Binders — Which of the following is an air-binding (air-setting) material that hardens primarily in air rather than under water?

Difficulty: Easy

Correct Answer: Gypsum

Explanation:


Introduction / Context:
Binders in construction are classified by their setting mechanism. Air-binding materials harden by physical drying or carbonation in air, while hydraulic binders set and gain strength in the presence of water. This distinction guides material selection for different environments.


Given Data / Assumptions:

  • Focus on principal, primary setting behavior.
  • Common building binders: gypsum, lime, hydraulic cements.


Concept / Approach:
Gypsum plasters harden and develop strength primarily by recrystallization and drying in air; they are unsuitable for prolonged wet exposure. In contrast, hydraulic cements (including specialized acid-resistant cements) set under water. Quick lime must first be slaked to form lime putty; hardened lime mortars rely on slow carbonation in air (air lime), but the option listed is specifically “quick lime,” which is not the binder in its usable state.


Step-by-Step Solution:
Identify air-setting candidate: gypsum → sets and hardens in air, used for plastering.Exclude hydraulic binders: acid-resistant cement is a hydraulic system.Clarify quick lime: requires slaking; the active air-setting binder is the hydrated (fat) lime, not CaO itself.


Verification / Alternative check:
Practical experience: gypsum plasters fail in wet locations, confirming their air-binding nature.


Why Other Options Are Wrong:
Acid-resistant cement: designed to set under water/chemical exposure; not air-binding.Quick lime: not directly used as a binder until slaked; the air-setting form is hydrated lime (fat lime), which is not the option listed.All of these: incorrect because the list mixes air- and hydraulic-type materials.


Common Pitfalls:
Conflating “lime” generally with “quick lime.” Terminology matters for the setting mechanism.


Final Answer:
Gypsum

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