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Hydration of cement and role of compounds

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.

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?

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)

2C3S+6H→C3S2H3+3Ca(OH)22C_3S + 6H \rightarrow C_3S_2H_3 + 3Ca(OH)_2 2C2S+4H→C3S2H3+Ca(OH)22C_2S + 4H \rightarrow C_3S_2H_3 + Ca(OH)_2 C3A+6H→C3AH6(with gypsum: ettringite)C_3A + 6H \rightarrow C_3AH_6 \quad (\text{with gypsum: ettringite})

The calcium silicate hydrate (C3S2H3C_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)2Ca(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

CompoundRateContribution
C3_3S (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
C2_2S (di-calcium silicate)SlowLater strength, after 28 days and up to a year; low heat
C3_3A (tri-calcium aluminate)InstantaneousFlash set unless gypsum is added; small strength at 1 day; high heat; weak against sulphates
Strength
  |                              ........ C2S (keeps rising,
  |                      ......''           overtakes later)
  |      _______________.'______________ C3S (levels off
  |     /          ...''                     after ~28 days)
  |    /      ...''
  |   /  ...''
  |  /.''
  +-------------------------------------> age (days)
   1   7   28        90           365

Thus C3_3S is responsible for the early strength and C2_2S for the long-term strength. By choosing the proportions of these compounds, rapid-hardening, low-heat or sulphate-resisting cements are produced.

✓ Answer checked by Claude Opus 5.5 (high effort)

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