Civil Engineering Reference
In-Depth Information
19
Cements with reduced hydration heat
evolution
The hydration of inorganic binders is an exothermic process, accompanied by the release
of heat of hydration. As a consequence, the temperature of the cement paste (or
mortar/concrete mix) increases in the course of hydration. At the same time the generated
heat is also carried off into the environment.
The generation of heat of hydration may cause problems, if bulk concrete structures are
being erected. Here—after placing the fresh mix—the temperature of the concrete may
increase significantly, owing to the very low rate at which the generated heat is
eliminated into the environment. Upon subsequent cooling, thermal gradients that
develop within the hardened concrete generate tensile stresses, which may cause cracks
within the material. To minimize such danger, measures must be taken to reduce the
amount of heat liberated in the hydration. This may be done by selecting a binder with a
low heat of hydration, and by reducing the amount of the binder within the mix as far as
possible.
Another factor of importance is the rate at which the heat of hydration is released. In
general, a faster rate of heat release will result in a higher maximum temperature of the
concrete, and will increase the risks of crack formation. The rate at which the hydration
heat is released, just like the total amount of heat generated, will depend on the phase
composition of the binder. However, in addition to the composition of the binder, the rate
of heat release will also depend on its fineness, and may be reduced by a coarser grinding
of the cement.
Additional factors that affect the increase of temperature within the concrete body and
the maximum temperature achieved include:
• the composition of the fresh concrete mix;
• the presence of admixtures that retard or accelerate the progress of hydration;
• the starting temperature of the mix;
• the ambient temperature;
• the geometry of the concrete body to be produced.
In some instances cooling of the concrete mix (for example by adding crushed ice, or by
the use of pipes with circulating cold water, incorporated into the concrete mix) may be
indicated, to prevent excessive temperature increase within the hardening concrete. If ice
is added, it must be completely melted prior to setting, and the mix must be remixed. The
addition of ice will increase the original water/cement ratio, and this must be taken into
consideration.
P
ortland cement
is a multi-phase binder in which the various phases possess different
