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Combined/bound
water is chemically combined water forming a definite part of hydrated compounds. It is estimated
that about 23% by weight of cement is required
for chemical reactions.
Free
water: is the water present in the capillaries-it is available for hydration & it is beyond the range of the
surface forces of the solid phase.
Gel
water: water present in the gel pores-it is not available for hydration. It is estimated that about 15% by weight
of cement is required to fill up the gel pores.
Bound water & gel water are
complementary to each other.
If the quantity of water is insufficient to fill up the gel pores, the
formation of gel itself will stop & if the formation of gel stops there is
no question of gel-pores being present.
During the hydration process, water is chemically
combined with cement. The products of hydration, however, do not form a
solid mass. Rather, their physical appearance is more like steel wool with a
lot of empty space between the crystals. Within this mass lie both
hydrated and unhydrated cement. Also within this mass is the water
that is physically bonded (gel water).
The physically bonded water exists in the spaces between hydrated cement crystals.
For cement to hydrate, the cement grains must come in
physical contact with water. Although
the pore structure of cement paste is microscopic in size, its relative volume
can be quite large. There is a lot of space in which water can be present
without having every molecule of water in direct contact with cement.
Therefore, to ensure contact between unhydrated cement and water there must be
more water present than the minimum required for complete chemical bonding with
the cement.
It has been determined that the amount of water for
complete hydration is equal to the chemically bonded water plus extra
water (physically bonded) that is required to maintain contact with all
the cement. In addition, there must be an accounting for the water lost
from the system through evaporation. This would require the water cement
ratio to be something higher than 0.38
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