Utilization of municipal sewage sludge as additives for the production of eco-cement
Yiming Lin
Lin Yiming
W18B
Dec-2
Original Oct-28 Results & Discursion
Count words
No target
2340
S/paragraph
No target
5
Words/S
Target <
26
Nov 3 turn in to Dr. Ho
20
Passive
No target
28%
Reading ease
Target >
47
60
Grammar &
Issue:
Description:
style
3 Results and discussion
3.1 Particle size distribution of raw meals and eco-cement clinkers
The size distributions of raw mealsand eco-cement clinkers are presented in Fig. 1. The diameter of particles
with percentages by volume less than 10%, 50% and 90% were 1.87, 10.07 and 48.76 µm, respectively. As
shown in Fig. 1(a), the maximum size of sample was 82.19µm and the particle size of the raw meals mainly
distributed in 0.39-71.11μm, which attained to the requirements of production of burning cement clinker
(retained on 80 mesh sieves less than10%). On the other hand, it was shown from Fig 1.(b)that the percentages
by volume of particles with diameter less than 3、10、30、60、90µm were 3.52%、27.50%、69.66%、89.61%
and 98.16%, respectively. The median diameter (D50) of eco-cement clinkers (the sewage sludge content was
5%) was 17.70µm. In addition, the diameters between 0 and 30µm, 10 and 40µm, 30 and 60µm, more than
60µm are 27.50%, 42.16%, 19.20% and 1.84%, respectively. Therefore, the size distributions of raw meals and
clinkers met the industrial requirements of production of cement.
3.2 Characterization of the eco-cement clinkers
Fig.2 shows the X-ray diffraction (XRD) pattern results for the plain clinker and the eleven eco-cement
clinkers. The result indicated that the major components in the eco-cement clinkers were C3S (Ca3SiO5 and
Ca3(SiO4)O), C2S (Ca2SiO4 and Ca2(SiO4)), C3A(Ca3Al2O6 and Ca3(Al , Fe)2O6), C4AF(Ca4Fe2Al2O10) and
were similar to those in ordinary Portland cement. Moreover, the phase formation of Ca54MgAl2Si16O90 was
identified in all the clinkers. However, the C2S (α-C2S or β-C2S) phase formation increased with the increase in
sewage sludge content. The XRD phase-identification study also demonstrated that the increase in C2S was not
obvious until sewage sludge content was below 10% (per raw meal by weight), as well as easily found higher at
12% and 15%. In addition, some components containing heavy metals (Cr, Zn, Ni, Cu, et.al) could also be
found when sewage sludge content was greater than 5%. It is also shown in Fig. 2 that peaks increases and
become more complex with the increase in sewage sludge content. In general, the general trend was that C2S
and components with heavy metals increased with the rise in sewage sludge content in raw meal.



 :Ca3(Al,Fe)2O6

 
0%

:C3A;:C2A;:CaAl2Fe4O10;

 







3.0%
Intensity
Intensity
1.0%






8.0%
1.5%
10.0%
2.0%
12.0%
2.5%
15.0%
20

 
:Ca3(Al,Fe)2O6


5.0%
0.5%
10
:C3A;:C2A;:CaAl2Fe4O10;



30
2θ
40
50
60
70
10
20
30
2θ
40
50
60
Fig.2. XRD patterns of eco-clinkers.
3.3 Microstructure of co-cement clinkers by SEM
To verify the X-ray diffraction (XRD) pattern results for the plain clinker and the eleven eco-cement clinkers,
SEM examinations were performed. Fig. 3 shows the microstructure of clinkers with sewage sludge content of
0% (a), 2.5% (b), 5.0% (c), 10.0% (d) and 15.0% (e). It was found that different addition of sewage sludge led
to discrepancies in the microstructure of the clinkers. As shows in Fig 3a, C3S, C2S and intermediate phase
structure are clearly seen. However, as the addition of sewage sludge C3S structures decreased, which can be
seen from Fig b and Fig c that are less than Fig a, as well as Fig d and Fig e have the least. On the other hand,
C2S structures increased with the increases of sewage sludge content. In addition, as shown in Fig. 3(e), it can
also be observed that the microstructure of mixture containing 15.0% sewage sludge content showed significant
differences in the morphology compared with other samples and a lager amount of pores distributed in the
clinker. Results from the X-ray diffraction (XRD) pattern showed that C2S was one of the major content in the
sample and one possible explanation was that the amount of components in sludge led to the increases in pores.
Furthermore, reports showed that it was most likely that the doped heavy metals (especially Zn) changes the
viscosity of liquid phase of clinkers during the burning process and it had an influence on the total porosity and
the distribution of pores [19].
70
Fig.3. SEM micrographs of the eco-cement clinkers with the addition of different sewage sludge in raw meals: (a) 0%, (b) 2.5%,
(c) 5.0%, (d) 10.0% and (e) 15.0%.
3.4. Setting times of the eco-cement pastes
The setting times for the twelve types of the plain paste and the eco-cement pastes are given in Table 2. The
results showed that the initial setting time and final setting time were 136 and 185 min, respectively. All the
eco-cement pastes had a bigger initial setting times and final setting times as well than plain paste in
comparison with the plain paste. These times increased as sewage sludge content in raw meal increased. It may
be due to a coagulant structural network forms from the residual cement, which resulted hydration slows [20].
Moreover, other possible explanation was that it may be attributed primarily to the increase in C2S (α-C2S or
β-C2S), which affected the rate of the pozzolanic reactions. On the other hand, these delays could also be due to
some minor elements (Zn, Cr et al.) presented in sewage sludge, which may dissolve in the pore solution and
then affect the hydration of the cement [8, 19, 21].
3.5 Strength development of plain paste and eco-cement pastes
Flexural and compressive strengths of the plain paste and eco-cement pastes after 3, 7 and 28 days are also
shown in Table 2. It could be seen that the flexural and compressive strengths of the plain paste and eco-cement
pastes developed when the curing age extended from 3 to 28 days. In comparison with the plain paste, the
results demonstrated that the flexural and compressive strengths of eco-cement paste were similar to that of
plain paste with the same conditions. However, it could be seen that the increase in sewage sludge content in
raw meals led, for curing age of 3 and 7 days, to a greater decrease in flexural strength, as well as a little
decrease in compressive strength. But, from 7 to 28 days, the flexural and compressive strengths increased and
became similar to that of plain paste. Moreover, the strengths were decreased in all hydration times when the
sewage sludge content in raw meals is 15%.
The decrease in early strengths (at 3days) may be partly due to the changes in content of pozzolanic active
matters in sewage sludge [3], which are thought to be less effective in pozzolanic reaction and delays the early
hydration. As mentioned above, the amounts of C2S (α-C2S or β-C2S) and initial setting times increased as the
sewage sludge content in raw meal increased. So the lower early strength may be responsible for the delay in
the process of hydration. On the other hand, as sewage sludge is a hazardous solid waste which contains
various matters, especially minor elements, it would led to the changes of clinkers and form news compounds
in the burning process when it was added in the raw meal. Further study is needed in the next section.
Table 2
Setting time, Flexural strength and Compressive strength of plain and eco-cement mortars
Setting time
Flexural strength (MPa)
(hour : minute)
Curing time(days)
Start
Final
3
7
28
3
7
28
0
2:16
3:05
6.02
7.56
9.65
31.42
45.27
63.15
0.5
2:19
3:12
5.98
7.55
9.62
30.28
44.79
62.56
1.0
2:20
3:13
5.75
7.62
9.67
32.59
43.66
64.01
1.5
2:28
3:25
5.74
7.69
9.59
32.63
42.55
61.72
2.0
2:35
3:35
5.65
7.59
9.62
31.78
43.51
65.44
2.5
2:40
3:45
5.43
7.53
9.62
28.07
43.33
63.52
3.0
2:42
3:46
5.41
7.51
9.65
30.02
42.58
63.48
5.0
2:46
3:49
5.22
7.52
9.56
31.25
43.21
62.58
8.0
2:55
3:55
5.21
7.56
9.61
30.02
41.85
61.95
10.0
3:10
4:08
5.16
7.35
9.49
29.95
42.68
60.64
12.0
3:12
4:17
5.08
7.16
9.45
29.86
40.28
60.98
15.0
3:20
4:23
4.98
7.02
9.55
28.64
39.33
60.48
Sewage
sludge
content (%)
Compressive strength (MPa)
3.6 Changes of the element content of eco-cement clinkers
The element concentration of Al, Fe, Mg, Ti, Mn, Sr, Ba, Zn, Cr, Cu, Ni and Pb in the plain and eco-cement
clinkers was analyzed by ICP-AES. The results of these elements are shown in Fig. 4. It could be seen that Al
and Fe, known to be the main compounds of the cement, decreased slightly as sewage sludge content in raw
meal increasd. But study showed that properly increasing the amount of aluminum oxide could enhance early
hydration and increase the early strength of the paste [22], it may be partly led to the increase in the initial
setting time and the lower early strengths of the eco-cement pastes by contrast a decrease in these two elements.
Although a small amount of Ti and Mn oxide can increase the strength of the pastes, the results from
publications [23] showed that the increase of these elements slightly retard the hydration during the first 2 days
and their effect on the hydration rate tends to become negligible at 28days. As shown in Fig. 6, the Ti increased
as the sewage sludge content in raw meal increased while the Mn did not have obvious change. So these two
elements may play a role to the later ages (7 to 28 days) strengths. Moreover, the temperature and viscosity of
the formation in the liquid phase can be reduced when a small mount of Mg is added to the raw meal. However,
it was found from the Fig. 4 that the Mg decreased with increase in the sewage sludge content in raw meal,
therefore, it may has negative effects on the formation and development of the alite crystals [24] and it may be
partly due to the increase in C2S in the eco-cement clinkers. In addition, the Ba and the Sr can have an
influence on the ability of stabilizing β-phase and and the crystal size. Although Ca, Ba and Sr are the chemical
elements of the same clan and Ba increased as the sewage sludge content in raw meal increased, the Sr
decreased obviously and had a higher concentration than Ba. So the change of Sr and Ba may have a
contribution to the delay of hydration times and the early lower strengths.
The heavy metals Cr, Zn, Cu can have an influence on the strength of cement mortar and the initial setting or
hydration of the cement when their concentrations are higher than normally present in the clinker. Stephan [19,
21, 25, 26] found that Cr accelerated the hydration and the initial setting of the cement, lowered the strength
and caused changes of the content of free lime and the modification of C3S in the clinkers at higher
concentration. The results were similar to Shirasaka et al. [27] and Murat et al [28]. Moreover, the results from
publications [19, 23, 26, 28, 29] found that Zn retarded the heat liberation, delayed the hydration of cement and
also led to a retardation of the initial setting but also caused higher strength of the cement pastes. In addition,
the Cu also was found that it was the greatest delay of the hydration rate of cement in all the elements [23, 27].
As shown in Fig. 6, the Cr, Zn, Cu content increased as the sewage sludge content in raw meal increased and
the concentration of Zn is higher than that of Cr, Cu also is the lowest. Although the concentrations of these
three elements were low, the incorporation of them may partly have the mainly effect on the increase in the
initial setting time and the lower early strengths of the pastes, as well as the increase in C2S in the eco-cement
clinkers. On the other hand, most publications [19, 21, 23, 25, 26] showed that the addition of Ni in raw meal
did not affect the hydration of cement. Several heavy metals, known to be volatile, will immediately vaporized
during the melting process, such as lead and cadmium. Therefore, these elements are not analyzed in this study.
Generally, most of the elements changed in the cement clinkers could have an influence on the properties of the
cement pastes and further studies must to be carried out to verity it.
Fig.4. Changes of the element content of plain and eco-cement clinkers with different sewage sludge contents in raw meals.
3.7 TCLP test of eco-cement clinkers
Table 3 shows the leaching concentrations of heavy metals and the identification for the extraction procedure
toxicity criteria of China. It can be seen that only Ba, Cr, Pb and Sr are present but none of them exceeded the
regulatory thresholds. Moreover, , the increase in sewage sludge in raw meal resulted in higher amounts
leached for almost all heavy metals by comparing with the leaching data between each eco-clinker. This could
be due to the higher heavy metal content in the clinkers when using sewage sludge, leading to an increase in the
amount of heavy metal available for leaching. However, in fact, all of the heavy metals are barely detectable.
This result could be related to either a parts of these heavy metals (primarily volatile metals) evaporated during
the clinkers burning process, or a parts of them could substitute with the major elements (Ca, Si, Al, Fe) so that
they were remained and immobilized in the structure of clinker minerals. In all cases, all the eco-cement
clinkers produced in this study were leach-safe and harmless to the environment.
Table 3
TCLP leaching concentrations (mg/L) and the finally pH of mixed liquids for the plain and eco-cement clinkers
Sewage sludge
content (w/w %)
GB
Elements(mg/L)
pH
Ba
Cd
Co
Cr
Cu
Mn
Ni
Pb
Sr
Ti
V
Zn
0
11.68
1.14
ND
ND
0.39
ND
ND
ND
ND
6.06
ND
ND
ND
1.0
11.55
1.41
ND
ND
0.09
ND
ND
ND
0.02
6.52
ND
ND
ND
3.0
11.66
1.70
ND
ND
0.02
ND
ND
ND
ND
6.68
ND
ND
ND
8.0
11.49
2.09
ND
ND
0.32
ND
ND
ND
0.01
7.03
ND
ND
ND
12.0
11.21
2.02
ND
ND
0.83
ND
ND
ND
0.01
6.64
ND
ND
ND
—
100
1
—
5
100
—
5
5
—
—
—
100
5085.3-2007a
“a” Identification standards for hazardous wastes- Identification for extraction toxicity of China.