---
title: "Three phase system; binding medium phase"
subject: "Concrete Technology and Masonry Structure"
chapter: 2
key: "semester_5/concrete_technology_and_masonry_structure:three-phase-binding-medium"
source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/three-phase-binding-medium
site: BCT Engineering
---

# Three phase system; binding medium phase

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

- Asked in: 2072 Kartik, 6 marks
- 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/three-phase-binding-medium
- Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-2

## Question

Explain concrete as a three phase system and explain the binding medium phase in detail.

## Short answer

Concrete is a three-phase system: aggregate, hydrated cement paste (binding medium), and transition zone. The binding medium phase contains C-S-H gel (50-60% of solids), calcium hydroxide (20-25%), calcium sulphoaluminates (15-20%), and voids (gel and capillary pores). It governs strength, shrinkage, creep, and permeability.

## Answer

Concrete is a **three-phase composite**: (1) the **aggregate phase**, (2) the **binding medium (hydrated cement paste, hcp)** and (3) the **transition zone** between them.

```text
 +----------------------------+
 | hcp  .------------.  ITZ   |
 |     / aggregate    \       |
 |     '--------------'       |
 +----------------------------+
```

The aggregate (70-75% of the volume) gives bulk and stiffness; the ITZ (10-50 µm thick) is the weak, porous layer around the aggregate.

#### Binding medium phase (hydrated cement paste)
Cement reacts with water to produce the paste, which hardens into a stone-like binder.

**Solid products of hydration**
- **C-S-H gel** (50-60% of solids): very fine, with a high surface area; gives strength through van der Waals forces; stable.
- **Calcium hydroxide** (20-25%): crystalline; contributes little to strength; provides alkalinity; weak against acids.
- **Calcium sulphoaluminates** (15-20%): ettringite and monosulphate; a minor role in strength; vulnerable to sulphate attack.
- **Unhydrated cement** remaining in the paste.

**Voids**
- Interlayer (gel) space in C-S-H (below 2.5 nm); capillary pores (10 nm-50 µm), which depend on the w/c and the degree of hydration; air voids (entrapped about 1-3 mm, entrained 50-200 µm).

**Water**: capillary water, adsorbed water, interlayer water and chemically combined water.

**Significance**
- Strength depends on the volume of capillary pores (gel-space ratio); a low w/c and good curing give a strong, impermeable paste.
- The paste governs shrinkage and creep (loss of adsorbed water from the C-S-H) and permeability.
- It binds the aggregates by adhesion, holds the steel, and protects it by its high pH.

## Check yourself

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

1. The binding medium phase of concrete is...
   A. Aggregate
   B. Hydrated cement paste
   C. ITZ
   D. Reinforcement
   Answer: B. Cement reacts with water to form the paste that binds aggregates. (Key point: Binding medium)

2. Which voids lie inside C-S-H?
   A. Entrapped air
   B. Interlayer (gel) space below about 2.5 nm
   C. Entrained air
   D. Capillary pores
   Answer: B. Interlayer space is the smallest void; capillary pores are larger. (Key point: Gel pores)

3. Paste strength depends on...
   A. Capillary pore volume via gel-space ratio
   B. Aggregate colour
   C. Sand fineness only
   D. Air temperature only
   Answer: A. Low w/c and good curing reduce capillary pores. (Key point: Strength factor)

4. How does paste protect steel?
   A. High pH from calcium hydroxide
   B. Chlorides
   C. Low pH
   D. Carbonation
   Answer: A. Alkalinity passivates the steel. (Key point: Steel protection)

## Related in Concrete Technology and Masonry Structure

- [Concrete as a three phase system](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Athree-phase-system) — Concrete Technology and Masonry Structure, chapter 2, asked 5×
- [Three phase system and transition zone](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Athree-phase-transition-zone) — Concrete Technology and Masonry Structure, chapter 2, asked 8×
- [Three phase system and structure of hcp](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Athree-phase-hcp-structure) — Concrete Technology and Masonry Structure, chapter 2, asked 6×
- [Three phase system; role of aggregate phase](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Athree-phase-aggregate-phase) — Concrete Technology and Masonry Structure, chapter 2
- [Stress-strain of paste, aggregate, concrete](https://www.bctengineer.com/question.md?key=semester_5%2Fconcrete_technology_and_masonry_structure%3Astress-strain-three-phase) — Concrete Technology and Masonry Structure, chapter 4

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