--- title: "Solid phases in hcp; transition zone" subject: "Concrete Technology and Masonry Structure" chapter: 2 key: "semester_5/concrete_technology_and_masonry_structure:hcp-solids-transition-zone" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/hcp-solids-transition-zone site: BCT Engineering --- # Solid phases in hcp; transition zone Concrete Technology and Masonry Structure · Chapter 2: Structure of concrete - Asked in: 2082 Baisakh, 4+2 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/hcp-solids-transition-zone - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-2 ## Question Describe the solid phases present in hydrated cement paste and explain the effect of the transition zone on concrete properties. ## Short answer Solid phases in hcp: C-S-H gel (50–60%, main strength), CH (20–25%, alkaline, soluble), sulphoaluminates (15–20%), unhydrated grains. The ITZ is a porous, weak layer with oriented CH that lowers strength, modulus, and durability, causes nonlinear stress-strain, and increases permeability. Use pozzolans, low w/c, good curing to improve it. ## Answer #### Solid phases in hydrated cement paste (hcp) 1. **Calcium silicate hydrate (C-S-H)**: 50-60% of the volume of solids. A poorly crystalline gel (fibres, sheets), with a very high surface area; bonds by van der Waals forces and is responsible for the strength and other properties of the paste. 2. **Calcium hydroxide (CH, portlandite)**: 20-25%; large hexagonal crystals; low surface area so contributes little to strength; makes the paste alkaline (protects steel), but is soluble and attacked by acids. 3. **Calcium sulphoaluminates**: 15-20%; ettringite (needles), later monosulphate; small contribution to strength, and a source of sulphate attack. 4. **Unhydrated cement grains**: especially at low w/c; they hydrate later if moisture is available (long-term strength). Also present: voids (gel pores, capillary pores, air voids) and water. #### Effect of the transition zone on the properties of concrete The ITZ is a thin layer (10-50 µm) of paste around aggregate surfaces which has higher w/c, larger oriented CH and ettringite crystals, more porosity and microcracks. - It is the **weakest link**, so it limits the compressive and especially the tensile and flexural strengths. - It causes the **nonlinear stress-strain** curve (bond microcracks form at about 30-40% of ultimate load). - It **lowers the elastic modulus** below the value expected from the phases. - Its porosity gives paths for water, chlorides and sulphates, reducing **durability and increasing permeability**. - Improved by silica fume, fly ash, low w/c, superplasticiser and proper curing. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is true of calcium hydroxide in hcp? A. Main strength source B. Low surface area, little strength, gives alkalinity but soluble and acid-attackable C. Needle-shaped D. Insoluble Answer: B. CH (20-25%) forms large hexagonal crystals, protects steel via high pH but is attacked by acids. (Key point: CH properties) 2. Calcium sulphoaluminates (ettringite, monosulphate) are significant because... A. They give most strength B. They contribute little strength and are a source of sulphate attack C. They are 50-60% of solids D. They prevent corrosion Answer: B. They make up 15-20% with small strength role and sulphate vulnerability. (Key point: Ettringite) 3. Why is the stress-strain curve of concrete nonlinear? A. Aggregate is plastic B. Progressive bond microcracking in the ITZ C. Steel yielding D. Paste is viscous only Answer: B. Bond cracks form from about 30-40% of ultimate load and grow. (Key point: ITZ nonlinear) 4. How does the ITZ affect durability? A. Seals the concrete B. Porous interconnected zone lets water and ions in C. No effect D. Increases frost resistance Answer: B. It gives paths for water, chlorides and sulphates. 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