regulate the setting time of the finished product. Though gypsum constitutes only a small proportion of the cement mixture (typically 3–5% by weight), its function is crucial for ensuring proper workability, strength development, and overall quality of the cement.

Role of Gypsum in Cement

Gypsum acts as a retarding agent during the hydration process of cement. Its principal function is to control the setting time of cement, preventing it from setting too rapidly after mixing with water.
Without gypsum, cement would set almost immediately after the addition of water, making it unworkable for practical construction use.

The Hydration Process

When Portland cement is mixed with water, a series of chemical reactions called hydration takes place. Cement mainly contains four compounds, known as Bogue’s compounds:

The compound Tricalcium aluminate (C₃A) reacts very rapidly with water and produces a large amount of heat and calcium aluminate hydrate, causing instantaneous setting — known as flash set.

Chemical Role of Gypsum

To prevent this, gypsum is added to cement clinker during the final grinding process. It reacts with tricalcium aluminate to form a protective layer of calcium sulphoaluminate (ettringite) around the C₃A particles, which slows down their hydration rate.
Chemical reaction:
C3A+3CaSO4⋅2H2O+26H2O→C6ASˇ3H32C₃A + 3CaSO₄·2H₂O + 26H₂O → C₆AŠ₃H₃₂C3​A+3CaSO4​⋅2H2​O+26H2​O→C6​ASˇ3​H32​
(Tricalcium aluminate + Gypsum + Water → Calcium sulphoaluminate (ettringite))
This reaction delays the setting of cement, allowing enough time for mixing, transportation, and placing before hardening begins.

Optimum Amount of Gypsum

• The quantity of gypsum added during cement manufacturing is critical.
• Normally, 3% to 5% gypsum (by weight of clinker) is added.
• Less than 3% may lead to flash setting (too fast setting).
• More than 5% may lead to excessive retardation and formation of undesirable sulphate compounds, causing long-term strength loss or expansion cracks.

Thus, maintaining the correct gypsum content is essential for achieving the desired balance between setting time and strength.

Effects of Gypsum on Cement Properties

Types of Gypsum Used in Cement

1. Natural Gypsum:
   • Mined directly from deposits; most commonly used.
2. Synthetic Gypsum:
   • By-product of industrial processes such as flue gas desulphurisation (FGD) in thermal power plants.
   • Chemically identical to natural gypsum and widely used for sustainability and cost-effectiveness.
3. Anhydrite (CaSO₄):
   • A dehydrated form of gypsum that can also be used in small amounts for specific cement types.

Gypsum in Different Types of Cement

• Ordinary Portland Cement (OPC): Gypsum controls setting time and enhances workability.
• Rapid Hardening Cement: Requires slightly less gypsum to prevent excessive retardation.
• Low Heat Cement: Gypsum helps manage the heat of hydration in mass concrete applications.
• Sulphate-Resisting Cement: Gypsum content is carefully reduced to avoid excess sulphate reaction.
• Blended Cements (PPC, PSC): The presence of fly ash or slag alters the gypsum requirement for balanced setting.

Consequences of Incorrect Gypsum Proportion

1. Too Little Gypsum:
   • Rapid hydration of C₃A.
   • Flash setting and poor workability.
   • Reduced strength and durability.
2. Too Much Gypsum:
   • Formation of excess ettringite during later stages (secondary expansion).
   • Cracking, loss of strength, and structural instability.
   • Increased porosity and decreased durability.

Hence, precise gypsum addition during cement grinding is vital for quality control.

Testing and Standards

According to Bureau of Indian Standards (BIS – IS: 269) and ASTM C150, cement should have:

• Initial setting time: not less than 30 minutes.
• Final setting time: not more than 600 minutes.

The gypsum content is adjusted to ensure compliance with these setting time standards.

Industrial Process

During cement manufacturing:

1. Clinker formation: Limestone and clay are heated to about 1450°C in a kiln to form clinker nodules.
2. Clinker grinding: Clinker is ground with a controlled quantity of gypsum (3–5%) in ball mills.
3. Final product: The result is fine powdered cement with the desired setting properties.

Environmental Aspect

• The use of synthetic gypsum from industrial waste (e.g., desulphurisation in power plants) helps reduce mining and supports environmental sustainability.
• Recycling gypsum also minimises industrial waste and promotes circular economy practices in cement production.