eliz paula manfroi1,a malik cheriaf2,b* janaíde cavalcante...
TRANSCRIPT
Development of eco-efficient binders to encapsulation of heavy metals
Eliz Paula Manfroi1,a, Malik Cheriaf2,b*, Janaíde Cavalcante Rocha3,c
1, 2, 3 University Federal of Santa Catarina, Department of Civil Engineering
[email protected], [email protected], [email protected]
Keywords: binder, encapsulation, ettringite, red mud, FGD gypsum.
Abstract. The use of eco-efficient binders, ie binders produced with reduced environmental
impacts, such as: reduction of the extraction of raw materials, calcination energy and CO2
emissions; use of wastes as raw material and heavy metal encapsulation provides an alternative for
the sustainable development. In this work were produced binders with partial replacement of the
calcium aluminate cement by FGD gypsum and red mud. The partial replacement of calcium
aluminate cement for FGD gypsum was carried out with the aim of form ettringite, which was used
to encapsulate heavy metals from wastes FGD gypsum and red mud. For this, pastes and mortars
were produced with different proportions of aluminate cement, FGD gypsum and red mud.
Hydrated compounds and phases in pastes and mortars were investigated using X-ray diffraction
(XRD), differential scanning calorimetry (DSC), scanning electron microscopy (SEM). The results
showed that closer is the ratio CA/FGD (calcium aluminate cement/FGD gypsum) to the theoretical
ratio (stoichiometric ratio CA/FGD for optimized ettringite formation), greater is the amount of the
ettringite formed. Compressive tests, water absorption by capillarity and evaluation of the
encapsulation of heavy metals in mortars showed that the ratio CA/FGD most appropriate is 5,7.
The result of this research showed that calcium aluminate cement can be replaced partially by FGD
gypsum and red mud to produce eco-efficient binders.
Introduction
The FGD gypsum is a waste generated in the coal-fired power plants. This residue is produced
during the desulphurization process of flue gases. In this process, the reaction between sulfur
dioxide (SO2) from flue gas, calcium carbonate, limestone and oxygen form the FGD gypsum
(CaSO4.2H2O) as a by-product. The FGD gypsum is composed mainly of calcium oxide and sulfur
trioxide [1, 2].
In the alumina industry, the red mud (RM) is the residue that causes major environmental impacts.
This residue is from the refining of bauxite during the Bayer process for the production of alumina
and the posterior production of aluminum. The red mud from the Bayer process is composed mainly
of alumina, iron oxide and silica. Besides, the red mud is classified as a hazardous residue (class I),
because the red mud has chromium and selenium concentrations higher than the limits values
defined in annex F of the Brazilian standard NBR 10004 [3]. Studies have been developed to
valorization of the red mud for the production of cement composites [3, 4, 5, 6].
One of the effective ways to increase the use of FGD gypsum is used it as partial replacement of
calcium aluminate cement (CAC) for production of eco-efficient binders, because calcium sulfate,
main component of natural gypsum, it is also the main component of the FGD gypsum. When
calcium sulfate reacts with calcium aluminate from alumina cement and water, the ettringite is
formed [7, 8].
Researchers found that ettringite is able to partially encapsulate the chromium from galvanic sludge
[9]. This researchers detected the presence of chromium in the particles of bottom ash and the
ettringite needles by scanning electron microscopy analysis in the pastes. The potential of ettringite
in the encapsulation of heavy metals present in solid waste, was also investigated by other
researchers [10, 11, 12].
Given this context, eco-efficient binders were developed for FGD gypsum and calcium aluminate
cement for encapsulating heavy metals of the red mud and FGD gypsum in the ettringite structure.
This paper presents the results of the physical evaluation, chemical and mechanical properties of
mortars produced with these binders, and the evaluation of the eco-efficiency point of view of the
encapsulation of heavy metals.
Materials and Methods
Materials. The sample of FGD gypsum was collected in a coal-fired power plants in the state of
Rio Grande do Sul. The FGD gypsum was calcined at 150 °C (hemihydrate). According to the
measurements 100% of FGD gypsum particles are smaller than 0.25 mm (laser diffraction,
Microtrac S3500). The sample of red mud was collected in an alumina industry in the northern
Brazil. Pastes and mortars were produced with red mud calcined at 600 °C, based on previous
studies [3]. Before calcination, the red mud was dried in an oven at 105 ± 5°C (for 72 h). The
maximum particles size of red mut at 600 °C were smaller than 0.032 mm (Malvern laser
diffraction Mastersizer 2000 dispersant water). The pastes and mortars were produced with Fondu
calcium aluminate cement. The mortars were produced with standard sand. The chemical
characteristics of the raw materials were carried out using energy dispersive X-ray fluorescence
spectrometry (EDX, Model 700 HS, Shimadzu). The chemical composition of raw materials are
shown in Table 1.
Table 1: Chemical composition (wt.%) of the aluminous cement, FGD gypsum and red mud
Sample
Oxides (%)
Al2O3 CaO TiO2 Fe2O3 K20 SO3 SiO2
Cement 51.70 40.57 1.22 0.88 0.28 - 2.88
FGD gypsum 1.36 51.35 0.39 0.86 0.55 30.20 4.51
Red mud 23.94 1.22 5.14 31.57 0.08 0.07 13.70
The presence of the elements arsenic (As), cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni)
and zinc (Zn) from red mud and FGD gypsum were identified by inductively coupled plasma– mass
spectrometry analysis (PerkinElmer, model NexIon 300D). Table 2 shows the amount (mg/kg) of
these elements from red mud and FGD gypsum.
Table 2: Elements arsenic, cadmium, chromium, copper, nickel and zinc present in the FGD
gypsum and red mud
Sample
Elements (mg/kg)
Cr As Cd Cu Ni Zn
Red mud 297.27±5.22 24.24±1.20 1.63±0.10 23.00±0.24 6.00±0.15 64.25 ±1.53
FGD gypsum 12.06±0.12 <0.01 <0.005 3.58±0.09 9.33±0.08 <0.025
Methods.
Production of Pastes and Mortars. The investigation of the hydrated compounds and assessment
of the proportion of the phases were carried out in pastes composed by 5 to 35% of FGD gypsum, in
a mass substitution of the calcium aluminate cement. The pastes were produced with a water/binder
ratio of 0.4 and 0.3% of superplasticizer additive (polycarboxylate ether based).
In order to evaluate the potential encapsulation of heavy metals from the red mud and FGD gypsum
in the ettringite structure, mortars with 100% of calcium aluminate cement and 5 to 20% FGD
gypsum in replacement of calcium aluminate cement were produced (A0 to A4). Afterwards,
mortars were produced with 0 to 20% of FGD gypsum (amount calculated on the mass CACFGD
95% to 85% CACFGD) and 5% (B0 to B4), 10% (C1 to C4) or 15% (D0 to D4) of red mud in
substitution of the total mass of the cement plus FGD gypsum. Table 3 shows the mix proportions
of the different mortars.
Mortars were produced according the binder to sand ratio of 1:3 (in mass). The water/binder ratio
kept constant at 0.5 and 0.4% of superplasticizer additive (polycarboxylate ether based) was used.
Table 3: Composition of the mortars
Investigation of Hydrated Compounds and Proportion of the Mineralogical Phases. After the
hydration periods (28 days), pastes and mortars were ground (particle size < 0.15 mm) and analysed
by X-ray diffraction (Model X-Pert, Philips) and diferencial scanning calorimetry (Model SDT
Q600- TA). The microstructures of the pastes were investigated by scanning electron microscopy
analysis (Model JSM-6390LV, JEOL). The diffraction X-ray analysis and integrated intensity
methodology were used to estimate the proportions of the mineralogical phases of pastes produced
with calcium aluminate cement and FGD gypsum. The integrated intensities of the peaks in a
diffractogram of a phase were related to the amount of this phase in the sample. This relationship
has been used for many years in the quantitative analysis of mineralogical phases [13]. Researchers
used the method of the integrated intensity to estimate the amount of mineralogical phases present
in zeolites and estimate the crystallinity of the bottom ash [14, 15]. The proportion of each phase
was determined based on integration of the most intense peak of each phase [16].
Evaluation of Compressive Strength, Water Absorption by Capillarity and Encapsulation of
Heavy Metals in Mortars. The compressive strength was carried out in mortar in accordance with
the procedures described in Brazilian standard NBR 13279 [17]. In order to evaluate the water
absorption by capillarity of the mortars was used a procedure that consists in the measure of the
height variation of a water column contained in a graduated Mariotte tube in function of time [18].
The height variation of the water column is directly related with the amount of water absorbed by
the sample. The volume of water absorbed by section of the sample was called absorption index (I =
cm3/cm
2). The inclination of the straight (absorption index versus square root of time) corresponds
to sorptivity. In Brazil there are no standards on environmental assessment of monolithic materials
produced with waste. Thus, to evaluate the release of heavy metals from mortars the leaching test
was performed in accordance with the procedure established by Dutch standard NEN 7375 [19].
After 64 days, the leachate extract obtained was filtered and analyzed using energy dispersive X-ray
fluorescence spectrometry (Modelo 700 HS, Shimadzu).
Mortar Composition
CAC+FGD (%) CAC (%) FGD (%) CAC/FGD RM (%)
A0 100.0 100.0 0.0 0.0 0.0
A1 100.0 95.0 5.0 19.0 0.0
A2 100.0 90.0 10.0 9.0 0.0
A3 100.0 85.0 15.0 5.7 0.0
A4 100.0 80.0 20.0 4.0 0.0
B0 95.0 95.0 0.0 0.0 5.0
B1 95.0 90.3 4.8 19.0 5.0
B2 95.0 85.5 9.5 9.0 5.0
B3 95.0 80.8 14.3 5.7 5.0
B4 95.0 76.0 19.0 4.0 5.0
C0 90.0 90.0 0.0 0.0 10.0
C1 90.0 85.5 4.5 19.0 10.0
C2 90.0 81.0 9.0 9.0 10.0
C3 90.0 76.5 13.5 5.7 10.0
C4 90.0 72.0 18.0 4.0 10.0
D1 85.0 85.0 0.0 0.0 15.0
D2 85.0 80.8 4.3 19.0 15.0
D3 85.0 76.5 8.5 9.0 15.0
D4 85.0 72.3 12.8 5.7 15.0
D5 85.0 68.0 17.0 4.0 15.0
Results and Discussion
Mineralogy and Microstructures of the Pastes. Fig. 1 shows the diffractograms of the pastes at
28 days. The phases detected in the pastes with 100% calcium aluminate cement were monocalcium
aluminate - CA (1), calcium dialuminate- CA2 (2), gelenite- C2AS (3), C3AH6 (4), C2AH8 (5) and
gibbsite – AH3 (6).
Figure 1 – Diffractograms of the pastes produced with calcium aluminate cement and
FGD gypsum at 28 days
Differently from the pastes with 100% of calcium aluminate cement, pastes with 5% FGD gypsum
showed the phases gypsite (8) and CaSO4.0,5H2O (9). The hydration of the calcium sulfate
hemihydrate (CaSO4.0,5H2O) resulted in the formation of gypsite (CaSO4.2H2O). However, the
presence of CaSO4.0,5H2O phase shows that part of calcium sulfate hemihydrate (FGD 150°C)
added into the mixture did not react with the compounds of calcium aluminate cement and/or water
to form hydrated products. Furthermore, in the pastes with 5% of FGD gypsum was not detected the
presence of ettringite. The lack of the peak corresponding to ettringite in these pastes is related to
the low amount of calcium sulfate present in the mixture to react with the compounds calcium
aluminate cement and water to form ettringite.
As well as pastes produced with 95%CA+5%FGD, the pastes with 10 to 35% of FGD gypsum in
replacement of calcium aluminate cement showed the mineralogical phases: CA (1), CA2 (2), C2AS
(3), C3AH6 (4), AH3 (6), gypsite (8) e CaSO4.0,5H2O (9). However, in the pastes with 10 to 35% of
FGD gypsum was identified the presence of ettringite (7) and the absence of the C2AH8 (5).
Moreover, the higher the content of FGD gypsum, the higher the intensity of the peak of ettringite.
The lack of the C2AH8 phase is related to least amount of calcium aluminates available for
hydration due to reduction of calcium aluminate cement. Also due to the reactions between the
calcium sulfate from FGD gypsum and calcium aluminate from cement for formation of ettringite
Fig. 2 shows the percentage of the areas of the more intense peaks of the phases C2AH8, C3AH6 e
ettringite, which are related to the amount of formed phases [14, 15, 20]. In the pastes with 10 to
35% of gypsum, the higher the FGD gypsum content, the higher the peak area and consequently the
higher the amount of ettringite formed. The paste with 35% of FGD gypsum showed the highest
ettringite content (23.65%), in accordance with the calculations of the proportion of gypsum needed
0
1000
2000
3000
4000
5000
6000
7000
8000
9000
10000
5 10 15 20 25 30 35 40 45 50 55 60
100CAC 5FGD+95CAC 10FGD+90CAC 15FGD+85CAC
20FGD+80CAC 25FGD+75CAC 30FGD+70CAC 35FGD+65CAC
Inte
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54 6
42 61 1
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1 11 12 13 2 22 333
331
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Inte
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Inte
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Inte
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Inte
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Inte
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Inte
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2θ
54 6
42 61 1
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1 11 12 13 2 22 333
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Inte
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Inte
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Inte
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71
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3
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2θ
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1 11 12 13 22 333
3
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5
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2θ
54 6
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3
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6
4
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3 3 1 432 4 4 43
1 11 12 12 33
3
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1
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8
82
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6 11
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91
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25%FGD
12
2
220%FGD 7
91 3
84
83 2
2
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871 4
73
2θ
54 6
42 61 1
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3 3 1 432 4 4 43
1 11 12 13 2 333
3
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5
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7 7 7 7 7 74 2 44 4333
3
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11
742
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2θ
54 6
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6
4
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3 3 1 432 4 4 43
1 11 12 1 33
3
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1
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64 4874 4
4
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1 1 1
3
332 332
2 3
77
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77 7
4 4 4 4 46 6 6
63
1111
33
3
2
73 3
2θ
54 6
42 61 1
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32 1
3
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32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12 13 2 333
3
3
44 4 45
5
5
5 66
7 7
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7 7 7 7 7 74 2 44 4333
3
3 311
11
766
77
777
2θ
54 6
42 61 1
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32 1
3
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313
6
4
24 6
3 3 1 432 4 4 43
1 11 12 1
3
3
3
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1
55
5
5 66
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777
7 7 7 7 74
4
4 433
3
3
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11
766
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4
41
1 1 1
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332 332
2 3
77 7
77 7 7 7
424 4 4
4 4 4
3
6 6
63
1
2
1133
388 8
8
8
8 8
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12 13 2
33
3
3
44 4 45
5
5
5 66
7 7
777
7 7 7 7 7 74 24 4
33
3
3 311
11
766
77
777
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12
3
3
44 4 4
1
55
5
5 66
7 7
777
7 7 7 7 74
4
4 43
3
3
3 311
11
766
77
77
7
7 76
64 44 4
4
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1 1 1
3
332 332
3
8
7 7
77 7 7 7
7
7
4 4 4 46 6
63
131
6
1133
33
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12 3 2
3
34
4 4 4
85
5
5
5 66
7 7
777
7 7 7 74 24
33
3
3 311
11
766
77
777
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12
34
4 4 4
1
55
5
5 66
7 7
777
2
7 7 74
4
483
3
3
3 311
11
766
77
7 77
7
76
68
44 4
4
41
1 1 1
3
332 332
3
7
7 7
77 7 7 7
71
4 4 4 4 46 6
63
1
3
1133
3
42
88
88 88
7
77 7 83
77 72 7
77
8 883
8
32
84 8
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12 3
3
34
4 4 455
5
5 66
7 7
777
7
7 7 74 24
33
3
3 311
11
766
77
777
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 12
34
4 4 4
1
55
5
5 66
7 7
777
1
7 7 74
4
43
3
3
3 311
11
766
77
77
7
76
6 8 44 434
11 1 1
3
332 332
3
7
7
7
7
77 7 7 7
7
4 4 4 4
4
6 6
63
31
11
83
33
382
1
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 2 33
34
4 4 455
5
5 66
7 7
777
3
7 7 74 24
33
3
3 311
11
766
77
777
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 2
34 8
4 4
1
55
5
5 66
7 7
777
32
7 874
8
3
3
3 311 1
766
77
77
7
76
8
44 4
8
4
84
1 1 1
3
332 332
7
7
7
7
77 7 7 7
7
4 4 4 4 484
4
66311
33
3
72
4
88
8
8
8
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 2 33
34
4 455
5
5 66
7 7
77
91
3
782
8 87
3
3
3 311
2
176
6
77
777
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 2
34
2
4 4
1
55
5
5
916
7 7
77
72
91
3
74
3
3
3 311
4
176
6
77
7
7
7
76
4
44 4 4
4
1 1 1
3
332 332
7
7
4
7 7 7 7
71
4 4 4 4663
31
8
1133
3
2θ
54 6
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 2 3
83
4
4 455
5
56
7 7
77
7 83
2
7
8
2 3
3
311 1
76
2
7
4
7
2θ
54 6
1- CA 2- CA2 3- C2AS 4- C3AH6 5- C2AH8 6-AH3
7- Ettringite 8- Gypsite 9-CaSO4.0,5H2O
42 61 1
351 2 4 4
32 1
3
42
32 2 1 4
313
6
4
24 6
3 3 1 432 4 4 43
1 11 24
31
4
8
5
5
926
7 7
63
7
1
717
4
3
3
1176
71
7
7
6 7
7
64
448
8
1 1 1
3
33
332
7
7 7 7 7 7
7
42
4 46
63
8
1133
3
7
88
8
8
8
7
7777 7
878 8
8
82
84 84
42 44 46
84
4
6 1 11
31
1
3 73 34 7 1
22
220%FGD
84
83 2
2
432
86
831 4 8
787 4
63 3 1
84 3
871
30%FGD
6
8
4
83
4
87
82
14
73 7 44
87
35%FGD
15%FGD
10%FGD
5%FGD
100%CA
8
8
81
8
9
9
9
9
9
9
to form the ideal ettringite (theoretical) [16]. As the peaks of C2AS, CaSO4.2H2O, CA,
CaSO4.0,5H2O phases were overlapped was not possible to determine exactly the areas these peaks
and to estimate the amount of these phases in the samples.
Figure 2 - Percentages of the areas of the main peaks of the diffractograms of C2AH8, C3AH6 phases
and ettringite in pastes with calcium aluminate cement and FGD gypsum
The analysis of differential scanning calorimetry (DSC) carried out in the FGD gypsum (dry 50 °C),
reference paste (100% CAC) and the pastes with 5 to 35% FGD gypsum in replacement of calcium
aluminate cement at 28 days are shown in Fig. 3. The DSC curve of the paste with 100% of calcium
aluminate cement show the presence of an endothermic peak at 70ºC, which is associated to the loss
of moisture. The endothermic peak at 140ºC of the DSC curve of the paste with 100% de CAC
indicates the presence of the C2AH8 compound. The double endothermic peak approximately at 300
°C is related to the presence of the gibbsite and C3AH6. The peaks correspondent to the presence of
gibbsite and C3AH6 are more evident in the curve of the derivative thermogravimetry [16]. The
C2AH8, gibbsita e C3AH6 hydrated compounds in the pastes with 100% of CAC were also detected
by means of the analysis of X-ray diffraction.
Although endothermic peak correspondente to gypsum does not appear in the DSC curves of the
pastes with up to 15% of FGD gypsum (overlapping peaks), the results of the analysis of X-ray
diffraction confirmed that all pastes produced with the FGD gypsum have gypsite in their
mineralogical composition (Fig. 1).
The endothermic peaks at 150°C of the DSC curves of pastes with FGD gypsum containing more
than 20% of FGD gypsum are most evident. The DSC curves of pastes with 95% CAC + 5% FGD,
DSC curves of pastes with 10 to 35% FGD gypsum in replacement to calcium aluminate cement
showed endothermic peaks at 70 °C, 150 °C, 170 °C, 275 and 295ºC that are attributed to the loss of
moisture, presence of gypsite, CaSO4.0,5H2O, gibbsite and C3AH6, respectively. However,
differently from DSC curves of the pastes with 5% FGD gypsum in replacement to calcium
aluminate cement, the DSC curves of the pastes with 10 to 35% FGD gypsum showed an
endothermic peak at 100 °C associated to presence of ettringite and have not showed endothermic
peak at 140 °C associated to C2AH8. The presence of the gypsite, CaSO4.0,5H2O, gibbsite, C3AH6
and ettringite in these pastes were also detected by means of the analysis of X-ray diffraction.
0
5
10
15
20
25
30
35
40
0 5 10 15 20 25 30 35
C2AH8 C3AH6 Ettringite
Content of FGD gypsum (%)%
the p
eak a
rea o
f th
e m
inera
log
ical p
hase
Figure 3- Analysis of differential scanning calorimetry of the FGD gypsum, of the paste with 100%
CAC and of the pastes with 5 at 35% of FGD gypsum in replacement of calcium aluminate cement
The image of the microstructures of the paste with 65% of calcium aluminate cement and 35% of
FGD gypsum shows the presence of ettringite crystals (Fig. 4). This hydrated compounds was also
detected by means of the analysis of X-ray diffraction and differential scanning calorimetry.
Figure 4 – Image of the microstructure of the paste produced with 65% of calcium aluminate
cement and 35% of FGD gypsum at 28 days
Mineralogy of the mortars. The mortars with 5, 10 and 15% of red mud in replacement of
calcium aluminate cement (B0, C0 e D0) showed the same mineralogical phases than the mortar
A0: CAH10 (1), C2AH8 (2), C3AH6 (3), AH3-gibbsite (4), SiO2 – quartz (5), C2AS-gelenite (6) e
anortite (10); showing that the replacement of calcium aluminate cement and FGD gypsum by red
mud has not changed the mineralogical structure of mortars.
Fig. 5 shows the diffractograms of the mortars C0 to C4. Mortars B1, C1 e D1 showed the same
mineralogical structure than the mortar A1, which showed the phases C2AH8 (2), C3AH6 (3), AH3-
gibbsite (4), SiO2 – quartz (5), C2AS-gelenite (6), CaSO4.0,5H2O (8) and gypsite (9). However, the
sample A2 at A4, B2 at B4, C2 at C4 and D2 at D4 did not present the CAH10 and C2AH8 phases
and presented the ettringite phase. The lack of the calcium aluminate hydrates CAH10 and C2AH8 to
due the replacement of the FGD gypsum by calcium aluminate; reducing the amount of calcium
-5-4-3-2-10123456789
10111213141516
0 100 200 300 400 500 600 700 800 900
FGD_dry_50ºC 100CAC 5FGD+95CAC 10FGD+90CAC 15FGD+85CAC
20FGD+80CAC 25FGD+75CAC 30FGD+70CAC 35FGD+65CAC
Heat
flo
w /m
ass(m
w/m
g)
Temperature (ºC)
100%CAC
5%FGD
10%FGD
15%FGD
20%FGD
25%FGD
30%FGD
35%FGD
C3AH6
Gibbsite
C2AH8
Ettringite
Gypsite
C2AH8 Hem.
Hem.
Dihydrate Hemihydrate
Hemihydrate Soluble anhydrite (CaSO4.ƐH2O)
FGD dry 50ºC
Ettringite
aluminates available for hydration. Moreover, due to the calcium aluminates react with the calcium
sulfate hemihydrate and water to form ettringite.
Figure 5 - Diffractograms of the mortars C0 at C4 at 28 days
Evaluation of Compressive Strength of the Mortars. Fig. 6 shows the results of the average
compressive strength at 28 days of the mortars A0 to A4, B0 to B4, C0 to C4 and D0 to D4. The
mortars with replacement of the calcium aluminate cement by FGD gypsum and/or red mud (A1-
A4, B0-B4, C0-C4, D0-D4) showed lower compressive strength values than the sample with 100%
of calcium aluminate cement (A0), because there was a reduction in the formation of the calcium
aluminate hydrates due to low amount available calcium aluminates in the mixture caused by
replacement calcium aluminate cement. The samples B1-B4, C1-C4 e D1-D4 produced with
calcium aluminate cement, FGD gypsum and 5 to 15% of red mud showed compressive strength
values at 28 days which were on average similar to the compressive strength values of the mortars
A1-A4 (CAC and FGD), showing that the compressive strength of mortars presents significant
variation only when the ratio CAC/FGD changes. Moreover, of the mortars B1, C1 and D1, the
average compressive strength of the samples C1 and D1 (10 and 15% RM) remained constant,
indicating that the red mud has a filler effect.
In the mortars with ratio CAC/FGD varying from 9.0 to 19.0 (A1-A2, B1-B2, C1-C2, D1-D2), the
lower ratio CAC/FGD, the less compressive strength. However, the compressive strength of the
mortars increased when ratio CAC/FGD decreased to 5.7. The lower compressive strength of the
mortars with CAC/FGD equal to 9.0 (A2, B2, C2 and D2) is related to lower formation of calcium
aluminate hydrates, low ettringite content, presence of gypsite and also CaSO4.0,5H2O. As seen in
the results of the analysis of X-ray diffraction, the diffractograms peaks associated to ettringite of
these mortars have lower intensity than the same diffractograms peaks of the mortars produced with
CAC/FGD equal to 5.7 and 4.0, respectively. The closer the ratio CAC/FGD of the theoretical ratio
CAC/FGD (1.86) [16] the larger amount of ettringite formed and consequently a higher
compressive strength of the mortars.
0
500
1000
1500
2000
2500
3000
3500
0 5 10 15 20 25 30 35 40 45 50 55 60
C0 C1 C2 C3 C4
2θ
Inte
ns
ity
(cp
s)
11
2 3 4 5
5
56
6 3 3 4
1- CAH10 2- C2AH8 3- C3AH6 4-AH3
5-SiO2 6-C2AS 7-Ettringite 8-CaSO4.0,5H2O 9 - Gypsite 10-Anortite
23
4 5
5
5
96
6
6
6
6
6
6
6
3
3
3
3
3
3
3
4
4
45
5
5
5
5
5
3
3
3
3
4
4
4
4 5
5
57
7
77
7
8
7
7 8
8 8
8
8
8
8
8
8
9
9
9
9
9
9
9
9
9
9
9
10
10
10
Figure 6 - Average compressive strength of the mortars A, B, C and D at 28 days
Water Absorption by Capillarity. The variations of the sorptivity of the mortars B1 to B4, C1 to
C4 and D1 to D4 are shown in Fig. 7. In general, the mortars B2, C2 and D2 (ratio CAC/FGD =
9.0) showed sorptivity lower than the sorptivity of the mortars B3-B4, C3-C4, D3-D4 (ratio
CAC/FGD 5.7 and 4.0, respectively), due to samples with lower ratio CAC/FGD have larger
amount of ettringite formed; reducing the pore connectivity and the sorptivity of the mortars.
Figure 7 - Sorptivity of the mortars B1-B4, C1-C4 and D1-D4
In the mortars which have ettringite in its mineralogical composition, the mortars produced with
ratio CAC/FGD equal to 4.0 (B4, C4 and D4) showed a lower sorptivity. Moreover, the larger the
content of calcium aluminate cement replacement and FGD gypsum by red mud, the larger the
sorptivity of the mortar. This is related with the larger replacement of the calcium aluminate cement
and FGD gypsum by red mud, forming low amount of calcium aluminate hydrates and ettringite;
increasing connectivity between the pores.
Encapsulation of Heavy Metals in Mortars. The elements leached from mortars A0 to A4 and the
concentrations limits defined in annex F of the Brazilian standard NBR 10004 [21] are shown in
Fig. 8. The mortars produced with 100% of calcium aluminate cement showed concentrations of
arsenic, cadmium and chromium higher than the limits values of the Brazilian standard NBR 10004.
The elements arsenic (As), cadmium (Cd), chromium (Cr), iron (Fe), nickel (Ni), titanium (Ti) and
zinc (Zn) leached from mortar are derived from the raw materials used in producing of the mortars.
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
80
85
ARG-A ARG-B ARG-C ARG-D
ARG_0 ARG_1 ARG_2 ARG_3 ARG_4
Co
mp
res
siv
e s
tren
gth
(MP
a)
Sample
0,000
0,002
0,003
0,005
0,006
0,008
0,009
0,011
0,012
0,014
0,015
0,017
0,018
0,020
ARG-B ARG-C ARG-D
ARG_1 ARG_2 ARG_3 ARG_4
Sample
S (
cm
3/c
m2x
min
1/2
)
The mortar A1 showed concentrations of metals leached higher than the concentrations leached
from of the mortars A2 to A4. The highest concentrations of metals leached from mortar A1 are
related to lower amount of calcium aluminate hydrates formed. Moreover, despite of the mortars A3
and A4 showed concentrations leached of chromium similar to concentration leached from mortar
A2; the mortars A3 and A4 have a higher amount of chromium due to the larger replacement of the
calcium aluminate cement by FGD gypsum. However, in mortars with higher ettringite content was
lower release of chromium.
Figure 8- Concentrations of elements leached from mortars A0 to A4 and limits values of the
Brazilian standard
The mortars C1 to C4, B1 to B4 and D1 to D4, produced with calcium aluminate cement, FGD
gypsum and red mud showed concentrations of cadmium, arsenic and chromium higher than the
limits values defined in annex F of the Brazilian standard NBR 10004 [21]. The accumulated
concentrations at 64 days of elements leached from mortar D1 to D4 (15% RM) and the
concentration limits defined in Brazilian standard are shown in Fig. 9.
Figure 9- Concentrations of elements leached from mortars D1 to D4 and limits values of the
Brazilian standard
Fig. 10 shows the concentrations of chromium leached from mortar B, C and D as a function of the
ratio CAC/FGD. Mortars with ratio CAC/FGD equal to 19.0 and 5 to 15% of red mud in
replacement of the calcium aluminate cement and FGD gypsum showed concentrations of metals
leached higher than concentrations of metals leached from mortars produced with ratio CA/FGD
0
50
100
150
200
250
300
350
As Cd Cr Cu Fe Ni Ti Zn
A0 A1 A2 A3 A4 Lim._NBR 10004
Elements
Co
nce
ntr
ati
on
(m
g/L
)
0
50
100
150
200
250
300
350
As Cd Cr Cu Fe Ni Ti Zn
D1 D2 D3 D4 Lim._NBR10004
Elements
Co
nce
ntr
ati
on
(mg
/L)
equal to 9.0, 5.7, 4.0 and 5 to 15% of red mud. The higher concentrations of the metals leached
from mortars with ratio CAC/FGD equal to 19.0 is related to lack of ettringite in these samples and
the lower amount of calcium aluminate hydrates formed. In general, the mortars with ratio
CAC/FGD equal to 5.7 showed concentrations leached of arsenic, cadmium and chromium lower
than the concentrations leached these metals from mortars with ratio CAC/FGD equal to 4.0, due to
the mortars with ratio CAC/FGD equal to 5.7 showed lower amount of FGD gypsum in the mixture.
Figure10 – Concentrations accumulated leached at 64 days of chromium from mortars B, C
and D in function of the ratio CA/FGD
Conclusions
The mineralogical analyzes carried out in pastes and mortars showed that calcium sulfate from FGD
gypsum reacted with the calcium aluminates from calcium aluminate cement and water to form
ettringite. The closer the ratio CAC/FGD is of the ratio theoretical CAC/FGD, the larger the amount
of ettringite formed and the lower the amount of calcium aluminate hydrates.
The most appropriate ratio CAC/FGD to obtain larger amount of ettringite, higher compressive
strength and lower sorptivity of the mortars would be equal to 4.0 i.e. the ratio CAC/ FGD closer
to theoretical ratio CAC/FGD (1.86) evaluated in this study. However, due to FGD gypsum shows
high concentrations of chromium leached, the ratio CAC/FGD indicated is 5.7.
The ratio CAC/FGD equal to 5.7 was also efficient for the encapsulation of metals, because despite
the larger amount of FGD gypsum (higher metal content), the amount of ettringite formed in these
pastes resulted in the partial encapsulation of metals.
The results of the release of heavy metals from mortars produced with red mud showed that the
content of replacement of the calcium aluminate cement and FGD gypsum by red mud the most
appropriate is 5%. Lastly, the results of this research showed that is possible to use FGD gypsum
residue, red mud and calcium aluminate cement to produce mortar with partial encapsulation of
heavy metals in the ettringite structure.
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0
20
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140
160
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200
220
240
260
2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20
Cr_ARG_B Cr_ARG_C Cr-ARG_D
Co
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ntr
ati
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(m
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