HYDRATION:-
Clinker is anhydrous (without
water) having come from a hot kiln. Cement powder is also anhydrous if we
ignore the small amount of water in any gypsum added at the clinker grinding stage.
The reaction with water is
termed "hydration". This involves many different
reactions, often occurring at the same time. As the reactions proceed, the
products of the hydration process gradually bond together the individual sand
and gravel particles, and other components of the concrete, to form a solid
mass.
By the process of hydration
(reaction with water) Portland cement mixed with sand gravel and water produces
the synthetic rock we call concrete. Concrete is as essential a part of the
modern world as is electricity or computers.
HYDRATION IN CONCRETE:-
Concrete derives its strength by
the hydration of cement particles. The hydration of cement is not a momentary
action but a process continuing for long time. Of course, the rate of hydration
is fast to start with, but continues over a very long time at a decreasing rate
In the field and in actual work, even a higher water/cement ratio is used,
since the concrete is open to atmosphere, the water used in the concrete
evaporates and the water available in the concrete will not be sufficient for
effective hydration to take place particularly in the top layer.
If the hydration is to continue,
extra water must be added to refill the loss of water on account of absorption
and evaporation. Therefore, the curing can be considered as creation of a
favorable environment during the early period for uninterrupted hydration. The
desirable conditions are a suitable temperature and ample moisture.
Concrete, while hydrating,
releases high heat of hydration. This heat is harmful from the point of view of
volume stability. Heat of hydration of concrete may also shrinkage in concrete,
thus producing cracks. If the heat generated is removed by some means, the
adverse effect due to the generation of heat can be reduced. This can be done
by a thorough water curing.
Under normal conditions, once
cement and water are mixed the hydration process will begin and the concrete or
mortar will gain strength and increase in durability with the passage of time.
To some degree concrete is self-curing. However, the term "curing" has
come to mean more than "self-curing." In fact, curing has come to
mean managing the environment in which hydration takes place in order to
enhance and optimize the properties of hardened concrete.
The hydration
process is driven by three elements:
- temperature
- water
- the
availability of unhydrated cement
HEAT OF HYDRATION:-
It is the amount of heat released
or absorbed (i.e., change in enthalpy) when one mole of an anhydrous or a
partially hydrated salt combines with the required number of moles of water to
form specific hydrates.
Heat of Hydration for Concrete:-
The chemical process by which
cement, aggregates and water mix and form a new substance has distinct
properties.
The main product involved in the
binding of cement and water is heat, which is released when the concrete is
being hardened. This process is known as the heat of hydration. Heat of
hydration can be managed while designing and pouring concrete, during
the process of curing and hardening. However, if designers do not allow for the
heat, it can cause serious issues with cracking and can also disturb the
structural integrity of the concrete.
Chemical process of heat of hydration:-
The process of hydration begins
when the molecules of cement and water are mixed. Loss of mass on a molecular
level, necessarily results in an energy release. In this case, the bonding of
molecules results in an exothermic chemical reaction. This is known as the heat
of hydration.
Heat of hydration effects in concrete:-
* Heat cannot escape easily when the heat of hydration is not properly
controlled, especially with a large slab or structure.
* Heat of hydration can lead to very high internal temperatures
within a structure. This happens particularly in hot weather. This happens when
either a large amount of concrete is poured quickly, or due to a very high
cement to water ratio.
* During construction if the heat of hydration is not managed, it
can cause an expansion while the cement is in the process of hardening and
curing.
* If artificial or weather-related
cooling materials are combined with significant amount of heat during the
curing process, it can create significant cracks. This will further disturb the
structural integrity of the concrete due to cracks formed by expansion and
contraction of the concrete.
