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Mechanical strength of cement gel
There are two classical theories of hardening or
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development of strength of cement. That put forward
by H. Le Chatelier in 1882 states that the
products of hydration of cement have a lower solubility
than the original compounds, so that the
hydrates precipitate from a supersaturated solution.
The precipitate is in the form of interlaced
elongated crystals with high cohesive and adhesive
properties.
The colloidal theory propounded by W.
Michaëlis in 1893 states that the crystalline aluminate,
sulfoaluminate and hydroxide of calcium
give the initial strength. The lime-saturated water
then attacks the silicates and forms a hydrated
calcium silicate which, being almost insoluble,
forms a gelatinous mass. This mass hardens
gradually due to the loss of water either by external
drying or by hydration of the inner unhydrated
core of the cement grains: in this manner
cohesion is obtained.
In the light of modern knowledge it appears
that both theories contain elements of truth and
are in fact by no means irreconcilable. In particular,
colloid chemists have found that many, if not
most, colloids consist of crystalline particles but
these, being extremely small, have a large surface
area which gives them what appear to be different
properties from the usual solids. Thus colloidal
behaviour is essentially a function of the size of
the surface area rather than of the non-regularity
of internal structure of the particles involved.
1.42
In the case of Portland cement, it has been
found that, when mixed with a large quantity
of water, cement produces within a few hours a
solution supersaturated with Ca(OH)2 and containing
concentrations of calcium silicate hydrate
in a metastable condition.
1.2
This hydrate rapidly
precipitates in agreement with Le Chatelier’s hypothesis;
the subsequent hardening may be due to
the withdrawal of water from the hydrated material
as postulated by Michaëlis. Following the
dormant period, precipitation of calcium silicate
hydrate and Ca(OH)2 continues.
Further experimental work has shown that the
calcium silicate hydrates are in fact in the form of
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extremely small (measured in nanometres) interlocking
crystals
1.20
which, because of their size,
could equally well be described as gel. When cement
is mixed with a small quantity of water, the
degree of crystallization is probably even poorer,
the crystals being ill-formed. Thus the Le
Chatelier–Michaëlis controversy is largely reduced
to a matter of terminology as we are dealing
with a gel consisting of crystals. Moreover,
the solubility of silica increases very substantially
at a pH above 10, so that it is possible for the
Michaëlis mechanism to operate initially and for
that of Le Chatelier later on. A more detailed
discussion of the two mechanisms is offered by
Baron and Santeray.
1.94
The term ‘cement gel’ is considered, for convenience,
albeit not correctly, to include the crystalline
calcium hydroxide. Gel is thus taken to
mean the cohesive mass of hydrated cement in its
densest paste, i.e. inclusive of gel pores, the characteristic
porosity being about 28 per cent.
The actual source of strength of the gel is not
fully understood but it probably arises from two
kinds of cohesive bonds.
1.27
The first type is the
physical attraction between solid surfaces, separated
only by the small (less than 3 nm) gel pores;
this attraction is usually referred to as van der
Waals’ forces.
The source of the second type of cohesion is
the chemical bonds. Because cement gel is of the
limited swelling type (i.e. the particles cannot be
dispersed by addition of water) it seems that the
gel particles are cross-linked by chemical forces.
These are much stronger than van der Waals’
forces but the chemical bonds cover only a small
fraction of the boundary of the gel particles. On
the other hand, a surface area as high as that of
cement gel is not a necessary condition for high
strength development, as high-pressure steamcured
cement paste, which has a low surface area,
exhibits extremely good hydraulic properties.
1.14
We cannot thus estimate the relative importance
of the physical and chemical bonds but there
is no doubt that both contribute to the very considerable
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strength of the hardened cement paste.
It has to be admitted that the understanding of
the cohesive nature of the hydrated cement paste
and its adhesion to aggregate is still imperfect. As
Nonat and Mutin
1.92
put it, the microstructure has
not been related in a general way to mechanical
properties.
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