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Fig. 1.9. Diagrammatic representation of
volume changes on hydration of cement paste
with a water/cement ratio of 0.42
The values given above are only approximate
but, had the total amount of water been lower
than about 42 ml, it would have been inadequate
for full hydration as gel can form only when sufficient
water is available both for the chemical reactions
and
for the filling of the gel pores being
formed. The gel water, because it is held firmly,
cannot move into the capillaries so that it is not
available for hydration of the still unhydrated cement.
Thus, when hydration in a sealed specimen
has progressed to a stage when the combined water
has become about one-half of the original water
135
content, no further hydration will take place.
It follows also that full hydration in a sealed specimen
is possible only when the mixing water is
at least twice the water required for chemical reaction,
i.e. the mix has a water/cement ratio of
about 0.5 by mass. In practice, in the example
given above, the hydration would not in fact have
progressed to completion because hydration stops
even before the capillaries have become empty. It
has been found that hydration becomes very slow
when the water vapour pressure falls below about
0.8 of the saturation pressure.
1.23
Let us now consider the hydration of a cement
paste cured under water so that water can be imbibed
as some of the capillaries become emptied
by hydration. As shown before, 100 g of cement
(31.8 ml) will, on full hydration, occupy 67.9 ml.
Thus, for no unhydrated cement to be left and no
capillary pores to be present, the original mixing
water should be approximately (67.9 – 31.8) =
36.1 ml. This corresponds to a water/cement ratio
of 1.14 by volume or 0.36 by mass. From other
work, values of about 1.2 and 0.38, respectively,
have been suggested.
1.22
If the actual water/cement ratio of the mix, allowing
for bleeding, is less than about 0.38 by
mass, complete hydration is not possible as the
volume available is insufficient to accommodate
all the products of hydration. It will be recalled
that hydration can take place only in water within
the capillaries. For instance, if we have a mix
of 100 g of cement (31.8 ml) and 30 g of water,
the water would suffice to hydrate
x
g of cement,
given by the following calculations.
Contraction in volume on hydration is:
0.23
x
× 0.254 = 0.0585
x
.
Volume occupied by solid products of hydration
is:
Porosity is:
136
and total water is 0.23
x
+
wg
= 30. Hence,
x
=
71.5 g = 22.7 ml and
wg
= 13.5 g. Thus, the
volume of hydrated cement is
0.489 × 71.5 + 13.5 = 48.5 ml.
The volume of unhydrated cement is 31.8 – 22.7
= 9.1 ml. Therefore, the volume of empty capillaries
is
(31.8 + 30) – (48.5 + 9.1) = 4.2 ml.
If water is available from outside, some further
cement can hydrate, its quantity being such
that the products of hydration occupy 4.2 ml
more than the volume of dry cement. We found
that 22.7 ml of cement hydrates to ocupy 48.5 ml,
i.e. the products of hydration of 1 ml of cement
occupy 48.5/22.7 = 2.13 ml. Thus 4.2 ml would
be filled by the hydration of
y
ml of cement such
that (4.2 +
y
)/
y
= 2.13; hence,
y
= 3.7 ml. Thus
the volume of still unhydrated cement is 31.8 –
(22.7 + 3.7) = 5.4 ml, which has a mass of 17 g.
In other words, 19 per cent of the original mass
of cement has remained unhydrated and can never
hydrate because the gel already occupies all
the space available, i.e. the gel/space ratio (see p.
274
) of the hydrated cement paste is 1.0.
It may be added that unhydrated cement is not
detrimental to strength and, in fact, among cement
pastes all with a gel/space ratio of 1.0 those
with a higher proportion of unhydrated cement
(i.e. a lower water/cement ratio) have a higher
strength, possibly because in such pastes the layers
of hydrated paste surrounding the unhydrated
cement grains are thinner.
1.24
Abrams obtained strengths of about 280 MPa
(40 000 psi) using mixes with a water/cement
ratio of 0.08 by mass, but clearly considerable
pressure is necessary to obtain a properly consolidated
mix of such proportions. Later on,
Lawrence
1.52
made compacts of cement powder
in a die assembly under a very high pressure (up
to 672 MPa (or 97 500 psi)), using the techniques
of powder metallurgy. Upon subsequent
137
hydration for 28 days, compressive strengths up
to 375 MPa (or 54 500 psi) and tensile strengths
up to 25 MPa (or 3600 psi) were measured. The
porosity of such mixes and therefore the ‘equivalent’
water/cement ratio are very low. Even higher
strengths, up to 655 MPa (or 95 000 psi), were
obtained using very high pressure and a high temperature.
The reaction products in these compacts
were, however, different from those resulting
from normal hydration of cement.
1.89
In contrast to these compacts which had an extremely
low water/cement ratio, if the water/cement
ratio is higher than about 0.38 by mass, all
the cement can hydrate but capillary pores will
also be present. Some of the capillaries will contain
excess water from the mix, others will fill by
imbibing water from outside.
Figure 1.10
shows
the relative volumes of unhydrated cement,
products of hydration, and capillaries for mixes
with different water/cement ratios.
138
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