Construction and Building Materials 133 (2017) 459–467
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Effects of red mud and Alkali-Activated Slag Cement on efflorescence in
cement mortar
Suk-Pyo Kang a, Seung-Jun Kwon b,⇑
a
b
Architectural Engineering, Woosuk University, Jincheon 27841, South Korea
Department of Civil and Environmental Engineering, Hannam University, Daejeon 306-791, South Korea
h i g h l i g h t s
Quantitative evaluation of red mud effect on efflorescence.
Various micro observation test results like EDA, XRD, SEM, and TGA.
The relationship between efflorescence and the results (ion concentration, porosity).
Verification of efflorescence mechanism through micro observation test.
Mechanical and durability performance evaluation.
a r t i c l e
i n f o
Article history:
Received 27 June 2016
Received in revised form 1 November 2016
Accepted 21 December 2016
Available online 28 December 2016
Keywords:
Efflorescence
AASC
Red mud
Alkali activator
Binder effect
a b s t r a c t
Recently AASC (Alkali Activated Slag Cement) is utilized for construction materials in order to reduce
environmental load like abundant CO2 emission. Red mud which is a byproduct from Bauxite ores process
has a strong alkali component containing 10.0–15.0% of Na2O, so that it can be used for an alkali activator
or retarder for cement hydration. This work presents an evaluation of efflorescence characteristics in
cement mortar with AASC and red mud. For the work, OPC (Ordinary Portland Cement) and AASC are used
as matrix binder, and varying replacement ratios of red mud (0.0–30%) are prepared. In order to evaluate
the efflorescence characteristics in the binders with red mud, analysis of water absorption and porosity
are performed. The changing efflorescence areas with weight loss are also measured. The compounds in
efflorescence are quantitatively analyzed through various techniques such as EDS, XRD, SEM, TGA, and
alkali leaching test. In the work, the accelerated efflorescence mechanism and its characteristics are
quantitatively evaluated considering the effects of binder types and red mud replacement ratios.
Ó 2016 Elsevier Ltd. All rights reserved.
1. Introduction
Recently, AAC (Alkali Activated Cement) is recognized as a special cement. It can replace OPC (Ordinary Portland Cement) which
causes a significant CO2 emission [1,2]. Many researches have been
performed on AASC (Alkali-Activated Slag Cement) which uses
GGBFS (Ground Granulated Blast Furnace Slag) as a main binder
main and NaOH is usually utilized as an activator. AASC can provide an eco-friendly construction material since the furnace slag
is the byproduct obtained from the process of steel manufacturing
[3,4]. It also has engineering advantages like early strength development, low hydration heat, and excellent resistance to chemical
attack [3–7]. Aqueous solutions of sodium hydroxide (NaOH) and
⇑ Corresponding author.
E-mail addresses: ksp0404@woosuk.ac.kr (S.-P. Kang), jjuni98@hannam.ac.kr
(S.-J. Kwon).
http://dx.doi.org/10.1016/j.conbuildmat.2016.12.123
0950-0618/Ó 2016 Elsevier Ltd. All rights reserved.
sodium silicate (Na2SiO3) which are commonly used as activators
for AASC are verified to be effective but using activators in liquid
form on-site is very dangerous and uneconomical. Consequently
several studies have been performed in order to find another
alkali-activators for preventing hard-handling and high cost [8].
Red mud is an inorganic byproduct with pH over 11, obtained
from mineral processing of aluminum hydroxide-Al(OH)3 and aluminum oxide-Al2O3. Through poly-condensation as slag or
alumino-silicate, the minerals in the red mud are dissociated.
Red mud containing 10–15% of Na2O can be used for 1) an alkali
activator replacing the previous liquid sodium silicate or 2) a retarder replacing sodium phosphate for construction material. The
researches on the system with AASRC (Alkali-Activated Slag-Red
mud Cement) can be a representative work for an application of
red mud with high alkali to construction material and AASRC is
reported to have high strength in early age and better chemical
resistance compared with matrix with OPC [9–13].
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Efflorescence is the migration of a soluble Ca2+ salt to the surface of a porous material. It begin with an aesthetic loss and finally
goes to a strength reduction by leaching out the cement hydrates
[14,15]. The construction material based on AASC is very vulnerable to efflorescence. The binders with sodium activators are
reported to have more severe damage from efflorescence since
increasing Na2O in unreacted state causes a relatively easy movement of sodium ions in the alumino-silicate structure which is a
product of the alkali-activated binder [16,17].
This work presents an evaluation of efflorescence characteristics in AASC mortar with red mud, and several tests like porosity
measurement, moisture absorption coefficient, and elution analysis are performed considering varying red mud replacement. For
analysis of compounds in efflorescence, microscopic analysis like
SEM, TGA, and EDS are also performed.
2. Materials and experimental program
sludge are shown in Fig. 1, where Fig. 1(a) and (b) show dumping
stage and sludge-typed red mud, respectively. The red mud used
in the work is ground granulated powder with red color as shown
in Fig. 1(c). The sludge with 40.0–60.0% moisture content is dried
to about 10.0%, then ground into granulated type.
2.2. Experimental program
2.2.1. Mix design of mortar and red mud characteristics
The mix proportions for cement mortar are listed in Table 2. The
mortar samples are manufactured using a mix ratio of 1:3 (binder:
sand). Water to binder (W/B) ratio is fixed as 0.75. OPC and AASC
are used as binder, respectively and the red mud replacement ratio
is considered from 0.0% to 30%. In order to evaluate the red mud
characteristics, the tests of SEM and XRD are performed. Particle
size distributions are also carried out for evaluating the mean
diameter of red mud.
2.1. Materials
2.1.1. Binder
The physic-chemical properties of the OPC-noted as C and
AASC-noted as NC are compared with those of FA (Fly Ash) and
GGBFS (Ground Granulated Blast Furnace Slag) in Table 1. Compared to GGBFS, AASC contains low content of SiO2 and Al2O3,
but high content of SO3. The specific surface area is 4058 cm2/g
and the density is 2.83 g/cm3, which are similar as those in GGBFS.
AASC contains desulfurization gypsum below 10.0% by mass ratio
as activator and its main components are CaO (73.2%), SO3
(21.9%), and SiO2 (2.27%).
2.1.2. Red mud
Approximately two tons of red mud are produced in sludge
with 40.0–60.0% moisture content, when one ton of Al2O3 is produced through the Bayer process. An annual production amount
in Korea is approximately twenty tons. The photos of red mud
2.2.2. Compressive strength
Cubic mortar samples with 50 50 50 mm are prepared and
cured for 28 days in the room condition with 25 °C and 60% of R.H.
After curing for 28 days, compressive strength is evaluated referred
to KS F 2405 [18].
2.2.3. Efflorescence acceleration
After 28 days of curing, the side surface of the sample is sealed
with epoxy and about 6.0 mm of the bottom surface is immersed in
distilled water for 14 days in the condition of 7 °C of temperature
and 50% of R.H. The photos for efflorescence area are taken after
14 days of accelerating period and the area is quantitatively evaluated through software of Paint.NET. The efflorescence grading is
evaluated based on the recommendation of As/NZS 6656.6 in
Australia.
Table 1
Physical properties and chemical composition of binder.
Type of binder
Specific surface area (cm2/g)
Density (g/cm3)
Ig. loss
Chemical composition (%)
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
C
NC (AASC)
FA
GGBFS
3144
4058
4012
4254
3.15
2.83
2.13
2.91
1.32
2.23
2.50
0.23
21.7
22.1
49.5
33.6
5.7
8.9
31.9
14.5
3.2
1.4
5.9
0.7
63.1
54.9
2.9
43.5
2.8
3.3
0.9
5.2
2.2
5.2
0.5
1.4
C: Ordinary Portland Cement NC: Alkali-Activated Slag Cement.
(a) Slurry type of red mud
Fig. 1. The photos of red mud.
(b) Dried red mud
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Table 2
Mix design of mortar.
Sample
Binder (wt%)
Binder: sand
Water to
binder (%)
Cement
RM
C
CR0
CR5
CR10
CR20
CR30
100
95
90
80
70
0
5
10
20
30
1:3
1:3
1:3
1:3
1:3
75
75
75
75
75
NC
NCR0
NCR5
NCR10
NCR20
NCR30
100
95
90
80
70
0
5
10
20
30
1:3
1:3
1:3
1:3
1:3
75
75
75
75
75
RM: Red mud.
Fig. 2. XRD pattern of dried red mud.
2.2.4. Absorption coefficient
Construction materials with porosity like cement mortar can
absorb water through capillary suction. A large amount of water
is absorbed initially and the absorption becomes steady afterwards. The absorption coefficient was obtained from Eq. (1) based
on KS 2609 [19].
2.2.5. Alkali leaching test
The mortar samples are crushed after the accelerated efflorescence test and the powder is mixed with distilled water in a weight
ratio of 1:50 and exposed to the same conditions of the accelerated
efflorescence test for 48 h. After 48 h, 20 mL of the mixed water is
obtained through filtering. Since soluble Na+ and Ca+ are the major
ions for efflorescence, the concentrations of them are measured
through ICP (Inductively Coupled Plasma) analysis. The results
are compared with the results before accelerated efflorescence.
and Fe2O3 reaches about 80% of the total weight. Fe2O3 accounting
for red mud color (red brown) is evaluated to be 21.6%–23.6% of
the weight. The weight ratio of Na2O which produces strong alkali
and principally causes efflorescence is observed to be 9.0%–10.5%.
The density and moisture content show 2.0 g/cm3 and 50.2%–
50.6%, respectively. The physical properties of the dried red mud
are listed in Table 4. Since sodium hydroxide is used in the extracting process of aluminum from Bauxite ores, sodium hydroxide
solution still remains, which causes strong alkali over 11.0 of pH.
The results of XRD analysis for dried red mud shown in Fig. 2,
which reveals that the material is mostly gibbsite-Al(OH)3,
goethite-FeO(OH), and Hematite-Fe2O3. Some of the oxides are
detected by XRD with aluminum hydroxide and a complex Na5Al3CSi3O15. Fig. 3 shows the particle size distribution of the red mud,
indicating that the particle size is only 10.0% level compared with
the average grain diameter for OPC, FA, and GGBFS. The specific
surface area of the dried red mud is 23.53 m2/g and the average
grain diameter is 2.75 lm. The average grain diameter of red
mud is even smaller than that of micro-cement (4–6 lm). As a
usual the small particles can provide a pacing effect in binder
matrix and strength loss due to efflorescence can be compensated
[21,22]. SEM image of the red mud is shown in Fig. 4.
As illustrated by the SEM images shown in Fig. 4(a) and (b) the
individual particles are spherical or flaky-shaped, and agglomerates of about 10 lm are visible. The particles are fine and easily
combine with cement particles.
3. Result and discussions
3.2. Compressive strength
3.1. Physical and chemical properties of red mud
The results of the compressive strength considering binder type
and red mud substitution ratio are shown in Fig. 5. The compressive strengths for the NC samples after 4 weeks are in the range
of 29.0–51.9 MPa, which is relatively low compared with the result
m ¼ w t1=2
ð1Þ
where m (kg/m2) is water absorption amount, t is test period
(hours), and w (kg/m2h1/2) is water absorption coefficient. The
coefficient is usually utilized for assessing water tightness in inorganic construction materials [20]. In order to calculate the water
absorption coefficient, cubic samples with 50 50 50 mm are
dried until they keep a constant weight in the standard condition
(20 ± 2°C of temperature and 65 ± 5% of R.H). Except for top and
bottom sides, all the other sides of the samples are coated with
epoxy and 2.0 mm depth of the bottom is soaked in water for
10 min, 30 min, 1 h, 6 h, and 24 h repeatedly.
The test results of physical and chemical properties are listed in
Table 3. In the chemical composition, the summation of SiO2, Al2O3,
Table 3
Properties of dumped red mud.
Sampling time
Chemical composition (wt%)
Average from 4 sampling for one year
SiO2
Al2O3
Fe2O3
CaO
MgO
SO3
Na2O
K2O
38.8
16.1
22.8
3.4
0.2
0.0
10.0
0.4
Density (g/cm3)
Moisture content ratio (%)
2.0
50.4
Table 4
Physical properties of dried red mud.
Color
Moisture content ratio (%)
pH
Gravity (g/cm3)
Specific surface area (m2/g)
Average particle diameter (lm)
Red
10.2
11.0
3.50
23.53
2.75
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Fig. 3. Particle size distribution of dried red mud.
Fig. 5. Compressive strength for C and NC samples at 7 days and 28 days.
of C samples which contain 39.8–54.5 MPa of strength level.
Regardless of the binder types, both C and NC show decreasing
compressive strength with increasing red mud substitution. In
the case of the NC, the compressive strength at the age of 7 days
shows slightly increasing trend with increasing mud substitution
ratios. However the compressive strength after 4 weeks, NC shows
relatively larger reduction of compressive strength with increasing
red mud substitution ratio. The reason for strength loss with
increasing red mud ratio is the reduction of binder content which
can develop strength. In the previous researches [11,12], AASRC
with very high alkali activator such NaOH and glass water (Na2SiO3) shows higher compressive strength than OPC mortar, but
the results in Fig. 5 contains powder typed activator (red mud
powder and desulfurization gypsum) so that compressive strength
is slightly reduced compared with C binder. The relatively higher
strength is measured since the mortar samples are cured in 25 °C
and sand in dried condition is used.
Fig. 6. Porosity of binder C and NC.
3.3. Porosity and moisture absorption coefficient
The results of porosity measurement with red mud substitution
ratios are plotted in Fig. 6. The total porosity of NC is 22.1–28.1%,
which is relatively high compared with C (18.9–22.8%). With
increasing red mud substitution ratio, the porosity increases as
well regardless of the binder type. Fig. 7 shows the pore size distribution with the red mud substitution ratio. By substituting with
red mud, the number of 10–1000 nm diameter capillary pores
increase while relatively large pores of 1000–10,000 nm diameters
decrease. The reason is a packing effect which is that 1000–
10,000 nm diameter grain sizes are dominant in the red mud and
they fill the matrix pores of its size. The increase in the 10–
1000 nm diameter capillary pores by substituting red mud also
increases feasibility of efflorescence due to an increase in moisture
absorption. The reduced travel path of the dissolved salts within
the matrix causes more efflorescence to the concrete surface
Fig. 4. SEM image of dried red mud.
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(a) C binder
(b) NC binder
Fig. 7. Pore size distribution of binder C and NC with red mud replacement ratio.
Table 5
Quantification of efflorescence degree.
Fig. 8. Water absorption coefficient in binder C and NC.
[23]. When red mud is added, alkali components increase inside
the matrix and this leads more efflorescence changes in the pore
structure.
The moisture absorption coefficients with varying red mud substitution ratios are shown in Fig. 8. The secondary efflorescence in
mortar is caused by movement of the moisture. When capillary
pores are filled with water, the reactions with precipitated CO2
or CO2 in the atmosphere accelerate the secondary efflorescence.
Therefore, the feasibility of efflorescence increases when the moisture absorption coefficient increases. The moisture absorption
coefficient in NC is 0.22–0.46, which is relatively high compared
with C (0.03–0.17). The moisture absorption coefficient is observed
to increases with increasing red mud replacement ration regardless of the binder types. It can be explained through the increase
in 10–1000 nm diameter capillary pores.
3.4. Efflorescence characterization
As shown in Table 5, the degree of efflorescence is quantitatively categorized into 5 levels considering the efflorescence area.
The loss of weight in the sample due to efflorescence is evaluated
by measuring the weight after removing efflorescence. SEM, EDS,
and XRD analyses are performed in order to characterize the efflorescence components. Fig. 9 shows the efflorescence in the case of
C30 after accelerated efflorescence test.
Degree
Efflorescence area
I
II
III
IV
V
None
Less than 10%
Above 10%, less than 30%
Above 30%, less than 50%
Above 50%
Fig. 10 and Table 6 show the efflorescence characteristics with
binder types and the red mud substitution ratios. With increasing
the substitution ratio of red mud, efflorescence area and mass loss
of the mortar also increase. The efflorescence area up to 5.0% of the
replacement ratio keeps level II which is less than 10.0% of efflorescence area. However when replacement ratio increases over 10.0%,
efflorescence area rapidly increases. In particular, the weight loss
in C binder is evaluated to be remarkable compared with NC
binder.
Table 7 shows EDS analysis results of the efflorescence compounds for 30% of red mud replacement ratio. The efflorescence
compounds for C and NC binder contain Na+ the most since Na2O
of red mud is a major factor behind the observed efflorescence.
S compound content is only 0.69% for C while 17.37% is
observed in NC sample. C binder contains 2.2% of SO3 component
but NC binder has 5.2% of SO3 since industrial by-product desulfurization gypsum is used as the activator for GGBFS. NC binder contains a relatively large amount of SO3 content and the unreacted
sulfate is eluted to the surface.
The results of XRD analysis are shown in Fig. 11. In the efflorescence from C binder with red mud (30% of replacement ratio), Na2CO3H2O is mainly observed while Na2SO4 is dominant in NC
binder (30% of replacement ratio). This reveals that the efflorescence is mainly comprised of sodium compounds regardless of
the binder type. The efflorescence from binder with red mud shows
that CaCO3 in concrete based on OPC is partially observable [13]
but sodium compounds are dominantly observable. The efflorescence from binder C with red mud is generic alkaline carbonate
efflorescence (Na2CO3H2O) produced from the reaction between
Na+ from the red mud and CO2 gas in the atmosphere. This is similar to the mechanism for geo-polymer efflorescence generation,
which is caused by abundant Na+ from a sodium activator such
as NaOH and water-glass [15]. The formulations of the efflorescence generation in geo-polymer are expressed as Eqs. (2) and
(3), respectively.
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(a) Occurred efflorescence
(b) Image processing
(c) Removal of efflorescence
Fig. 9. Photos for efflorescence treatment.
(a) C binder with red mud
(b) NC binder with red mud
Fig. 10. Efflorescence area image in C and NC samples with red mud.
Table 6
Efflorescence characteristics.
Type of binders
Efflorescence
Red mud content (%)
0
5
10
20
30
C type
Area (%)
Degree
Weight loss (g)
0.3
II
0
8.2
II
0.09
19.4
III
0.11
37.7
IV
0.51
40.6
IV
1.3
NC type
Area (%)
Degree
Weight loss (g)
0.1
I
0
9.1
II
0.13
24.8
III
0.29
35.5
IV
0.46
38.1
IV
0.58
CO2 ðgÞ þ 2OH ðaqÞ ! CO2
3 ðaqÞ þ H2 O
ð2Þ
2Naþ ðaqÞ þ CO2
3 ðaqÞ þ 7H2 O ! Na2 CO3 7H2 O
ð3Þ
The efflorescence from NC binder is mainly made up with Na2SO4 from the reaction between Na+ supplied from red mud and SO3
from desulfurization gypsum used as the activator for GGBFS. In
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Table 7
EDS analysis results of the efflorescence (30% of replacement ratio).
Electron image
Element
Na
Ca
S
Al
Weight (%)
35.08
1.76
0.69
1.00
Weight (%)
22.27
0.87
17.37
0.57
C30
NC30
(a) C type
(b) NC type
Fig. 11. XRD analysis results of the efflorescence compound (30% of replacement ratio).
Fig. 12. SEM images of the efflorescence in C and NC binder (30% of replacement ratio).
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Fig. 11(a), TiO2 is observed but it is from the mortar containing red
mud when efflorescence compounds are taken from the mortar
surface by mistake.
SEM images of the efflorescence compounds are shown in
Fig. 12(a) for C binder and Fig. 12(b) for NC binder, respectively.
The surface of the compound is covered by formation of calcite
and sodium carbonate crystals. The particles are mostly spherical
with smooth or rough surfaces and heterogeneous size. In Fig. 12
(b) sodium sulfate and sodium carbonate particles due to
desulfurization gypsum are agglomerated with enlarged particle
size.
3.5. Alkali leaching content
The concentrations of Ca2+ and Na+ from the each binder are
presented in Fig. 13. The efflorescence in C binder is from the free
CaO in the cement matrix or Ca(OH)2 from the cement hydration
reaction, flowing out through the pores. The efflorescence formation increases as Ca(OH)2 from the hydration reaction becomes
greater. TGA analysis results of CR0 and NCR0 are shown in
Fig. 14, which shows that mass loss at 400–450 °C due to decomposition of Ca(OH)2 is about 1.36%, which is much higher compared
with NCR0 (0.77%). This is consistent with the results of Ca2+ leaching content, where Ca2+ leaching content in CR0 was 189.1 ppm
before efflorescence test while NCR0 shows only 27.6 ppm. The
Ca2+ leaching content is found to sharply decrease with increasing
red mud substitution ratio in the C binder due to reduction of
cement content. Furthermore soluble Ca2+ from the samples
increases compared with the results before the test. During efflo-
rescence test, the binders are cured for 28 days in 7 °C and afterwards the side surfaces are sealed with epoxy with the
immersed bottom surfaces. As a result, the moisture absorption
is unidirectional towards the top due to the capillary suction, so
that Ca2+ of the lower portion dissolves and moves to the upper
surface.
Very small amount of Na2O exists since CR0 and NCR0 has no
red mud addition, however Na2O increases with higher red mud
substitution ratio regardless of the binder type. Na+ leaching after
efflorescence test shows a different behavior from Ca2+ leaching. It
is thought that the efflorescence is still generated continuously
even if efflorescence formed on the samples is removed. Considering the leaching contents and efflorescence components, Na+ content is less than Ca2+ in the binder C with red mud while
Na2CO3H2O is dominant in the efflorescence. This implies that,
even if Na+ is small amount, Na+ still has a greater impact on efflorescence than Ca2+.
4. Conclusion
The conclusions on effects of red mud and Alkali-Activated Slag
Cement on efflorescence in cement mortar are as follows.
1) The results in compressive strength at the age of 28 days
show 29.0–51.9 MPa level in NC binder, which is relatively
low compared with those in C binder (39.8–54.5 MPa).
Regardless of the binder type, both C and NC binder shows
decreasing compressive strengths with increasing red mud
substitution ratio due to reduction of binder which can
develop pore densification and strength.
2) The increase in the 10–1000 nm diameter capillary pores
due to red mud replacement causes acceleration of efflorescence. The enlarged moisture absorption and increase in the
travel path of the dissolved soluble salts within the matrix
cause more efflorescence.
3) For the efflorescence compounds in C binder with red mud,
Na2CO3H2O is mainly observed while Na2SO4 is observed in
NC binder with red mud, which reveals that the efflorescence compounds are mainly comprised of sodium compounds, regardless of binder types. Despite of small
content of Na+, it has greater impact on efflorescence acceleration than Ca2+ due to high solubility.
Acknowledgements
Fig. 13. Results of alkali leaching test for C and NC binder.
This research was supported by a grant (16CTAP-C115206-01#)
from Infrastructure and transportation technology promotion
research Program funded by Ministry of Land, Infrastructure and
Transport of Korean Government. The authors also appreciate for
the support of Basic Science Research Program through the
National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT & Future Planning (No. 2015R1A5A1037548).
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