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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 2
Analysis of chloride diffusivity in concrete containing red mud
steady-state diffusion coefficients (Figure 8) are calculated.
A clear decrease in the flow of chloride ions is observed in samples
containing increasing amounts of waste. This is very positive because
it reveals a delay in the onset of the corrosion process caused by
the migration of chloride ions. These observations are in agreement
with findings reported SANTOS
[18] e AÏTCIN
[21], who showed the
tendency of supplementary cementitious materials (such as red mud)
to significantly reduce the mobility of chloride ions, reflecting the effect
of increased tortuosity and better pore diameter distribution resulting
from the pozzolanic reactions, which hinder ionic movement.
Moreover, red mud contains mineralogical phases such as sodium
aluminosilicates, known as sodalites, i.e., zeolite-type compounds
with an extremely high ion-exchange capacity, which makes red
mud a good absorber of heavy metals [22] and influences their
surface properties
[23], including the formation of compounds by
reaction with chloride ions. Other authors
[24] also cite the impor-
tance of the presence of aluminates, which play an important role
in anchoring the chloride ions, which would otherwise be free and
available to start the corrosion process.
The aspects discussed in the above paragraph are also reflected
in the steady-state and nonsteady-state diffusion coefficients of
chloride ions shown in Figure 8, both of which decreased due to
the addition of a higher red mud content.
Some authors [16, 18, 25] attribute reductions of the diffusion coef-
ficient to reductions in the water/binder (in this case, cement + red
mud). However, they report a decrease in total porosity as this ratio di-
minishes, which was not the case in the present study. Therefore, the
results cannot be correlated with a decrease in the water/binder ratio.
In an attempt to have more sensitive representations about the
benefits of the red mud use, the results of chloride penetration are
related with the service life of reinforced concrete. For this pur-
pose, the Fick’s second law of diffusion (equations E and F) was
used, according to proposed by recent studies [24, 26].
(5)
tD z
PC
s
.
)(2
=
(6)
o
S
o
cl
C C
C C z erf
-
-
-=
1 )(
where D
s
is the steady-state diffusion (cm
2
/year), t is the service life
(years), erf(z) is the Gaussian error function, CP (chloride penetra-
tion) is the depth at which the chloride reaches the threshold limit
concentration for rebar depassivation (cm), C
0
is the initial chloride
concentration (in this case, 0%), C
S
is the chloride concentration at
the surface (%), and C
Cl
is the chloride concentration as a function
of depth and time (%), by cement mass.
Additionally, in line with the literature [26, 27], several parameters
were fixed: C
S
= 1.8% and C
Cl
= C
dep
= 0.4% per weight of cement,
where C
dep
is the chloride threshold limit concentration to depas-
sivate the rebars (per weight of cement). The results obtained are
shown in Figure 9, which highlights the service life values for chlo-
ride penetration in structures with a concrete layer equal to 4 cm
thick (minimum required by standards for heavy environments, with
presence of chlorides). These values are best viewed in Figure 10.
The formulation without red mud shows a service life of 16.5 years.
As can be seen, the addition of red mud extends the service life of
the concrete to up to 35 years (twice the service life of the reference
concrete) to specimens containing 30 wt.% of red mud addition.
4. Conclusions
This research led to the following conclusions:
n
The time lag increases with increasing red mud content as a
result of the reduction in the relative amount of capillary pores;
n
The decrease in the interconnectivity between capillary pores
in samples containing red mud and the presence of typi-
cal mineralogical phases such as sodium aluminosilicates,
known as sodalites, are responsible for reducing the flow of
Figure 8 – Steady-state and nonsteady-state
diffusion coefficients of chloride ions as a
function of red mud content in concretes
cured for 28 days, estimated from migration tests
Figure 9 – Correlation between time and
depth of chloride penetration in concrete
containing different amounts of red mud
–
(to reach 0.4% Cl per weight of cement)