Freeze-thaw weathering is common in mountainous areas where the temperature is around freezing point. This type of weathering is caused by the expansion and contraction of water within porous rocks. As ice crystals form, they grow larger, attracting liquid water from the surrounding pores. The ice crystal growth weakens the rocks which can then crack, exfoliate or shatter with the stresses imposed.
Chalk is a quite porous material, containing numerous air pockets.
Soaking the chalk replaced the air with water, setting up a situation where the chalk could be broken by rapidly changing the temperature of the materials.
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concrete damage from freeze thaw weathering.
Reference:
Brady, J. E. & Holum, J. R. (1993). Chemistry. The Study of Matter and Its Changes.
John Wiley & Sons, New York