🧪 Magnesia Content Test of Cement

🔍 Purpose

The Magnesia Content Test determines the percentage of Magnesium Oxide (MgO) present in cement. This test is important because excessive magnesia in cement can cause unsoundness and delayed expansion, leading to cracks and reduced durability in concrete structures.

📘 Reference Standard

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

⚙️ Apparatus Required

• Beakers and measuring flasks
• Filter paper
• Burette and pipette
• Conical flask (250 ml)
• Volumetric flask (100 ml)
• Weighing balance (accuracy ±0.001 g)

🧪 Reagents Used

• Ammonium chloride (NH₄Cl)
• Ammonium hydroxide (NH₄OH)
• Standard EDTA solution (0.01 N)
• Eriochrome Black T indicator
• Buffer solution (pH ≈ 10)

🧾 Principle

Magnesium in the cement sample is precipitated as Magnesium Hydroxide (Mg(OH)₂) by the addition of ammonium hydroxide. The precipitate is then dissolved and titrated with standard EDTA solution using Eriochrome Black T indicator to determine the amount of magnesium oxide (MgO) present.

⚗️ Procedure

1. Weigh 1 g of cement sample accurately and dissolve it in 50 ml of distilled water and 10 ml of HCl.
2. Boil the solution for 10 minutes and filter it to remove insoluble matter.
3. Add 5 ml of NH₄Cl and 10 ml of NH₄OH to the filtrate to precipitate iron and aluminum hydroxides.
4. Filter again to obtain a clear solution containing magnesium.
5. Add buffer solution (pH 10) and 2–3 drops of Eriochrome Black T indicator.
6. Titrate the solution with 0.01 N EDTA until the color changes from wine-red to pure blue.

📐 Calculation

Magnesia (MgO) % = (V × N × 2.015 × 100) / W

Where:

• V = Volume of EDTA used (ml)
• N = Normality of EDTA solution
• W = Weight of cement sample (g)

Example:

If 15.2 ml of 0.01 N EDTA was used for 1 g of cement sample:

MgO % = (15.2 × 0.01 × 2.015 × 100) / 1 = 3.06%

📊 Permissible Limits (IS 269:2015)

📈 Significance

• Ensures cement does not contain excess magnesia that can cause unsoundness.
• Helps maintain long-term durability and dimensional stability of concrete.
• Part of chemical analysis for cement quality control.

⚠️ Effects of Excess Magnesia

• Leads to delayed expansion and cracking.
• Reduces soundness and durability of hardened cement.
• May cause volume instability during hydration.

✅ Precautions

• Use standard 0.01 N EDTA and fresh buffer solution.
• End point must be distinct color change (wine red → blue).
• Avoid contamination during filtration and titration.

📘 Conclusion

The Magnesia Content Test is a crucial chemical analysis that ensures the cement contains safe limits of magnesium oxide. As per IS 269:2015, the MgO content should not exceed 6%. This helps guarantee the cement’s long-term soundness and structural performance.