During the preparation of lime mortar, the process of "slaking" is mandatory for quicklime (CaO). What is the primary physical hazard if under-slaked lime is inadvertently used in a plastering finish?
- AThe plaster will immediately suffer from severe efflorescence.
- BUnhydrated particles will slowly absorb moisture over time, expanding and causing popping or blowing of the plaster surface.Correct
- CThe mortar will set too rapidly leading to extreme shrinkage cracks.
- DThe plaster will completely lose its adhesive bond with the masonry substrate.
Solution
Concept
Lime slaking and plaster popping defects
Formula
CaO + H2O -> Ca(OH)2 + Heat (Volume Expansion)
Step-by-step solution
1. Quicklime (CaO) undergoes vigorous hydration (slaking) upon adding water, forming calcium hydroxide Ca(OH)2 with substantial heat release and volume increase. 2. If slaking is incomplete, unhydrated quicklime core particles remain embedded in the plaster mortar. 3. After application, these residual particles slowly absorb ambient moisture, expand, and blow off overlying plaster patches, leaving small conical craters. 4. This surface defect is termed popping or blowing of plaster. Answer: B
Explanation
Quicklime requires complete hydration (slaking) prior to use in mortar and plaster applications. If unslaked or under-slaked calcium oxide particles remain in the applied plaster, they slowly absorb moisture from the air and hydrate over weeks or months. This delayed hydration is accompanied by significant volume expansion, which blows off local plaster patches and creates conical craters known as popping or blowing.
Why the other options are wrong
A) Efflorescence is caused by the crystallization of soluble salts on the plaster surface. C) Under-slaked lime does not cause rapid setting; rather, it delays hydration expansion. D) Adhesive debonding across entire walls is caused by poor substrate preparation, not unslaked particles.
Shortcut
Under-slaked lime = Delayed moisture hydration -> Volume expansion -> Plaster popping/blowing.
Common mistake
Attributing plaster popping to rapid setting or efflorescence rather than delayed moisture-induced volume expansion.
Correct answer: B) Unhydrated particles will slowly absorb moisture over time, expanding and causing popping or blowing of the plaster surface.