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

# Ingredients of cement and functions

Civil Engineering Materials · Chapter 5: Cement

- Asked in: 2080 Baisakh, 4 marks; 2071 Chaitra, 4 marks; 2069 Chaitra, 3 marks; 2078 Kartik, 1 marks; 2059 Poush (old course), 2+6 marks; 2057 Chaitra (old course), 6 marks; 2070 Chaitra (old course), 3 marks (asked 7 times)
- Video: [Functions Of The Ingredients Of Cement - Important Engineering Materials - Engineering Chemistry 1](https://www.youtube.com/watch?v=-IaIu-7BdEk) — Ekeeda
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_3/civil_engineering_materials/ingredients-of-cement-functions
- Whole chapter: https://www.bctengineer.com/md/semester_3/civil_engineering_materials/chapter-5

## Question

What are the ingredients of (Portland) cement? Explain their functions.

## Short answer

Lime (CaO, 60–67%) gives strength, excess unsoundness. Silica (SiO₂, 17–25%) gives strength, excess slows set. Alumina (Al₂O₃, 3–8%) quickens setting, flux. Iron oxide (0.5–6%) gives colour, flux. Magnesia (0.1–4%) adds hardness, excess unsoundness. Gypsum (2–3%) controls setting time.

## Answer

Portland cement is made from raw materials rich in lime, silica, alumina and iron. After burning, the ingredients in cement are:

| Ingredient | Percentage | Function |
|---|---|---|
| Lime (CaO) | 60-67 | Main constituent; controls strength and soundness. Excess causes expansion and unsoundness; less lowers strength and speeds setting |
| Silica (SiO₂) | 17-25 | Gives strength through calcium silicates. Excess slows setting |
| Alumina (Al₂O₃) | 3-8 | Quick setting; acts as flux. Excess lowers strength |
| Iron oxide (Fe₂O₃) | 0.5-6 | Gives colour and acts as flux, helps in fusion |
| Magnesia (MgO) | 0.1-4 | Gives hardness and colour; excess causes unsoundness |
| Sulphur trioxide (SO₃) | 1-3 | Makes cement sound; excess unsoundness |
| Calcium sulphate (gypsum) | 2-3 | Added during grinding to control setting time |
| Alkalies (Na₂O, K₂O) | 0.4-1.3 | Excess causes efflorescence and alkali–aggregate reaction |

## Check yourself

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

1. What is the effect of excess lime (CaO) in cement?
   A. Very quick loss of colour
   B. Expansion and unsoundness
   C. Higher efflorescence only
   D. Faster flash set only
   Answer: B. Lime is the main constituent controlling strength and soundness; excess causes expansion and unsoundness, while too little lowers strength. (Key point: Lime content)

2. Why is calcium sulphate (gypsum) added during grinding?
   A. To increase the lime content
   B. To give colour
   C. To control setting time
   D. To lower the burning temperature
   Answer: C. Gypsum, 2-3%, controls the setting time of cement. (Key point: Gypsum function)

3. Excess alkalies (Na2O, K2O) in cement mainly cause which problems?
   A. Flash set and heat
   B. Low early strength only
   C. Dark colour
   D. Efflorescence and alkali-aggregate reaction
   Answer: D. Alkalies in excess lead to efflorescence and alkali-aggregate reaction. (Key point: Alkalies)

4. Which pairing of ingredient and function is correct?
   A. Silica gives strength through calcium silicates; alumina gives quick setting
   B. Silica gives quick setting; alumina gives strength
   C. Iron oxide gives strength; magnesia gives quick setting
   D. Lime gives colour; iron oxide gives soundness
   Answer: A. Silica forms strength-giving calcium silicates (excess slows setting); alumina gives quick setting and acts as flux. (Key point: Silica and alumina)

## Related in Civil Engineering Materials

- [Harmful ingredients of cement](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aharmful-ingredients-opc) — Civil Engineering Materials, chapter 5, asked 2×
- [Clinker compounds and their functions](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Acement-clinker-compounds-functions) — Civil Engineering Materials, chapter 5, asked 9×

## Related in other subjects

- [Microstructure of ordinary Portland cement](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Amicrostructure-opc) — Concrete Technology and Masonry Structure, chapter 2