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IBRACON Structures and Materials Journal • 2012 • vol. 5 • nº 2
Analysis of chloride diffusivity in concrete containing red mud
1. Introduction
The huge volumes of industrial waste produced today represent
one of the world’s greatest environmental problems, with an annu-
al volume of up to 25 million tons of waste generated in São Paulo
alone [1]. The red mud studied in the present paper is a by-product
of aluminum production from bauxite ore produced by the Bayer
process. The vast quantities of wastes call for the search for widely
consumed target products into which they can be incorporated,
such as civil construction.
The world’s production of bauxite in 2009 was 205 million tons,
and the main producing countries were Australia, China, Brazil,
Guinea, India and Jamaica. Ranking third in worldwide production
in 2009, Brazil produced 26.6 million tons of bauxite. It also has
the world’s third largest bauxite ore reserves (around 3.5 billion
tons), concentrated mainly in the northern part of the country (state
of Pará)
[2].
Roughly 0.3 – 1.0 tons of red mud waste are generated per ton of
aluminum produced. Brazil has discarded about 10.6 million tons/
year of caustic red mud in recent years and the worldwide genera-
tion of red mud exceeds 117 million tons/year
[3].
Approximately 35–40% of bauxite ore is discarded in the form of
strongly alkaline RM slurry
[4]. This mud contains about 60 vol. % of
solid content in the form of superfine particles. Therefore, this mate-
rial has a large surface área and a strong water absorbing capacity.
Alkaline matrices such as those provided by Portland cement in
mortars and concrete are commonly used in waste conditioning.
They are inexpensive, have an extensively documented history of
safe use, and are a draw-upon readily-accessible technology. Al-
kalinity greatly reduces the solubility of many hazardous inorganic
species and inhibits microbiological processes. Moreover, since
these matrices require water for hydration, they may readily incor-
porate wet wastes
[5] such as red mud.
The search for an economically and environmentally viable alterna-
tive has led to the study of red mud for various applications, such
as adsorbent for the removal of heavy metals from aqueous solu-
tions
[6], building materials such as bricks
[7], ceramics and tiles [8],
ceramic glazes [9], as polymer-based composites to replace wood
[10], iron-rich cement [11, 12], component of clinker [4, 11, 12], and
the addition to mortar and concrete formulations was also reported
[13]. The use as common building material has been suggested as
an alternative that ensures high rates of consumption
[14].
It is widely known that chloride ions cause local breakdown of the
passive layer and subsequent corrosion of reinforcing steel bars
(rebars) in concrete structures.
Several studies [15, 16, 17] report the use of migration tests to
evaluate the resistance of concrete to penetrating chloride ions.
Initially, these tests were used to evaluate this penetration based
on the total penetrated load, according to the ASTM C 1202/1992
standard, and to estimate the diffusion coefficient in steady-state
conditions, as suggested by ANDRADE
[15]. More recently, sev-
eral authors have used migration tests to calculate the diffusion
coefficient in nonsteady-state conditions [16, 17]. Depending on
the proposed objective, these tests may vary in terms of procedure
and parameters, but all are based on the induction of the move-
ment of ions under the action of an external electric field [18].
The migration test is based on the principle of applying a poten-
tial difference between two cells: one containing a chloride solu-
tion (cathode) and the other without chlorides (anode), which are
placed on each side of the concrete sample under analysis. The
externally applied electric potential forces the passage of chloride
ions through the concrete sample from the first (cathode) to the
second (anode) cell. By this method, the passage of chloride ions
through the specimen is induced by the electric current generated
by the potential difference of 12 volts applied to a direct current
source through the electrodes contained in each cell.
The positive cell (anode) is filled with distilled water to prevent cor-
rosion-induced deposition of chlorides. The negative cell (cathode)
is composed of a solution containing a concentration of 1 M of
sodium chloride (NaCl).
2. Materials and methods
2.1 Materials
Ordinary CP-II 32 Z Portland cement (OPC), according to the Brazil-
ian NBR 11578 standard, commercially available in São Carlos, Bra-
zil, was selected as reference in all the tests. The coarse aggregate
was dense, crushed granitic stone and the sand was supplied from a
river deposit commercially available in São Carlos, Brazil.
The red mud came from Poços de Caldas-MG and was supplied by
ALCOA Brazil. It is a mixture containing about 60% of solids, collect-
ed immediately after alumina recovery from the digestion process.
2.2 Methods
2.2.1 Materials characterization and Concrete Dosage
The materials characterization involved X-ray diffraction (Rigaku
Geirgeflex ME 210GF2 Diffractometer) and X-ray fluorescence
(Philips PW1480 X-ray Fluorescence Spectrometer) analyses,
while physical parameters such as the specific surface area (es-
timated by BET, using a Micrometrics Gemini 2370 V1.02 equip-
ment) and specific gravity (Helium Pycnometer Accupyc 1330
V2.01 from Micrometrics) were also determined. Similar determi-
nations were performed on sand and on the red mud.
The concrete formulation used as reference was prepared in a
1.0 : 1.5 : 1.3 : 0.5 (Portland cement + red mud : sand : coarse ag-
gregate : water) weight ratio. The mortar content was 65.8% and
the cement consumption was 540 kg/m
3
compared to a reference
mixture. Distinct concrete mixtures in which cement was partially
replaced by red mud (10, 20, and 30% in weight) were analyzed.
All the specimens were cylindrical. To perform the migration tests,
specimens of 50 mm in diameter and 100 mm in length were mold-
ed, from which 40 mm thick slices were cut, discarding the outer
top and bottom parts in order to minimize heterogeneities and en-
sure water saturation.
2.2.2 Chloride Migration Testing
The American ASTM C-1202/97 standard (Standard Test Method
for Electrical Indication of Concretes Ability to Resist Chloride Ion
Penetration) advocates the use of vacuum saturated samples be-
fore submission to migration tests. This procedure has been ad-
opted by other researchers [11, 16] o ensure that the penetration of
chlorides into the sample was caused predominantly by diffusion.
In this work, vacuum saturation was not used; instead, the samples