--- title: "Hydration of cement and role of compounds" subject: "Concrete Technology and Masonry Structure" chapter: 1 key: "semester_5/concrete_technology_and_masonry_structure:hydration-compounds-strength" source: https://www.bctengineer.com/q/semester_5/concrete_technology_and_masonry_structure/hydration-compounds-strength site: BCT Engineering --- # Hydration of cement and role of compounds Concrete Technology and Masonry Structure · Chapter 1: Introduction to concrete and concrete materials - Asked in: 2067 Magh (old course), 1+4 marks; 2067 Ashadh (old course), 2+3 marks; 2065 Shrawan (old course), 5 marks (asked 3 times) - 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/hydration-compounds-strength - Whole chapter: https://www.bctengineer.com/md/semester_5/concrete_technology_and_masonry_structure/chapter-1 ## Question Explain hydration of cement. How do the different compounds of cement (tri-calcium silicate, di-calcium silicate, tri-calcium aluminate) play a role in strength gaining of concrete? ## Short answer Hydration is the reaction of cement with water forming C-S-H gel (main strength) and Ca(OH)2. C3S hydrates fast, giving early strength up to 28 days; C2S hydrates slowly, providing long-term strength; C3A reacts instantly, causes flash set and adds little strength, controlled by gypsum. ## Answer **Hydration of cement** is the chemical reaction between the cement compounds and water that produces new hydrated compounds, which set and harden into a rigid, strong mass. It is exothermic (it releases heat). #### Hydration reactions (simplified) $$ 2C_3S + 6H \rightarrow C_3S_2H_3 + 3Ca(OH)_2 $$ $$ 2C_2S + 4H \rightarrow C_3S_2H_3 + Ca(OH)_2 $$ $$ C_3A + 6H \rightarrow C_3AH_6 \quad (\text{with gypsum: ettringite}) $$ The calcium silicate hydrate ($C_3S_2H_3$, called **C-S-H gel**) is the main binding product and makes up about 50-60 per cent of the volume of solids in the paste. Calcium hydroxide, $Ca(OH)_2$ (about 20-25%), is crystalline; it adds little strength but keeps the concrete alkaline. About 23 per cent water (by mass of cement) is chemically combined and about 15 per cent is held as gel water, so roughly 38 per cent (w/c about 0.38) is needed for complete hydration. #### Role of the compounds in strength gain | Compound | Rate | Contribution | |---|---|---| | C$_3$S (tri-calcium silicate) | Fast (hours to 28 days) | Early strength up to 7 days and a large part of the 28-day strength; high heat | | C$_2$S (di-calcium silicate) | Slow | Later strength, after 28 days and up to a year; low heat | | C$_3$A (tri-calcium aluminate) | Instantaneous | Flash set unless gypsum is added; small strength at 1 day; high heat; weak against sulphates | ```text Strength | ........ C2S (keeps rising, | ......'' overtakes later) | _______________.'______________ C3S (levels off | / ...'' after ~28 days) | / ...'' | / ...'' | /.'' +-------------------------------------> age (days) 1 7 28 90 365 ``` Thus C$_3$S is responsible for the early strength and C$_2$S for the long-term strength. By choosing the proportions of these compounds, rapid-hardening, low-heat or sulphate-resisting cements are produced. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Hydration of cement is: A. An exothermic reaction of cement compounds with water forming hardened products B. A physical drying process C. An endothermic reaction D. Evaporation of water from paste Answer: A. Hydration releases heat and forms binding products. (Key point: Definition) 2. The main strength-giving hydration product is: A. Ca(OH)2 B. C-S-H gel C. Gypsum D. Free lime Answer: B. C-S-H is about 50-60% of paste volume and the binder; Ca(OH)2 adds little strength. (Key point: Main product) 3. About how much water (w/c) is needed for complete hydration? A. 0.23 only B. 0.60 C. 0.38 D. 1.0 Answer: C. About 23% chemically combined plus 15% gel water gives 0.38. (Key point: Water requirement) 4. Long-term strength after 28 days comes mainly from: A. C3S B. C3A C. Gypsum D. C2S Answer: D. C2S hydrates slowly and gains strength up to a year. 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