Civil Engineering Reference
In-Depth Information
Some initial studies on the role of some accelerators for CSA have been
undertaken (Paglia et al., 2001).
Cements with very large amounts of yeelimite may present shrinkage
problems, and this effect is strongly correlated with porosity. In order to
prevent water evaporation causing drying shrinkage, it is advisable to decrease
the surface tension of water in paste pores, thereby lowering the capillary
tension within the pore structure. As discussed above, CSA cements require
larger amounts of water due to C
4
A
3
S
hydration, thus evaporation of water
is also facilitated. The use of some shrinkage-retardant admixtures will
modify pore structure in order to prevent the release of water (Ambroise et
al., 2009).
18.4.6 Performances of blended CSA/OPC cements
Blending cements may enhance performances and achieve new properties,
and moreover valorize waste materials. oPC with CSA cement as additive
are classified as type K cements which present expansive properties (Klein,
1966). The fundamental behavior of these cements is based on the following
reaction:
C
4
A
3
S
+ 8C
S
H
2
+ 6CH + 74H Æ 3C
6
A
S
3
H
32
[18.13]
In reaction [18.13], yeelimite is combined with gypsum and portlandite to
yield ettringite. Portlandite is released to the system mainly by C
3
S hydration
(from the oPC). The presence of portlandite in the system makes the reaction
pathway [18.13] more favorable than [18.1]. Consequently, each unit of
C
4
A
3
S
would produce three moles of ettringite instead of one, resulting in
a larger potential for expansion (Kurdowski and Thiel, 1981). Moreover, the
crystal size of ettringite is also affected by the presence of portlandite in
the hydrating environment. In fact, ettringite crystals coming from reaction
[18.13] are significantly smaller than those produced by the hydration of
yeelimite with gypsum, reaction [18.1], causing a different microstructure of
the hydrating paste (Mehta, 1973; Gastaldi et al., 2011). CSA/oPC (80/20
mass ratio) blends with limestone filler and superplasticizer were studied
for self-leveling applications and glass-fiber-reinforced composites (Pera and
Ambroise, 2004). Finally, CSA has also been used to improve the strength
of high-grade slag cement composed of granulated blast-furnace slag, oPC
and anhydrite. The only studied parameter was the gypsum ratio which led
to different hydrates (Michel et al., 2011).
BCSAF cement performances may also be enhanced by its blending with
oPC (Janotka and Krajci, 1999; Janotka et al., 2003; Pelletier et al., 2010,
Pelletier-Chaignat et al., 2011). BCSAF cements may present very short
initial setting times, however, by blending with 15 wt% of oPC setting
time is sufficiently enlarged for mortar-making technological procedures.
