Civil Engineering Reference
In-Depth Information
employed to characterize the compounds involved.
At appropriate liquid/solid ratios the resultant cementitious mix will exhibit the
consistency of a plastic paste or a thick suspension. Its
fluidity
(also called its
consistency
or
workability
) will depend on this ratio, and will improve with increasing
amounts of the added liquid. Prior to setting, such a system is called a
fresh cement
paste
. Over time the rheological properties of the paste will alter, and its viscosity will
increase as a consequence of chemical reactions taking place in the system. Ultimately
the paste will lose its plastic properties and will convert to a non-plastic porous solid
body. This conversion is called
setting
. The
setting time
—that is, the time between
mixing and loss of plasticity—depends on the quality of the cement, on the initial
liquid/cement ratio (it lengthens as this ratio increases), on the existing temperature (it
shortens as the temperature increases), on the presence of chemical admixtures, and on
other factors. The product of this reaction is called
set cement paste
.
As the chemical reactions in the paste will continue even after setting, the hardness and
strength of the paste—which are low immediately after setting—will increase, and a
hardened cement paste
will be formed. Ultimately the hardening reaction in the paste
comes to an end, owing to the complete consumption of the original binder or insufficient
amounts of mixing liquid, and a
mature hardened cement paste
results.
The observed changes in consistency, setting, and hardening are the result of physico-
chemical reactions taking place in the cement paste. In many but not all cementitious
systems the formation of reaction products is associated with the consumption of the
liquid phase (usually water) and thus with the decline of the actual liquid/solid ratio. In
the setting/hardening process aggregates of solids are created in which the newly formed
reaction products act as a glue, keeping the residua of the non-reacted material together.
Initially these aggregates are not interconnected, and the paste preserves its fluid or
plastic consistency, even though its viscosity increases. Eventually, however, the solid
phases interconnect, and a three-dimensional solid network develops, causing the loss of
fluidity or plasticity of the paste. The strength of the material—that is, its ability to resist
external mechanical forces—subsequently increases as the chemical reactions continue,
and thus the solid network becomes denser and more interconnected.
In cement suspensions with an excessively high liquid/solid ratio the relevant chemical
reactions will still take place, but the amount of reaction products formed will be too
small to result in the creation of a three-dimensional solid network and thus cause setting.
By contrast, if the amount of the liquid phase is too low, it may not be sufficient to wet
the surface of the cement particles, to fill the intraparticular space, and to yield a system
of paste consistency. Nevertheless, a strength development may also be observed here if
the material has been sufficiently compacted.
The chemical reactions causing setting and hardening may be many and varied. In the
simplest case, setting and hardening of a mix of plastic consistency occurs as a
consequence of the loss of part of the liquid present, for example by evaporation, or by
absorption by a porous base. The particles of the binder—initially separated spatially—
enter into mutual contact, and bonds develop between them, causing a loss of plasticity
and a development of strength. Such bonds may be created as a consequence of
recrystallization, in which the area of the existing solid/liquid interface declines and—as
a result—the free enthalpy of the system also declines. Examples of such cementitious
