---
title: "Microstructure of ordinary Portland cement"
subject: "Concrete Technology and Masonry Structure"
chapter: 2
key: "semester_5/concrete_technology_and_masonry_structure:microstructure-opc"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/microstructure-opc
site: BCT Engineering
---

# Microstructure of ordinary Portland cement

Concrete Technology and Masonry Structure · Chapter 2: Structure of concrete

- Asked in: 2066 Chaitra (old course), 5 marks
- Video: [Hydration of Portland Cement](https://www.youtube.com/watch?v=M3-T3_XmMm4) — ETHZ - Physical Chemistry of Building Materials
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/microstructure-opc
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-2

## Question

Explain the microstructure of ordinary Portland cement.

## Short answer

Microstructure of OPC paste consists of C-S-H gel (50-60%, high surface area, strength-giving), calcium hydroxide plates (20-25%), ettringite/monosulphate (15-20%), unhydrated cores, and pores (gel pores <3 nm, capillary pores 10 nm–µm). Strength depends on gel/space ratio, increasing with hydration; w/c and curing control porosity.

## Answer

The **microstructure of ordinary Portland cement paste** is the structure of the cement paste after hydration, as seen under an electron microscope.

#### Unhydrated cement
Cement is a fine powder (particle size about 1-100 µm) of the clinker compounds C$_3$S, C$_2$S, C$_3$A and C$_4$AF, with gypsum. The grains have an irregular shape.

#### Hydrated structure
On mixing with water, the grains dissolve and hydration products precipitate in the water-filled space between the grains:
1. **C-S-H gel** (50-60% by volume of solids): at first fibrous or honeycomb-like, later massive, with an enormous surface area (about 100-700 m$^2$/g). It forms from the hydration of C$_3$S and C$_2$S and gives the strength.
2. **Calcium hydroxide (portlandite)** (20-25%): large hexagonal plate crystals that fill spaces.
3. **Ettringite** (needles, at early ages), which changes into **monosulphate** plates (15-20% together); formed from C$_3$A (and C$_4$AF) with gypsum.
4. **Unhydrated cores** of cement grains, especially the large ones.
5. **Pores**: gel pores (below about 3 nm), capillary pores (about 10 nm to a few µm), and air voids.

```text
 Early (hours)   ->   Later (days)
  o   o   o           oooo C-S-H
 grains in water      fills spaces,
  + needles           CH hexagons
```

The strength depends on the amount of solids and how well they fill the space (gel/space ratio), so it rises with age as the hydration products grow and the capillary pores shrink. The w/c ratio (a higher w/c gives more capillary pores) and curing decide the porosity, and thus the strength, permeability and durability.

## Check yourself

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

1. C-S-H forms mainly from hydration of...
   A. C3A and gypsum
   B. C3S and C2S
   C. C4AF only
   D. Gypsum only
   Answer: B. Silicates produce C-S-H, the strength-giving gel. (Key point: C-S-H origin)

2. Ettringite and monosulphate come from...
   A. C3A + gypsum
   B. C3S
   C. Sand
   D. Water only
   Answer: A. Ettringite needles form early and later convert to monosulphate. (Key point: Ettringite)

3. Why does strength rise with age?
   A. Products fill space so capillary pores shrink
   B. Water increases
   C. Pores grow
   D. CH dissolves
   Answer: A. Greater gel-space ratio with hydration raises strength. (Key point: Gel-space ratio)

4. Higher w/c gives...
   A. Fewer capillary pores
   B. More capillary pores, lower strength, higher permeability
   C. Stronger paste
   D. Less porosity
   Answer: B. Extra water leaves pores. (Key point: w/c effect)

## Related in Concrete Technology and Masonry Structure

- [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
- [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

## Related in other subjects

- [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×
- [Precautions while applying cement mortar](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Aprecautions-applying-cement-mortar) — Civil Engineering Materials, chapter 6, asked 3×
- [Why pozzolana is used in PPC](https://www.bctengineer.com/question.md?key=semester_3%2Fcivil_engineering_materials%3Awhy-pozzolana-in-ppc) — Civil Engineering Materials, chapter 5