Admixtures MCQ with answer and solution

Concept

Identification of concrete ingredients, specifically distinguishing between main structural components (aggregates, cement, water) and chemical/mineral additives (admixtures).

Step-by-step solution

1. Understand the definition of a concrete admixture: materials added other than portland cement, water, and aggregates. 2. Evaluate option (A) CaCl2: It is a chemical admixture used as an accelerator. 3. Evaluate option (B) Fly ash: It is a mineral admixture (pozzolana) added to concrete. 4. Evaluate option (C) Gravel: It is a coarse aggregate, which forms the skeleton of concrete, not an admixture. 5. Evaluate option (D) Sugar: It is sometimes used as a chemical admixture (retarder) in very small quantities. 6. Conclude that gravel is the correct answer.

Explanation

Admixtures are chemical or mineral substances added to concrete batches during mixing to modify its properties such as workability, setting time, or strength. Calcium chloride is a set accelerator, fly ash is a mineral admixture, and sugar acts as a set retarder. Gravel, on the other hand, is a coarse aggregate used as a primary inert bulk material in concrete, not an admixture.

Why the other options are wrong

A: CaCl2 is a well-known accelerating chemical admixture. B: Fly ash is a widely used mineral admixture. D: Sugar acts as a set-retarding chemical admixture in controlled small doses.

Shortcut

Remember the basic constituents of concrete: Cement, Water, Fine Aggregate, and Coarse Aggregate (Gravel/Crushed Stone). Anything else added for modification is an admixture.

Common mistake

Students sometimes confuse sugar or calcium chloride as foreign contaminants rather than intentional admixtures.