What are the reasons of occurrence of dusting and scaling of concrete surface?

Dusting refers to the formation of loose powder arising from the disintegration of concrete surface. For dusty concrete surface, they can be easily scratched by nails. The cause of dusting can be related to finishing works carried out before completion of concrete bleeding. The working back of bleeding water to concrete surface produces a low strength layer with high water cement ratio. It may also arise owing to inadequate curing and inadequate protection of freshly placed concrete against rain, wind and snow.

Scaling is the occurrence of peeling of hardened concrete surface as a result of freezing and thawing effect. Scaling occurs in non-air-entrained concrete as air entrainment is normally adopted to protect concrete against freezing and thawing. Spaying of sodium chloride deicing salts is also another common cause of scaling.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.