The addition of CaCl&sub2; in concrete results in which of the following? (i) increased shrinkage (ii) decreased setting time (iii) decreased shrinkage (iv) increased setting time
Correct Answer: D. Only (i) and (ii)
📚 Detailed Explanation: CaCl2 Increases Shrinkage AND Decreases Setting Time
Why D (Only i and ii) is correct: Calcium chloride has two well-documented effects on concrete: it decreases setting time (because it is an accelerator) and it increases shrinkage (because the accelerated and modified hydration microstructure has a higher potential for drying shrinkage). Effects (iii) and (iv) are the opposite of what CaCl2 actually does, so they are wrong.
Analysis of All Four Statements
| Statement | True or False? | Explanation |
|---|---|---|
| (i) Increased shrinkage | ✓ TRUE | CaCl2 alters the hydration microstructure; produces more capillary pores with specific geometry; increases drying shrinkage by 10–25% compared to non-accelerated concrete |
| (ii) Decreased setting time | ✓ TRUE | CaCl2 is an accelerator by definition; it accelerates C3S and C3A hydration; initial setting time reduced from ≈45–60 min to ≈20–30 min with 2% CaCl2 |
| (iii) Decreased shrinkage | ✗ FALSE | Opposite of (i); CaCl2 INCREASES shrinkage, not decreases it |
| (iv) Increased setting time | ✗ FALSE | Opposite of (ii); CaCl2 DECREASES setting time; this would describe a retarder, not CaCl2 |
Why CaCl2 Increases Shrinkage
| Mechanism | Detail |
|---|---|
| Accelerated C3S hydration | More rapid formation of C-S-H gel and Ca(OH)2; the rapid precipitation creates a finer, more porous microstructure |
| Modified pore structure | CaCl2-accelerated concrete has more gel pores; gel pores hold more adsorbed water; loss of this water on drying causes more shrinkage |
| Formation of calcium chloroaluminate | CaCl2 reacts with C3A to form calcium chloroaluminate; alters paste microstructure and shrinkage characteristics |
Summary of CaCl2 effects:
Setting time: DECREASES (accelerator effect)
Shrinkage: INCREASES (by 10–25%)
Dosage limit: 2% by wt. cement (IS 456 Cl- limit)
Banned in: Pre-stressed concrete (stress-corrosion cracking risk)
Setting time: DECREASES (accelerator effect)
Shrinkage: INCREASES (by 10–25%)
Dosage limit: 2% by wt. cement (IS 456 Cl- limit)
Banned in: Pre-stressed concrete (stress-corrosion cracking risk)
- CaCl2 → (i) increased shrinkage and (ii) decreased setting time — both correct.
- CaCl2 does NOT decrease shrinkage or increase setting time — those are retarder effects, not accelerator effects.
- Typical dosage restricted to 2% by weight of cement (IS 456); prohibited in PSC.