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Capillary pores: represent that part of the gross volume which has not been
filled by the products of hydration.
At w/c ratio> 0.38 the volume of gel is not sufficient to
fill all the space available to it. So there will be some volume of
capillary pores left even after hydration has been completed. The volume of capillary pores
decreases with the progress of hydration whereas the total volume of gel pores
increases alongwith the volume of gel as the hydration proceeds.
Size of capillary pores is of the order of 10 to 50 nm,
although it may be larger (3 to 5 mm) for higher w/c ratio.
Larger voids affect strength and
permeability, whereas smaller
voids impact shrinkage
Interconnected capillary pores are
primarily responsible for permeability & frost vulnerability of hardened
cement paste.
By choice of appropriate w/c ratio &
sufficient moist curing, the continuity of capillaries can be broken by
gel.
However, if w/c ratio is > 0.7,
even complete hydration will not produce enough gel to block capillaries i.e. an
impermeable concrete can not normally be made.
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High
Permeability
(Capillary Pores Interconnected in C-S-H Framework)
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Low-Permeability
CapillaryPores (Segmented and only Partially Connected in C-S-H Framework)
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The area represented by blue colour
represents " Capillary pores" spread in framework of C-S-H
(shown in black & white ). Note the interconnectivity of pores in Ist
figure.
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Schematic diagram of concrete paste microstructure
at the boundary of an air void:
The solid portion of the
hydrated cement gel is depicted as small black spheres. The interstitial
spaces between the spheres are the gel
pores. The capillary pores are denoted with a “C.”
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