Civil Engineering Reference
In-Depth Information
(1.6)
In such instances the liquid phase just acts as a medium for ion transport, and does not
participate in the chemical reaction.
Finally, setting and hardening may also be caused by reactions that cannot be included
in any of the categories mentioned so far. An example of such a reaction is the
setting/hardening reaction in alkali silicate cements:
(1.7)
In the hardening/setting process the constituent(s) of the cement dissolve in the liquid
phase, and the reaction product(s) precipitate from it. This occurs because the solubility
of the reaction product(s) in the liquid phase is lower than that of the starting material(s).
If the rate-controlling step of this process is the precipitation of the reaction product
rather than the initial dissolution of the starting constituent, the liquid phase quickly
becomes oversaturated with respect to the reaction product, and precipitation occurs
randomly in the whole liquid volume. We call this a
through-solution reaction
. When,
conversely, the rate-controlling step is the dissolution of one of the starting constituents,
the reaction product tends to precipitate right at, or close to, the surface of the solid phase
that dissolves most slowly, before oversaturation can be reached in the whole volume of
the liquid phase. We call such a process a
topochemical reaction,
as the formation of the
end-product is confined to a specific location, rather than occurring randomly throughout
the whole volume of the paste.
All setting/hardening reactions of inorganic binders occur at ordinary or only
moderately elevated temperatures. All of them are exothermic: that is, they are associated
with a liberation of the heat of reaction. The amount of heat liberated in the
setting/hardening process may vary greatly in different cements, just like the kinetics of
the heat release. Periods of faster and slower rates of heat liberation may alternate. An
increased temperature, increased fineness of the binder and an initial agitation of the mix
generally increase the rate of reaction.
The volume of reaction products formed equals roughly, but not exactly, the sum of the
volumes of the solid and liquid phases entering the reaction. Usually a loss of a fraction
of the original volume takes place, and this phenomenon is known as
chemical
shrinkage
.
The volume of the hardened paste may decrease further upon losing free water located
in the pore system (by evaporation, for example), whereas it may increase if water is
taken up from the environment. However, this phenomenon may not be entirely
reversible, and may contain a reversible and an irreversible component.
