When testing the durability of building stones against weathering, the ‘crystallization test’ is performed using sodium sulphate. What specific natural weathering mechanism does this laboratory test simulate?
- AThe chemical dissolution of calcium carbonate by acid rain.
- BThe mechanical disruptive force caused by the freezing and expansion of water in the stone's pores.Correct
- CThe abrasive action of wind-blown sand particles on the exposed face.
- DThe thermal expansion and contraction due to severe diurnal temperature variations.
Solution
Concept
Salt crystallization test for rock durability
Formula
NONE
Step-by-step solution
1. The sodium sulphate (or magnesium sulphate) crystallization test evaluates rock soundness and weathering resistance. 2. Submerging stone specimens in salt solution followed by drying causes salt crystals to grow within the pores. 3. Salt crystal expansion exerts mechanical outward pressure on pore walls, replicating the expansive stress exerted by freezing ice. 4. Thus, the test simulates the mechanical disruptive force of freezing and expansion of water in stone pores. Answer: B
Explanation
The sodium sulphate crystallization test simulates the destructive physical forces of frost action in rock pores through accelerated salt crystallization. As the salt solution evaporates and crystallizes within capillary pores, crystal growth exerts expansive pressure identical to freezing ice. This standardized soundness test evaluates the resistance of building stones against mechanical weathering.
Why the other options are wrong
A) Chemical dissolution by acid rain is evaluated using acid solubility tests, not salt crystallization. C) Wind-blown sand abrasion is evaluated using mechanical abrasion tests. D) Diurnal thermal expansion is tested via rapid cyclic heating and cooling tests.
Shortcut
Salt crystallization test = Laboratory simulation of freeze-thaw frost action.
Common mistake
Confusing salt crystallization with chemical dissolution by acid rain or thermal shock.
Correct answer: B) The mechanical disruptive force caused by the freezing and expansion of water in the stone's pores.