Assertion (A): Flash set is the stiffening of the cement paste within a few minutes after mixing. Reason (R): Flash set occurs due to insufficient gypsum to control the rapid reaction of C3A with water.
- ABoth A & R are true and R is the correct explanation of ACorrect
- BBoth A & R are true but R is not the correct explanation of A
- CA is true but R is false
- DA is false but R is true
Solution
Concept
Flash Set of Cement
Formula
No formula available
Step-by-step solution
1. Understand that C3A reacts rapidly with water, causing rapid stiffening (flash set). 2. Note that gypsum is added to regulate C3A hydration; inadequate gypsum allows this rapid reaction to occur. 3. Conclude that Assertion (A) is true, Reason (R) is true, and R correctly explains A, making option A correct.
Explanation
Flash set refers to the fast stiffening of cement paste within minutes of mixing water. This happens because tricalcium aluminate (C3A) reacts very quickly with water. Gypsum is normally added to slow down this reaction; without enough gypsum, the rapid C3A reaction causes a flash set, making both the assertion and reason true.
Shortcut
Remember: "Flash = Fast (C3A), Gypsum = Guard." Without the Guard (Gypsum), C3A causes a Fast (Flash) set, so both A and R are true and R explains A.
Common mistake
Students often confuse flash set with false set, or fail to realize that insufficient gypsum directly causes the rapid reaction of C3A, leading them to select Option B instead of recognizing Reason (R) as the correct explanation for Assertion (A).
Correct answer: A) Both A & R are true and R is the correct explanation of A