Civil Engineering Reference
In-Depth Information
∑ The consumption of portlandite due to the pozzolanic reaction, which
implies that a smaller amount of CO
2
is required to carbonate the remaining
hydrates. It has been shown that the depth of carbonation is greater when
the amount of Ca(OH)
2
present is lower (Bier, 1986). Consequently the
presence of SCMs could result in more rapid carbonation.
∑ Modification of the porosity and permeability of concrete (Sections 8.2.1
and 8.2.2), which could lead to an improvement in the transport properties
of pozzolan concretes in certain conditions (active pozzolan, long curing
time, etc.). The denser microstructure, refined and more tortuous pores,
and reduced permeability encountered in pozzolan concretes tend to
slow down the carbonation rate.
Depending on the dominant effect, pozzolan concretes can be more or less
carbonated than control mixtures at a given age.
Although numerous papers comparing the carbonation of Portland cement
with and without SCMs have been published, generalizations remain difficult.
This could be partly due to different experimental conditions existing in the
literature, e.g. accelerated test vs. natural carbonation, driven by the CO
2
content and the relative humidity. According to Massazza (1998), accelerated
tests tend to overestimate the depth of carbonation of pozzolan concretes
compared to long exposure on site. So care should be taken when interpreting
the results of accelerated tests.
The overall trend found in the literature is that the depth of carbonation
increases with the use of pozzolan in concretes (A to B on Fig. 8.11)
(Massazza, 1998; Neville, 1995). However, some authors affirm that there
is no significant difference in carbonation depth if the concretes with or
without pozzolan have a similar strengths (Matthews, 1984; Hobbs, 1988;
Kokubu and Nagataki, 1989). This affirmation is not always supported,
as shown by Kim et al. (2007) with metakaolin and silica fume. All other
things being equal, the carbonation of pozzolan concretes increases relative
to control mixtures when the relative strength decreases (Shi et al., 2009).
Compressive strength of concrete over 50 MPa can significantly reduce
the depth of natural carbonation (indoor or outdoor), not only for ordinary
Portland cement (OPC) but also for fly ash concretes (Sulapha et al., 2003).
However, this is not always the case for accelerated conditions (CO
2
> 4%),
for which even high-strength concretes with very active pozzolans can remain
vulnerable to carbonation (Kim et al., 2007, 75 MPa concretes). For relatively
low pozzolan content, the differences might be negligible (Collepardi et al.,
2004), but for high pozzolan content, it could be necessary to decrease the
water-binder ratio to maintain the same carbonation performance (Cengiz
Duran, 2003). One important parameter that greatly influences the result is the
curing of concretes, which can be very problematic for pozzolan concrete.
Although concretes containing SCMs are sometimes considered as less
