Civil Engineering Reference
In-Depth Information
21
Expansive cements
21.1
DEFINITION
Cement pastes generally undergo small but distinct volume changes upon hydration and
in the course of curing. Volume changes of this kind are usually of little significance, but
may become critical under some special conditions.
In the hydration of Portland cement (and most other cements) the anhydrous cement
constituents react with the mixing water to form hydration products. Even though the
total volume of solids increases under these conditions, the volume of the formed
hydrates is less than the sum of the volumes of the original cement and water consumed
in the hydration reaction:
In a typical ordinary Portland cement paste this
chemical shrinkage
amounts to 0.03-
0.08 ml per gram of cement at a
w/c
of ~0.50 (Köster and Odler, 1986; Justines
et al.,
1997; Paulini
et al.,
1997; Takahashi
et al.,
1997). As long as the cement paste is still
plastic the chemical shrinkage results in a corresponding decrease of its external volume.
After setting, when the paste is not able to deform freely anymore, the
external
shrinkage
(also called
atogenious shrinkage
) represents only a fraction of the total
chemical shrinkage that has occurred (Takahashi
et al.,
1997). The rest of the chemical
shrinkage (and the resultant volume change) manifests itself as a corresponding increase
of the internal porosity of the hardened material.
External shrinkage of the cement paste may be avoided by moist curing. Under these
conditions an uptake of water from the environment causes both a complete filling of the
pore system and even a limited overall expansion of the paste. However, upon subsequent
curing at normal humidity or in dry air the hardened paste gradually loses water and
exhibits a
drying shrinkage
that outweighs the original expansion that took place under
moist curing conditions. The resulting tensile stresses may cause cracks in the hardened
concrete, especially under restrained conditions.
If the hydration of the paste takes place without moist curing the stresses in the set
paste caused by chemical shrinkage combine with those caused by drying shrinkage, and
the probability of crack formation increases correspondingly. Figure 21.1 shows
schematically the volume changes taking place in an ordinary cement due to hydration
and curing.
Expansive cements are inorganic binders that generate expansive (compressive)
stresses in the hardened paste in the course of hydration, counter-acting the tensile
