---
title: "Harmful ingredients of cement"
subject: "Civil Engineering Materials"
chapter: 5
key: "semester_3/civil_engineering_materials:harmful-ingredients-opc"
source: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/harmful-ingredients-opc
site: BCT Engineering
---

# Harmful ingredients of cement

Civil Engineering Materials · Chapter 5: Cement

- Asked in: 2059 Poush (old course), 2 marks; 2057 Chaitra (old course), 2 marks (asked 2 times)
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/harmful-ingredients-opc
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-5

## Question

What are the harmful constituents (ingredients) of OPC?

## Short answer

Excess free lime, magnesia (>6%), and SO3 (>2.5%) cause unsoundness by late expansion. High alkalis (>0.6% Na2O eq) cause efflorescence and alkali-aggregate reaction. Insoluble residue and loss on ignition beyond limits indicate poor burning or adulteration, reducing strength.

## Answer

Harmful constituents are the compounds in cement that, if present in excess, reduce strength, cause cracking or damage the concrete in the long run.

1. **Excess free lime (CaO):** left unburnt, it hydrates slowly after setting and expands, causing unsoundness and cracks.
2. **Excess magnesia (MgO):** above 6% (IS 269) it also hydrates late and causes expansion and cracking.
3. **Excess sulphur trioxide (SO3):** more than 2.5% (3% when C3A is over 5%) forms excess ettringite, causing unsoundness and expansion.
4. **Alkalis (Na2O and K2O):** limited to 0.6% (as Na2O equivalent); they cause efflorescence and alkali-aggregate reaction.
5. **Insoluble residue and loss on ignition:** kept within limits (about 5% each for OPC in IS 269:2015); high values indicate poor burning, adulteration or stale (partly hydrated) cement.
6. **Excess alumina/iron oxide and uncombined silica:** upset the compound balance and lower strength.

Their limits are fixed in IS 269 and checked by chemical analysis.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Why are excess free lime and magnesia harmful?
   A. They make the cement set instantly
   B. They hydrate late, expand and crack the hardened concrete
   C. They reduce the cement's colour
   D. They lower specific gravity
   Answer: B. They hydrate slowly after setting, causing expansion, unsoundness and cracks. (Key point: Free lime and MgO)

2. What is the limit of alkalies (as Na2O equivalent) per IS 269 and the problem they cause?
   A. 6%; flash set
   B. 2.5%; unsoundness
   C. 0.6%; efflorescence and alkali-aggregate reaction
   D. 4%; slow strength
   Answer: C. Alkalies are limited to 0.6% because they cause efflorescence and alkali-aggregate reaction. (Key point: Alkali limit)

3. What does a high insoluble residue or loss on ignition indicate?
   A. Excellent quality
   B. High C3S content
   C. Fine grinding
   D. Poor burning or adulteration
   Answer: D. Both are limited to 4% each; high values show poor burning or adulteration. (Key point: Insoluble residue)

## Related in Civil Engineering Materials

- [Harmful ingredients in brick earth](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aharmful-ingredients-brick-earth) — Civil Engineering Materials, chapter 3, asked 3×
- [Ingredients of cement and functions](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aingredients-of-cement-functions) — Civil Engineering Materials, chapter 5, asked 7×
- [7 and 28 day strength of 53 grade OPC](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aopc-53-grade-strength) — Civil Engineering Materials, chapter 5
- [Cement manufacture with flow diagram](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acement-manufacture-flow-diagram) — Civil Engineering Materials, chapter 5, asked 5×

## Related in other subjects

- [OPC versus PPC](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Aopc-vs-ppc) — Concrete Technology and Masonry Structure, chapter 1
- [Physical properties of OPC and concrete behaviour](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Acement-physical-properties) — Concrete Technology and Masonry Structure, chapter 1