Civil Engineering Reference
In-Depth Information
According to Ramlochan et al. (2003, 2004), one of the major roles
of pozzolan in controlling DEF expansion is related to the availability of
reactive Al
2
O
3
, often found in SCMs such as fly ash, metakaolin or GGBS.
The release of additional alumina from SCMs favours the precipitation of
AFm instead of AFt (Taylor et al., 2001), as confirmed by the experimental
work of Ramlochan et al. (2004), by decreasing the SO
3
/Al
2
O
3
ratio. This
ratio is sometimes proposed to estimate the risk of DEF occurrence (Heinz
and Ludwig, 1987; Grabowski et al., 1992; Odler and Chen, 1996). The
presence of supplementary alumina moves the ratio away from the pessimum
(around 0.8 in mass, according to Grabowski et al., 1992, Heinz et al., 1999,
and Zhang et al., 2002), under the critical value for DEF to occur. Heinz
and Ludwig (1987) and Heinz et al. (1999) affirm that blended cements with
appropriate amounts of GGBS and siliceous fly ash must keep the SO
3
/Al
2
O
3
ratio under 0.45-0.55 to avoid DEF.
Particular effects of SCMs
Most SCMs show a beneficial effect in reducing DEF expansion when used
in sufficient amounts (Fig. 8.6). Their efficiency could be related to several
parameters, including their available aluminium content (MK, GGBS and
class F fly ash) (Ramlochan et al., 2003, 2004). For
GGBS
, the minimum
content varies between 25 and 35%, depending on the author (Fu, 1996;
Kelham, 1999; Ramlochan et al., 2003), but it can reach 40% (Santos Silva
et al., 2010) or higher levels when it is used with cements having very high
sulphate or alkali contents (Ramlochan et al., 2003). Santos Silva et al.
Fly ash
(high-
calcium)
GGBS
Fly ash
(low-calcium)
Silica fume
Metakaolin
100
80
60
40
20
0
-85%
-20
10 15 20 25 30 35 40 50
10 15 20 25 30 40 60
15 25 30
5 8 10 15 20
5 8 10 15 20
Mineral admixture content (%)
8.6
Reduction of DEF expansion (in %) due to the use of SCMs. Data
from Ramlochan et al. (2003), Ghorab et al. (1980), Santos Silva et al.
(2006, 2010), Fu (1996), Kelham (1999), Kurdowski and Duszak (2004).
