🧪 Free Lime Content Test of Cement

🔍 Purpose

The Free Lime Content Test of cement determines the amount of uncombined calcium oxide (CaO) present in the clinker or cement. Excess free lime can lead to unsoundness, expansion, and cracking of hardened concrete. The test ensures complete combination of lime with silica, alumina, and iron oxide during manufacturing.

📘 Reference Standards

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

⚙️ Apparatus and Chemicals Required

• Weighing balance (accuracy ±0.001 g)
• Beakers, pipettes, burettes, and conical flasks
• Ethylene glycol (solvent)
• Phenolphthalein indicator
• Standard hydrochloric acid (HCl) solution – 0.1 N
• Ammonium chloride (NH₄Cl) solution

📐 Principle

Free lime in cement reacts with hydrochloric acid to form calcium chloride and water. The unreacted HCl is then titrated with standard sodium hydroxide (NaOH). The amount of acid consumed gives the free lime content.

Chemical Reaction:

CaO + 2HCl → CaCl₂ + H₂O

🧮 Formula for Free Lime Content

Free Lime (%) = (V₁ - V₂) × N × 2.8 / W

• V₁ = Volume of HCl added (in mL)
• V₂ = Volume of NaOH used in titration (in mL)
• N = Normality of HCl solution
• W = Weight of sample in grams
• 2.8 = Equivalent weight factor of CaO

🧫 Procedure

1. Weigh accurately about 1 g of cement sample.
2. Transfer it into a 250 mL beaker and add 50 mL of ethylene glycol.
3. Heat gently for 30 minutes to dissolve free lime completely.
4. Add a few drops of phenolphthalein indicator.
5. Titrate the hot solution with 0.1 N hydrochloric acid (HCl) until the pink color disappears.
6. Record the volume of acid used (V₁).
7. In a blank titration (without cement sample), use the same amount of HCl and titrate with 0.1 N NaOH (record as V₂).
8. Calculate the free lime percentage using the above formula.

📊 Example Calculation

Given data:

• Weight of sample (W) = 1.0 g
• Volume of HCl (V₁) = 25.0 mL
• Volume of NaOH (V₂) = 19.0 mL
• Normality (N) = 0.1

Free Lime (%) = (25 - 19) × 0.1 × 2.8 / 1 = 1.68%

✅ Result

Free lime content of the given cement sample = 1.68%

📏 Permissible Limits (as per IS 269)

• Free Lime in Clinker: Not more than 2.0%
• Free Lime in Cement: Preferably less than 1.5%

⚠️ Effects of Excess Free Lime

• Causes unsoundness and expansion in hardened cement.
• Leads to delayed cracking and disintegration.
• Indicates incomplete burning or improper raw mix proportioning in clinker.

🧭 Precautions

• Ensure complete dissolution of free lime using ethylene glycol.
• Perform titration while the solution is hot.
• Avoid exposure of cement to moisture before testing.
• Use freshly standardized HCl and NaOH solutions.

📘 Conclusion

The Free Lime Content Test is vital for assessing the completeness of clinker burning and overall soundness of cement. Maintaining the free lime content within limits (below 2%) ensures durable and stable concrete performance without expansion or cracking issues.