🧪 Sulphur Trioxide (SO₃) Content Test of Cement

🔍 Purpose

The Sulphur Trioxide (SO₃) Content Test determines the percentage of sulphates (SO₃) in cement, which primarily come from the addition of gypsum (CaSO₄·2H₂O). The SO₃ content controls the setting time of cement. However, excess SO₃ may lead to unsoundness and expansion in hardened concrete.

📘 Reference Standards

• IS 4032:1985 – Methods of Chemical Analysis of Hydraulic Cement
• IS 269:2015 – Specification for Ordinary Portland Cement

⚙️ Apparatus Required

• Beakers and conical flasks (250 ml)
• Filter paper
• Burette and pipette
• Glass funnel and measuring cylinder
• Weighing balance (accuracy ±0.001 g)

🧪 Reagents Used

• Hydrochloric acid (HCl)
• Barium chloride (BaCl₂) solution (10%)
• Distilled water
• Silver nitrate (AgNO₃) – for confirming complete precipitation

🧾 Principle

The sulphate present in cement is precipitated as barium sulphate (BaSO₄) by adding barium chloride solution to the acidified filtrate of the cement solution. The amount of BaSO₄ formed is proportional to the sulphur trioxide (SO₃) content, which is then determined gravimetrically.

⚗️ Procedure

1. Weigh 1 g of cement sample accurately and place it in a beaker.
2. Add 50 ml of distilled water and 10 ml of concentrated HCl.
3. Boil gently for 10 minutes and cool slightly.
4. Filter the solution to remove insoluble residue and collect the filtrate in another beaker.
5. Add 10 ml of BaCl₂ solution slowly to the hot filtrate with constant stirring.
6. Allow the solution to stand for 2 hours for complete precipitation of BaSO₄.
7. Filter the precipitate using filter paper and wash with hot water until free of chloride (test with AgNO₃).
8. Dry the filter paper and residue, then ignite it in a weighed crucible at 800–900°C for 30 minutes.
9. Cool in a desiccator and weigh the residue (BaSO₄).

📐 Calculation

SO₃ (%) = (Weight of BaSO₄ × 34.3) / Weight of sample

Example:

If 0.045 g of BaSO₄ is obtained from 1 g of cement sample:

SO₃ (%) = (0.045 × 34.3) / 1 = 1.54%

📊 Permissible Limits (IS 269:2015)

📈 Significance

• Ensures cement contains adequate gypsum to regulate setting time.
• Prevents unsoundness caused by excessive sulphate content.
• Helps maintain cement quality and long-term durability.

⚠️ Effects of Excess Sulphur Trioxide (SO₃)

• Causes expansion and cracking due to formation of calcium sulphoaluminate (ettringite).
• Reduces soundness and durability of hardened cement.
• Interferes with hydration of tricalcium aluminate (C₃A).

✅ Precautions

• Ensure complete precipitation of BaSO₄ before filtration.
• Do not overheat the sample during ignition (avoid decomposition).
• Test for absence of chlorides using AgNO₃ before weighing.

📘 Conclusion

The Sulphur Trioxide (SO₃) Content Test is essential for assessing the sulphate content in cement. Excess SO₃ can cause volume instability and cracks, while insufficient SO₃ affects setting time. As per IS 269:2015, the permissible SO₃ content in OPC should not exceed 3.5%. This test ensures the cement is chemically balanced for safe and durable construction.