🧪 Insoluble Residue Test of Cement

🔍 Purpose

The Insoluble Residue (IR) Test determines the amount of material present in cement that is insoluble in hydrochloric acid (HCl) and nitric acid (HNO₃). These insoluble substances are generally inert impurities like sand, clay, and undissolved minerals that do not participate in hydration. The test ensures the purity and uniformity of cement.

📘 Reference Standard

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

⚙️ Apparatus Required

• Beakers and glass rod
• Filter paper (Whatman No. 40 or equivalent)
• Hot plate
• Hydrochloric acid (HCl) – 1:1 solution
• Nitric acid (HNO₃) – concentrated
• Weighing balance (accuracy ±0.001 g)
• Wash bottle with distilled water

🧪 Procedure

1. Weigh about 1 g of cement sample accurately (W₁).
2. Transfer it into a beaker and add 25 ml of 1:1 HCl solution.
3. Boil gently for 10 minutes while stirring with a glass rod.
4. Add a few drops of concentrated HNO₃ and continue boiling for 5 minutes to ensure complete reaction.
5. Filter the solution through pre-weighed filter paper and wash thoroughly with hot distilled water.
6. Dry the filter paper and residue in an oven at 105°C to constant weight.
7. Cool in a desiccator and weigh again (W₂).

📐 Formula

Insoluble Residue (%) = (W₂ / W₁) × 100

Example:

If 1.000 g of cement (W₁) leaves a residue of 0.008 g (W₂):

IR = (0.008 / 1.000) × 100 = 0.8%

📊 Permissible Limits (IS 269:2015)

📈 Significance

• Indicates purity and quality of raw materials used in cement manufacturing.
• High insoluble residue suggests contamination or presence of unreactive impurities like sand or clay.
• Ensures cement conforms to IS specifications for chemical composition.

⚠️ Causes of High Insoluble Residue

• Improper grinding or incomplete burning of raw materials.
• Contamination with dust, sand, or other inert matter during manufacturing or storage.
• Use of poor-quality raw materials.

✅ Precautions

• Use clean glassware and accurate weighing balance.
• Ensure complete dissolution of cement in acid mixture.
• Do not overheat the solution to prevent spattering.
• Wash the residue thoroughly before drying to remove soluble materials.

📘 Conclusion

The Insoluble Residue Test is an important chemical analysis that indicates the cleanliness and purity of cement. A lower insoluble residue value (≤ 0.75% for OPC) ensures high-quality cement with minimal inert materials. Excessive residue signifies contamination or poor manufacturing control.