--- title: "Three phase system and structure of hcp" subject: "Concrete Technology and Masonry Structure" chapter: 2 key: "semester_5/concrete_technology_and_masonry_structure:three-phase-hcp-structure" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/three-phase-hcp-structure site: BCT Engineering --- # Three phase system and structure of hcp Concrete Technology and Masonry Structure · Chapter 2: Structure of concrete - Asked in: 2081 Bhadra, 2+4 marks; 2079 Bhadra, 6 marks; 2079 Baisakh, 6 marks; 2076 Ashwin, 6 marks; 2075 Ashwin, 2+4 marks; 2074 Chaitra, 3+2 marks (asked 6 times) - 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-hcp-structure - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-2 ## Question Describe concrete as a three phase system with necessary sketches and explain the structure of the hydrated cement paste (hcp) phase and its effect on concrete properties. ## Short answer Concrete consists of aggregate, hydrated cement paste (hcp), and the ITZ. In hcp, C-S-H gel (50–60%) provides strength; CH (20–25%) gives alkalinity; sulphoaluminates (15–20%) contribute little. Capillary pores, determined by w/c and hydration, govern strength and permeability. Lower w/c improves strength and reduces shrinkage/creep. ## Answer #### Concrete as a three-phase system At the microscopic level, concrete consists of: 1. **Aggregate phase** (70-75% of volume): gravel or crushed stone and sand; provides volume, stiffness and dimensional stability. 2. **Hydrated cement paste (hcp)**: the binder phase. 3. **Transition zone (ITZ)**: a thin (10-50 µm) layer of paste next to the aggregate surfaces; the weakest part. ```text +---------------------------+ | paste ___ITZ___ paste | | / AGGREG- \ | | \ ATE / | | --------- | | ( ) air voids . . pores| +---------------------------+ ``` #### Structure of hydrated cement paste **Solid phases** - **Calcium silicate hydrate (C-S-H)**: 50-60% of the solids; a poorly crystalline gel with a huge surface area (100-700 m$^2$/g); the main source of strength, through van der Waals forces. - **Calcium hydroxide (CH, portlandite)**: 20-25%; large hexagonal crystals; low strength and surface area; soluble; gives alkalinity (pH about 12.5) but reduces chemical resistance. - **Calcium sulphoaluminates**: 15-20%; ettringite (needle-like) and monosulphate; play a minor role in strength, and monosulphate makes the paste vulnerable to sulphate attack. - **Unhydrated cement grains**: remain in the paste, especially when the w/c is low. **Voids in hcp** - **Interlayer space in C-S-H**: about 0.5-2.5 nm; too small to affect strength. - **Capillary pores**: 10 nm to 50 µm; spaces left by water not filled by hydration products. Their volume depends on the w/c and the degree of hydration, and they control strength and permeability. - **Air voids**: entrained air bubbles of about 50-200 µm; entrapped air voids from about 1 mm up to 3 mm. **Water in hcp**: capillary water, adsorbed water, interlayer water and chemically combined water. #### Effect on concrete properties - Strength is inversely related to the capillary porosity (Powers: $f = f_0 x^3$, where $x$ is the gel-space ratio). - A lower w/c and longer curing reduce capillary pores, so strength and durability rise. - Drying shrinkage and creep come from the loss of adsorbed and interlayer water from the C-S-H. - Permeability and durability depend on how continuous the capillary pores are; CH is vulnerable to acids and leaching, and the sulphoaluminates to sulphate attack. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which solid in hcp is chiefly responsible for strength? A. Calcium hydroxide B. Ettringite C. C-S-H gel (50-60% of solids) D. Unhydrated cement Answer: C. C-S-H is a high-surface-area gel bonding by van der Waals forces. (Key point: C-S-H role) 2. Which voids control strength and permeability of hcp? A. Interlayer space in C-S-H B. Capillary pores C. Chemically combined water D. Aggregate pores Answer: B. Capillary pores (10 nm to 50 µm) depend on w/c and hydration degree and control strength and permeability; interlayer space does not affect strength. (Key point: Capillary pores) 3. In Powers' relation f = f0 x^3, x is... A. Water-cement ratio B. Gel-space ratio C. Aggregate fraction D. Air content Answer: B. Strength rises with the gel-space ratio, i.e. falls with capillary porosity. (Key point: Powers relation) 4. Drying shrinkage and creep of paste arise mainly from... A. Loss of adsorbed and interlayer water of C-S-H B. Loss of aggregate water C. Ca(OH)2 melting D. Steel corrosion Answer: A. Removing water held by C-S-H causes shrinkage and creep. 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