The followings are the harmful constituents of brick.
It causes crystallization and disintegration of brick particles on drying.
These are mainly chlorides and sulphates of calcium,magnesium,sodium and potassium causing dark greenish hue on brick surface on drying.It also cause brick to twist,fuse and warp.It absorb moisture which cause efflorescence.
It prevents mixing of clay thoroughly and uniformly.It makes brick porous and weak.
4.Vegetation and Organic Matter
These leave pores in brick on burning.
Excess lime cause brick to fuse and hence shape is lost.Lime in the form of lime stone and Kankar nodule very harmful.Because limestone(CaCO3) leaves CO2 in burning and in contact with water it gets hydrated and swells and cause the brick to lose shape.Kankar also cause similar type of damage to brick.but lime is very important in brick.So proper amount of lime should present in brick.