Mitigation of Leachates in Blast Furnace Slag
Aggregates by Application of Nanoporous Thin
Films
J.F. Muñoz, J.M. Sanfilippo, M.I. Tejedor, M.A. Anderson, and S.M. Cramer1
Abstract. The reutilization of slag materials as aggregates is seriously limited by
the production of contaminant leachates rich in heavy metals and sulfur when these
materials are contacted by water. A unique type of thin-film nanotechnology was
used to ameliorate this problem. The surface of the slag was altered by depositing
a thin-film comprised of nanoporous oxides. The deposition was performed by
coating the aggregates with a suspension containing nanoparticles. Once the water
evaporated, a nanoporous thin-film (<0.5 µm) remained firmly attached to the surface of the slag. Different leachate experiments under semi-anoxic conditions were
performed using three different nanoparticles oxides films including silica, and titanium. These films were compared against a control. The preliminary results
demonstrated that samples coated with one layer of these oxides can decrease the
amount of sulfur and calcium in the leachate by 70 and 80%, respectively.
1 Introduction
The combination of different factors such as the aim to reduce greenhouse gas
emissions, the achievement of a sustainable development, and simply economical
J.F. Muñoz
University of Wisconsin-Madison
e-mail: [email protected]
J.M. Sanfilippo
University of Wisconsin-Madison
e-mail: [email protected]
M.I. Tejedor
University of Wisconsin-Madison
e-mail: [email protected]
M.A. Anderson
University of Wisconsin-Madison
e-mail: [email protected]
S.M. Cramer
University of Wisconsin-Madison
e-mail: [email protected]
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J.F. Muñoz et al.
constraints, have motivated a growing interest to explore new applications for materials that in the recent past have been considered as waste. This is the case of
blast furnace slag emanating from iron and steel production that, depending on its
gradation, is used as traditional supplementary cementitious materials or as a potential substitute for natural aggregates. However, when the material is contacted
by water, the application of slag materials as aggregates is seriously limited by the
production of contaminant leachates rich in heavy metals and sulfur (H2S) [1-4].
The hydration of the high amounts of lime present in the aggregates triggers a rise
in pH that leads to a degradation of the amorphous silicates that otherwise binds
the sulfur and the other contaminants to the aggregate.
The objective of this research is to explore the capacity of nanoporous thinfilms comprised of nanoparticulate oxides, judicially located on the surface of the
aggregates, to eliminate or ameliorate the leachate produced when air cooled blast
furnace (ACBF) slag aggregates are used as base layer materials.
2 Materials and Sample Preparation
The slag materials selected for this research were ACBF slag aggregates obtained
from iron production with ¾ inch size gradation. These aggregates were coated
with two types of basic nanoparticle coatings using a “dip coating” method. One
of thin-film coatings was a nanoporous silica dioxide and the second a nanoporous
titanium dioxide. Both oxide solutions were prepared using standard sol-gel processes [5, 6]. The coating of the slag has been done by immersing baskets of slag
aggregate into the sols and later draining the sol from the aggregate at a constant
speed. The slag was coated with one layer of either materials and left to dry. As
the last steep in the process, the coated samples were heated at 300 ºC for 3 hours.
The sintering temperature was selected taking into account the thermal stability of
the aggregates and the capability for the sintering particles to themselves and to
the slag. The thermal stability of the slag was determined from thermogravimetric
and differential thermal analysis of the slag heated in air.
3 Methodology
The methodology initially chosen to study the leaching of these slag materials was
based on the standards 1027 and 212-02T from the Ohio State and the Indiana Departments of Transportations, respectively. These two methods were selected
since they are commonly used to evaluate the leaching capacity of the ACBF slag
aggregates. In order to save time and material, it was decided to scale-down
specimen size. In our modified method, 500 g of slag aggregate was used. In
these leaching experiments, the samples were kept immersed in deionized water
inside 500 ml propylene bottles under almost anoxic conditions as little overhead
air remained in the container. These experimental conditions were chosen in order
to minimize the lost of the water phase through evaporation. Aliquots of
the leachate were extracted and filtered after 24 and 48 hours from preparing the
Mitigation of Leachates in Blast Furnace Slag Aggregates
323
initial samples. Three different analyses were performed on the samples: i) color
evaluation of the filtrate and the solid retains in the filter paper, ii) pH measurements of the filtrate, and lastly, iii) a measurement of the calcium and sulfur concentration in the filtrates using inductively coupled plasma (ICP).
From an analysis of the results, we could see some limitations of these particular initial analytical protocols. As a result, a more thorough methodology for the
analysis of leached calcium and sulfur was developed in order to quantitatively determine the capability of the thin film coatings to ameliorate the leaching of the
ACBF slag aggregates. A scheme of the final methodology used to quantify the
leachates is represented in Figure 1. The results predicted by the corresponding
chemical equilibrium diagrams were corroborated by applying the analytical protocol to a solution of 0.25 M of Ca(NO3)2 and K2SO4. The analyses done by ICP
indicated that the recovery of calcium and sulfate by using the proposed methodology is very close to 100%.
Sample
(5 ml)
5 ml of H2O2 5% v/v
Dilute with KOH 10-2 M
pH = 12
Fd = 5
Fd = 5
Solution 1
(25 ml)
5 ml of Solution 1
10 ml Ba(NO3)2 10-1 M
Add HNO3 1N to pH = 2
Fd = 5
5 ml of Solution 1
10 ml K2C2O4 10-1 M
Add NaOH 1N to pH > 11.75
Solution 2
(25 ml)
Centrifugation
Solution 3
(25 ml)
15 ml of Solution 3
Add HNO3 1N to acid pH = 2
Centrifugation
Measure Calcium
& Magnesium
Measure Sulfur
1 ml of Solution 3
To 25 ml (MQ water)
Add HNO3 1N to acid pH = 2
Centrifugation
Measure Silica
Fig. 1 New Proposed Analytical Protocol of Leachates
4 Results and Discussion
4.1 Analysis of Leachates Using Standards 1027 and 212-02T
The results obtained from the three types of analysis are summarized below. None
of the filtrates showed any color. The color of all solids retained by the filter
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matched a light brown color labeled as 10YR 8/2 in the rock-color chart. Exceptions were the samples coated with titanium that exhibited a darker brown color
(10YR 5/4).
The measured pH values of the filtrates were different for coated and uncoated
samples as can be seen in Figure 2. The pH in control samples oscillated from basic (~ 10) to acid (~ 2) values during the first 48 hours of leaching. A similar
trend but over a different pH range and also smaller in magnitude was observed in
the samples coated with silica. The samples with a coating of titanium showed a
different behavior. The pH shifted to more basic values during the first 48 hours
but the shift was very small when compared with the other two systems. The low
values of pH in most of the filtrates can be explained by the leaching of sulfur as
sulfide or polysulfide. This is to be expected as the leaching test was performed
under rather anoxic conditions. During these extraction and filtration procedures,
sulfur species were exposed to atmospheric oxygen. Under these conditions, the
sulfides can easily oxidize to sulfates, as it is expressed in equation 1.
−
H 2 S + 4 H 2 O ↔ HSO4 + 9 H + + 8e −
(1)
The oxidation of one mole of sulfides produces nine moles of protons that explains the acidification observed in the filtrate. The fact that the 24 hours filtrate
sample has a very basic pH can have several explanations: a smaller leaching of
sulfur than in the rest of the systems; most of the sulfur being retained on the filter
as colloidal polysulfates; and even a third explanation that larger quantities of Ca
leaching into solution could increase the buffer capacity of the filtrate. Further
hypothesis await additional studies. However, one thing seems clear, the coated
slag produces a more similar pattern of pH values in the filtrates than do uncoated
slag samples. Thus, it can be concluded that nanoporous coatings on slag result in
quite different leaching behaviors.
This first evaluation of the potential of the coatings to ameliorate ACBF slag
leachate concluded by determining the concentration of calcium and sulfur in the
12.0
10.0
pH
8.0
6.0
4.0
2.0
0.0
Control
SiO2
TiO2
Fig. 2 pH Values of the Filtered Water Measured at 24 (
) and 48 (
) Hours
Mitigation of Leachates in Blast Furnace Slag Aggregates
0.80
3.0
a
325
b
0.60
M o l/L
M o l/L
2.0
0.40
1.0
0.20
0.00
0.0
Control
SiO2
TiO2
Control
Fig. 3 Concentration of Calcium (a) and Sulfur (b) at 24 (
Leachate
SiO2
) and 48 (
TiO2
) Hours in Slag
filtrates. These two elements were chosen as tracers of the leaching activity of the
slag aggregates. The results are represented in Figure 3. The values obtained for
the concentration of calcium did not show any significant difference with respect
to leaching time of coated versus uncoated slag. On another hand, Figure 3b
shows a more than 50% reduction in the concentration of sulfur in the filtrates associated with silica and titanium dioxide coated slag after 48 hours of leaching.
In the leaching mechanism proposed by Schwab et al. [7], the amount of sulfur
liberated is directly dependant of the amount of soluble calcium originating during
the hydration of the lime. Originally, the sulfur is trapped inside some of the inter-granular amorphous silica matrix. The hydration process of the lime triggers
the resulting basic pH of the system to dissolve this matrix. Therefore, a lower
amount of leached sulfur from the coated samples should be accompanied with a
lower amount of calcium. This correlation was not observed in the analysis of the
filtrates of Figure 3. The homogeneity in the values of calcium concentration
could be explained if the soluble calcium was controlled by the solubility product
of some calcium salt present as a solid phase in the leachate. In this case, the
leached calcium will be the sum of the calcium in the filtrate and the calcium on
the filter. Therefore, an analysis of leachated solutes in the filtered will not allow
one to evaluate the total leached calcium. A similar problem can be encountered
when measuring sulfate in the filtrates, as some of the sulfates can be present in
the leachate as an insoluble phase. Under the anoxic conditions of the test, the
sulfur is as sulfide that could easily be in the formation of polysulfides. The polysulfide particles are colloidal in nature and could be retained and/or adsorbed by
the paper filter. Furthermore, the sulfates can form calcium sulfates, which is not
very soluble. Despite the limitations of these analytical protocols, there are some
encouraging signs indicating a different behavior for coated and uncoated slag
with respect to leaching.
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J.F. Muñoz et al.
4.2 Analysis of Leachates Using the New Analytical Protocol
The new analysis protocol was applied to leachates taken from the three systems
mentioned above, 60 days after mixing the slag with water. It is worthwhile
to mention that, after 2 months of near anoxic conditions, only the bottles of the
control displayed a characteristic green color, indicative of higher presence of
polysulfides.
0.50
0.40
Mol/L
0.30
0.20
0.10
0.00
Control
SiO2
TiO2
Fig. 4 Leaching Concentration under Anoxic Conditions of Calcium ( ), Magnesium ( ),
and Sulfur ( ) Measured at 60 Days
Results of these analyses, shown in Figure 4, indicated that coating the slag
with a thin-layer of either oxide significantly decreased the amount of calcium
and sulfur leached under anoxic conditions. The amount of calcium leached with
SiO2 and TiO2 is only 28% and 14% of the one leached in the control system.
The same trend is true for the quantity of leached sulfide. The results illustrate
the higher capacity of the titanium oxide versus the silica coating to ameliorate
leaching.
5 Conclusions
These results clearly suggest that the thin-film nanotechnology has the potential to
stop or significantly decrease the production of environmental unfriendly
leachates of ACBF slag aggregates. The experiments have proved that the
nanoporous coatings of metal oxides can be used as an effective barrier to avoid
this diffusion and ultimately decrease the leaching in ACBF slags. Therefore, it is
worthwhile to study this subject matter in more detail, for example, the influence
of the number of coatings, different oxide coatings, etc.
This new way of applying nanoparticles in concrete processing opens the door
to the possibility of managing and manipulating the physical-chemical properties
Mitigation of Leachates in Blast Furnace Slag Aggregates
327
of aggregates depending on specific needs. In other words, this technology could
be applied in concrete to improve the flexural and tensile strengths, permeability
of concrete in addition to its resistance to alkali silica reaction.
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