Civil Engineering Reference
In-Depth Information
21.3.4
Expansive sulfobelite cements
Sulfobelite cements contain belite ( -C
2
S), calcium sulfoaluminate (C
4
A
3
), and calcium
sulfate (C or C H
2
) as their main constituents. They may be produced in an expansive
or non-expansive form by varying the proportions of these constituents. In general, the
capacity of the cement to expand will increase with increasing content of sulfoaluminate
and calcium sulfate. At the same time the content of free lime in the binder must be kept
low. For additional data on sulfobelite cements see section 4.2.
21.3.5
Other expansive cements based on ettringite formation
Expansive calcium sulfoaluminate cement contains the phases C
4
A
3
, C
5
S
2
(sulfospurrite), and C (anhydrite) in variable ratios. Mixes of this cement made with
relatively low water/cement ratios exhibit expansion due to ettringite formation,
associated with an uptake of water from the environment. For additional data on calcium
sulfoaluminate cements see section 4.5.
Expansive sulfoferrite cement consists of ordinary Portland clinker, calcium sulfate,
and a separately produced sulfoferrite-bearing clinker, which contains the high-basicity
calcium sulfoferrite phase C
3
FS as its main constituent. The expansion in the hydration of
this cement is due to the formation of a ferric AFt phase (ferric ettringite, C
3
F.3C .31H).
For more information on calcium sulfoferrite cements see section 15.6.
Wu and Wang (1980) developed an expansive cement on the basis of alunite. It
consists of Portland clinker, anhydrite, fly ash or blast furnace slag, and natural alunite
rock [with 20-50% alunite, K
2
SO
4
.Al
2
(SO
4
)
3
.4Al(OH)
3
]. The expansion of such a
cement is due to the formation of ettringite from Al
3+
and SO
4
2−
ions from the alunite
and calcium hydroxide liberated in the hydration of tricalcium silicate present in the
clinker. Most of the ettringite is formed within the first few days of hydration, but its
formation continues slowly even after 7 days. Owing to the rather good solubility of
alunite, the ettringite phase is formed predominantly or exclusively in a through-solution
reaction, and as a result the free expansion of the hardened cement paste is comparatively
small. The ettringite precipitates in the free pore space of the paste and improves its
impermeability and strength at later ages. The texture and properties of the paste are
improved if the hydration takes place under restraint.
An expansive cement consisting of a combination of calcium aluminate cement and
gypsum, called high-alumina/gypsum expansive cement, has been developed (Xue
et al.,
1986; Stark and Chartschenko, 1997). At sufficiently high gypsum contents—that is,
above about 40%—ettringite and hydroaluminate gel are formed as the sole products of
hydration:
(21.4)
