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Difficulty: MediumChemistry in Daily Life and Industrial Applications

Consider the following statements regarding the raw materials and chemical compounds involved in the manufacture and setting of Portland cement:

1. Gypsum (CaSO4⋅2H2OCaSO_4 \cdot 2H_2O) is added to cement clinker during final grinding to retard the rate of setting.
2. Dicalcium silicate (2CaO⋅SiO22CaO \cdot SiO_2) hydrates rapidly with water and is primarily responsible for the early strength development of cement within the first 24 hours.
3. Tricalcium aluminate (3CaO⋅Al2O33CaO \cdot Al_2O_3) undergoes rapid hydration and causes flash setting if setting retarders are omitted.

Which of the statements given above is/are correct?

  1. 1 and 3 onlyAnswer
  2. B
    2 only
  3. C
    1 and 2 only
  4. D
    1, 2 and 3

Answer

Statements 1 and 3 are correct.
Gypsum is added to cement clinker to prevent rapid flash setting caused by tricalcium aluminate (C3AC_3A). Tricalcium silicate (C3SC_3S), not dicalcium silicate (C2SC_2S), is responsible for early strength development.

Step-by-Step Solution

1
Analyze Statement 1 regarding Gypsum.
Gypsum (CaSO4⋅2H2OCaSO_4 \cdot 2H_2O) is interground with cement clinker (around 2–3%) specifically to control and retard the rate of setting, allowing sufficient time for mixing, transporting, and placing concrete. Thus, Statement 1 is correct.
Gypsum reacts with tricalcium aluminate to form calcium sulfoaluminate (ettringite), which coats aluminate grains and retards early hydration.
2
Analyze Statement 2 regarding Dicalcium Silicate (C2SC_2S).
Tricalcium silicate (3CaO⋅SiO23CaO \cdot SiO_2 or C3SC_3S) hydrates rapidly and is responsible for early strength development (7 to 14 days). Dicalcium silicate (C2SC_2S) hydrates slowly and contributes to long-term progressive strength (beyond 28 days). Thus, Statement 2 is incorrect.
Confusing the hydration rates and strength contribution timelines of C3SC_3S and C2SC_2S is a common error in industrial chemistry.
3
Analyze Statement 3 regarding Tricalcium Aluminate (C3AC_3A).
Tricalcium aluminate (C3AC_3A) reacts violently and instantaneously with water, generating a high heat of hydration and leading to immediate hardening known as 'flash set'. Thus, Statement 3 is correct.
Without gypsum, C3AC_3A leads to premature flash setting, rendering the cement unworkable.

Key Concept

Chemical Composition and Hydration Reactions of Portland Cement
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Topics

Chemistry in Daily Life and Industrial Applications

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