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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 ( 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
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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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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
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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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