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Water held in hydrated cement paste
The presence of water in hydrated cement has
been repeatedly mentioned. The cement paste is
indeed hygroscopic owing to the hydrophilic
character of cement coupled with the presence of
sub-microscopic pores. The actual water content
of the paste depends on the ambient humidity. In
particular, capillary pores, because of their comparatively
large size, empty when the ambient relative
humidity falls below about 45 per cent,
1.25
but water is adsorbed in the gel pores even at very
low ambient humidities.
We can thus see that water in hydrated cement
is held with varying degrees of firmness. At one
extreme, there is free water; at the other, chemically
combined water forming a definite part of
the hydrated compounds. Between these two categories,
there is gel water held in a variety of other
ways.
The water held by the surface forces of the gel
particles is called adsorbed water, and that part of
it which is held between the surfaces of certain
planes in a crystal is called interlayer or zeolitic
water. Lattice water is that part of the water of
crystallization which is not chemically associated
with the principal constituents of the lattice. The
diagrammatic representation of
Fig. 1.13
may be
of interest.
148
Fig. 1.13. Probable structure of hydrated silicates
1.53
Free water is held in capillaries and is beyond
the range of the surface forces of the solid phase.
There is no technique available for determining
how water is distributed between these different
states, nor is it easy to predict these divisions
from theoretical considerations as the energy of
binding of combined water in the hydrate is of the
same order of magnitude as the energy of binding
of the adsorbed water. However, investigations
using nuclear magnetic resonance suggest
that gel water has the same energy of binding as
interlayer water in some swelling clays; thus the
gel water may well be in interlayer form.
1.54
A convenient division of water in the hydrated
cement, necessary for investigation purposes,
though rather arbitrary, is into two categories:
evaporable and non-evaporable. This is achieved
by drying the cement paste to equilibrium (i.e.
to a constant mass) at a given vapour pressure.
The usual value is 1 Pa at 23 °C, obtained over
Mg(ClO4)2.2H2O. Drying in an evacuated space
which is connected to a moisture trap held at –79
°C has also been used; this corresponds to a vapour
pressure of 0.07 Pa.
1.48
Alternatively, the
evaporable water can be determined by the loss
upon drying at a higher temperature, usually 105
°C, or by freezing out, or by removing with a
solvent.
All these methods essentially divide water according
to whether or not it can be removed at a
certain reduced vapour pressure. Such a division
is perforce arbitrary because the relation between
149
vapour pressure and water content of cement is
continuous; in contrast to crystalline hydrates, no
breaks occur in this relationship. However, in
general terms, the non-evaporable water contains
nearly all chemically combined water and also
some water not held by chemical bonds. This water
has a vapour pressure lower than that of the
ambient atmosphere and the quantity of such water
is, in fact, a continuous function of the ambient
vapour pressure.
The amount of non-evaporable water increases
as hydration proceeds but, in a saturated
paste, non-evaporable water can never become
more than one-half of the total water present. In
well-hydrated cement, the non-evaporable water
is about 18 per cent by mass of the anhydrous material;
this proportion rises to about 23 per cent
in fully hydrated cement.
1.1
It follows from the
proportionality between the amount of non-evaporable
water and the solid volume of the cement
paste that the former volume can be used as a
measure of the quantity of the cement gel present,
i.e. of the degree of hydration.
The manner in which water is held in a cement
paste determines the energy of binding. For instance,
1670 J (400 calories) are used in establishing
the bond of 1 g of non-evaporable water,
while the energy of the water of crystallization of
Ca(OH)2 is 3560 J/g (850 cal/g). Likewise, the
density of the water varies; it is approximately
1.2 for non-evaporable water, 1.1 for gel water,
and 1.0 for free water.
1.24
It has been suggested
that the increase in the density of the adsorbed
water at low surface concentrations is not the result
of compression but is caused by the orientation,
or ordering, of the molecules in the adsorbed
phase due to the action of the surface
forces,
1.12
resulting in a so-called disjoining pressure.
The disjoining pressure is the pressure expected
to maintain the film of adsorbed molecules
against external action. A confirmation of
the hypothesis that the properties of adsorbed water
are different from those of free water is afforded
by measurements of the absorption of microwaves
150
by hardened cement paste.
1.64
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