-
Oof
heenta444
Accepted 2 December 2011Available online 11 January 2012
Keywords:Sewage sludge
n se
pastes fabricated with various mass ratios of aluminate/cement
has been analyzed in terms of mechanical
However, a potential concern is that the PC-based
formulationsare much more viable for inorganic wastes, but not
suitable for theorganic-high wastes such as waste activated sludge.
It is becauseorganic matters had detrimental effects on the
hydration reactionsof cement and accordingly lowered the S/S
performance [12,16,18].
trast, the strength of specimens solidied by
cementbentonitemixture was similar to, and in some cases, 5268%
lower thanthose obtained with cement only. Another study by Luz et
al.[14] employed sulfoaluminate cement and bottom ash as theblended
binder to modify a galvanic sludge. They observed thatthe
introduction of bottom ash induced the increased porosity
ofsolidied matrixes, which was more detrimental than benecialto the
strength development. Furthermore, Malliou et al. [18] uti-lized
Portland cement with both CaCl2 and Ca(OH)2 as acceleratorsto
solidify sludge. Results showed that 10% or so increment in the
Corresponding author. Tel.: +86 21 6598 0609; fax: +86 21 6598
0041.E-mail addresses: [email protected] (G. Zhen),
[email protected]
(X. Lu), [email protected] (X. Cheng), [email protected] (H.
Chen), [email protected]
Construction and Building Materials 30 (2012) 675681
Contents lists available at
Construction and B
ev(X. Yan), [email protected] (Y. Zhao).The increasing
number of waste sludge devoted to waste immo-bilization reects a
genuine need for cost-effective solutions forrestoring a safe and
green environment. Solidication/stabilization(S/S) technologies are
expected to increase in treating hazardouswastes, like heavy
metal-contained wastes before land disposal.A major factor in
applying this process to wastes is that it improvesthe physical and
chemical characteristics and reduces the mobilityof contaminants.
In this process, the identication of binders ableto assume the
xation of contaminants is essential for the successof the technique
[14]. Portland cement (PC) is considered as themost common
amendment media due to its low cost and largeavailability
[3,6,10,22].
properties of solidied sludge revealed that the grease, oil and
phe-nol had the signicantly negative effect on binders. The
presence of8 wt.% grease and oil resulted in 50% reduction in the
28 day-strength of cement-y ash samples, also 54% and 92%
decrementwere observed for the cement only and cement-y ash pastes
with8 wt.% phenol addition, respectively.
Therefore, recently various additives have been utilized
duringthe cement-based S/S process in order to counteract the
inuencesfrom the organic matters in activated sludge. Researchers
havestudied the feasibility of using bentonite as the additive due
toits high adsorbability to organic substances [12]. Yet no
apparentlypositive effect on the strength development was observed.
In con-Solidication/stabilizationModierEttringiteLeaching test
1. Introduction0950-0618/$ - see front matter 2011 Elsevier Ltd.
Adoi:10.1016/j.conbuildmat.2011.12.049strength, hydration products,
microstructure and leaching characteristics. The incorporation of
aluminatesignicantly improved the cement-based S/S performance. The
solid matrix obtained with the aluminate/cement ratio of 4/6 and
binder addition of 10 wt.% presented 28 day-strength of 157.2 kPa,
in contrast,25.1 kPa or so obtained for the cement only-sludge mix.
X-ray diffraction (XRD), thermogravimetrydifferential scanning
calorimetry (TG-DSC) and scanning electron microscopy (SEM)
analysis revealedthat the presence of aluminate counteracted the
interference from organic matters, favored the formationof
crystalline phases viz. ettringite C6AS3H32, calcite CaCO3 and
quartz, and therefore the strength devel-opment. Environmental
assessment of the nal products in compliance leaching tests
demonstrated thatthe concentration of heavy metals were below the
certain legal limits (GB 5085.3-2007) set in China,though the
addition of aluminate slightly fell the resistance of solidied
mortars to acidic environments.
2011 Elsevier Ltd. All rights reserved.
A study by Minocha et al. [16] on the impact of grease, oil,
hexa-chlorobenzene, trichloroethylene and phenol on the
geotechnicalReceived 14 January 2011Received in revised form 9
November 2011
stabilization (S/S). A novel aluminate 12CaO7Al2O3 was prepared
as accelerator in order to improve theperformance of cement-based
S/S making the sludge disposal and recycling possible. The behavior
ofHydration process of the aluminate 12Cacement-based
solidication/stabilization
Guangyin Zhen a, Xueqin Lu b, Xiaobo Cheng c, Hua Ca The State
Key Laboratory of Pollution Control and Resource Reuse, School of
Environmb School of Environmental and Chemical Engineering,
Shanghai University, Shanghai 200c Shanghai Municipal Sewage
Company Limited, Shanghai 200233, China
a r t i c l e i n f o
Article history:
a b s t r a c t
The high organic content i
journal homepage: www.elsll rights reserved.7Al2O3-assisted
Portlandsewage sludge
n c, Xiaofei Yan c, Youcai Zhao a,l Science and Engineering,
Tongji University, Shanghai 200092, China, China
wage sludge is the main obstacle to Portland cement-based
solidication/
SciVerse ScienceDirect
uilding Materials
ier .com/locate /conbui ldmat
-
28-day strength was achieved when adding 3 wt.% CaCl2 and2 wt.%
Ca(OH)2 of cement. But the presence of chloride ion re-tarded the
reuse of nal products in some elds, especially inbuilding
industries attributed to chloride-induced steel corrosionin
reinforced concrete structures [13]. In addition, other
pozzolanicadditives like jarosite, alunite [3], Na2SiO35H2O, Na2CO3
[1], aswell as lime and y ash [20] have also been added to or
partiallyreplaced PC for sludge S/S, however, the published data
were lesssatisfactory.
Mayenite (12CaO7Al2O3), one among aluminates, can be de-tected
in some high performance cement [7,14]. It owns the out-
2.4. Analytical methods
2.4.1. Unconned compressive strength (UCS)UCS was measured in
compliance with SL237-020-1999 (China). And the spec-
imens were mechanically tested in 3, 7, 14 and 28 days. For each
mortar and curingage, three specimens were tested.
2.4.2. X-ray diffraction (XRD) analysisThe hydrations of samples
were stopped by immersion in acetone solution for
24 h, followed by drying at 65 C. Then a D8 Advance powder
diffractometer (Bru-ker AXS Inc., Germany) was employed to identify
the crystalline products present.The accelerating voltage was 40 kV
and the current was 40 mA. The samples(
-
2.4.3. Scanning electron microscopy (SEM) analysisScanning
electron microscopy (Quanta 200 FEG, FEI Company, USA) was used
for the morphological and chemical analysis of the hydrated
samples. The crushedsamples were mounted on Al-stubs and gilt with
Au, with a working voltage of15 kV.
2.4.4. Thermal analysisThe thermogravimetrydifferential scanning
calorimetry (TG-DSC) was per-
formed with a SDT Q600 (TA, USA). The samples were heated from
50 C to1000 C at a heating rate 10 C/s under nitrogen ow (100
mL/min).
2.4.5. Acid neutralization capacity (ANC)The hydration products
in the aluminate/PC-sludge systems have a signicant
inuence on the ANC, and therefore the leachability of solidied
pastes [15]. TheANC tests, combined with the leaching of heavy
metals from the solidied sludge,was carried out using a procedure
recommended by Chen et al. [4] and Lampriset al. [15]. The pastes
were dried at 60 C, crushed and sieved (
-
In order to investigate the characteristic morphology transfer
as
matrix appeared extremely loose, and thus it was possible to
verifynumerous voids visible. The high porosity and low
crystallinity ofhydration products provided the basilic evidence
for the lowstrengths of samples A0/1010 (Fig. 2) at this stage.
Fig. 5c shows a special SEM photo of the paste A4/610.
SEMobservations conrmed the presence of a large amount of
vitreousand prismatic-shaped ettringite with high crystallinity
[24,26],homogenously distributed in the paste. This nding
correspondedwell with those observed from XRD and DTG/DSC analysis,
asshown in Figs. 3 and 4. When added, aluminate 12CaO7Al2O rap-idly
reacted with CSH2 to form ettringite gels through Eq. (2),which
covered the sludge particles, reduced and even offset theharmful
organics interference providing a safe environment forPC hydration.
As a result of this, the PC hydration and ettringitegeneration were
considerably accelerated. These hydration phaseslled the pores in
the mortar [2] and interconnected the sludgeparticles as physically
or chemically as possible. The lling effectdecreased the porosity,
altered the morphological characteristicsand subsequently made the
mortar denser and stronger in com-
uildie.g. 37.13 wt.% of dried sludge. Whereas a very interesting
ndingwas that the similar organic contents were measured in
AC2/810and AC3/710 samples (around 39.04 and 40.66 wt.%,
respectively).It became very clear that the presence of aluminate
12CaO7Al2O3eliminated the interference fromorganic compounds, and
enhancedthe formation of ettringite through Eqs. (1) and (2)
[8,11,25].The crystalline compound was able to ll the pores of the
pastesmaking a contribution to the setting [14] and onspicuous
strengthdevelopment.
C3A 3CSH2 26H! C6AS3H32 1
C12A7 3CSH2 53H! C6AS3H32 3AH3 3C3AH6 2
with : C CaO; C CO2; A Al2O3; S SO3; H H2O:Regarding AC4/610 and
AC5/510 pastes, more appreciable
strengthwas developed at 28 days, however, X-ray diffraction
failedto identify ettringite. Gu et al. [8] and Yang et al. [25]
pointed outthat the formation and stability of ettringite was
closely related tothe ratio of sulfate and aluminates in binders as
well, implying thatthe sulfate from PCwas the limiting reactant.
Moreover, the conver-sion of ettringite to monosulfoaluminate
hydrate C4ASH12, C4AH13and C2AH8 might take place due to the
presence of superuous12CaO7Al2O3 (Eq. (3)) [25] in aforementioned
pastes. Additionally,since the samples have been cured at ambient
temperature, thecarbonation of samples might have made ettringite
turn to calcite(Eq. (4)) [17,23], leading to disappearance of
ettringite. This couldbe partially conrmed by the XRD analysis that
higher peakintensities of calcite could be detected in AC4/610 and
AC5/510pastes in comparison with those in AC2/810 and AC3/710, as
shownin Fig. 3a. However, XRD failed to identify calcium-bearing
hydratestoo, i.e. C4ASH12, CSH, C4AH13 and C2AH8, probably due to
theirsmall amount and low crystallinity.
C12A7 C6AS3H32 34H! 3C4ASH12 1=2C4AH13 2C2AH8 5=2AH3 amorphous
3
3C6AS3H32 C! 3CSH2 3CC A XH 26 XH 4The main hydration products
with the same aluminate/PC ratio
and different additions of blended binder in
aluminate/PC-sludgesystems seemed much more similar, as shown in
Fig. 3a and b.The only difference was that C3S peaks appeared in
the pastes withthe 20 wt.% addition of binder. The observation that
C3S remainedinactive until the termination of curing revealed that
C3S hydrationhad been delayed or inhibited owing to the poison of
sludge, andthat an incompatibility between the aluminate and PC
hydrationproducts took place when addition of binder was higher
than20%. From these ndings it can be assumed that both the
consump-tion of ettringite and delay of C3S hydration might be
especiallyresponsible for the considerable decrease in the
strengths of AC4/620 and AC5/520 pastes (Fig. 2).
3.3. Thermal analysis
Fig. 4 compares DTG/DSC data of S/S products with
differentaluminate/PC ratios after 28 days of curing. As in the XRD
results,the thermal analysis reected the differences in phase
compositionof hydrated pastes.
The weight loss occurred at about 120 C, exhibiting a weakpeak
in the DTG curves, which might be attributed to the evapora-tion of
free and bound water of sludge. When temperature was upto 160 C, a
weight loss was detected in AC3/710, but not in other
678 G. Zhen et al. / Construction and Bsamples, indicating that
considerable ettringite has formed in thespecimen [14],
corresponding exactly with the results from XRDanalysis. The peaks
at 250350 C referred to the pyrolysis ofa function of aluminate/PC
ratio, and also to conrm the resultsfrom the XRD and DTG/DSC
analysis, the pastes with the formula-tion aluminate/PC = 0/10, 3/7
and 4/6 after 28 days of hydrationrespectively were subjected to
SEM analysis. The results are dis-played in Fig. 5. In the case of
low-aluminate binder (Fig. 5a), thehydrate phases covering the
sludge particles exhibited the muchlower crystallization and no
enough of the gel-like hydration prod-ucts has formed to ll the
pore space and to embed sludge parti-cles, suggesting that the
presence of organic materials introducedfrom sludge retarded the
hydration of PC. As a result of this, theorganics from sludge and
dehydration of CSH with much lowerlevel of crystallization [3,12].
Also, several other peaks appearedfor the solidied pastes except
AC0/1010 at around 450 C, whichwas mainly because of the water loss
of portlandite in traces[2,24] formed by hydration of free lime
from the synthesizedaluminate. Depending on the aluminate/PC ratio,
the peaks withvarious intensities were visible at 700 C or so in
the DTG curves,relating to CO2-loss of calcium carbonate [18,24]
and pyrolysis ofsome inorganic matters within sludge.
3.4. Scanning electron microscopy (SEM) analysis
Fig. 4. DTG/DSC curves of the various mortars at 28 days.
ng Materials 30 (2012) 675681pression. The low porosity of the
gel-like matrix and the tight con-nection between the sludge
particles might be the reason for thesignicantly high compressive
strength of mortar A4/610.
-
Fig. 5. SEM images of the samples A0/1010 (a), A3/710 (b),
and
G. Zhen et al. / Construction and Buildi3.5. Acid neutralization
capacity
The ANC can be used to evaluate the ability of a certain
materialto neutralize acid [19] and the likely performance of
contaminantimmobilization in an acidic environment [4], since
precipitation/dissolution of metals is directly affected [19]. The
amount of acidadded in each solution was converted into equivalents
of acidper dry weight of the material so as to obtain ANC. Fig. 6
showsthe ANC curves obtained from the 10% binder matrixes cured
for28 days. It is obvious that the starting pH values were
extremelylow (pH 8.87.8) in part due to the absence of hydrates
Ca(OH)2(Fig. 6), with a steep initial gradient to pH 7, followed by
a plateauat pH 76.
The ANC shall be reported to a reference pH, and in our
caseAccording to two pH 7 (referred to as ANCpH7) and 4 (ANCpH4)
wereconsidered to evaluate their acid resistance [19]. As depicted
inFig. 6, though the similar ANC curves were obtained for all
thesolidied pastes, the capability of resisting to acid attack
variedslightly, dependent on aluminate/PC ratio. The ANCpH4
values(6.16.3 meq g1) were relatively similar for all the solidied
sam-ples, however, a small reduction in ANCpH7 values was
observed.The corresponding ANCpH7 values were around 2.7, 2.6,
2.2,2.1 meq g1 for samples AC2/810, AC3/710, AC4/610 and
AC5/510,respectively, which were somewhat lower than that of
referenceAC0/1010 (3.2 meq g1). The buffering capability of
calcite, lowCa/Si ratio CSH and silica gel were, due to their low
dissolutioncharacteristics, mostly responsible for the ANC of
solidiedmortars,which was observed to occur between pH 75 [4]. The
introductionof aluminate in the present study was observed to
induce thegeneration of ettringite (C6AS3H32) and yet result in low
calcite,CSH and silica gel contents as previously stated. This
presumablyFig. 6. ANC curves of the 10 wt.% binder samples.aroused
the small decrement of ANC values and lowered theresistance to
long-term acid attack.
3.6. Leaching behavior of heavy metals in solidied sludge
exposed tonitric acid
In order to evaluate the inuence of pH on leaching behavior
oftoxic heavy metals, we plotted the leachate concentration at
keypoints in the ANC curves versus pH as illustrated in Fig. 7.
Thesedata clearly depict that leaching levels showed to be
stronglypH-dependent, and the presence of aluminate caused slightly
high-er solubility of Pb, Cd, Ni and Cr in a wide range of pH 36.
For in-stance, 20% replacement of cement with aluminate
increasedleachable metal concentrations for metals such as Pb and
Cd upto about 300400% at pH around 3.5 compared with the PC
only-sludge mixture. While the dissolution/retention of heavy
metalsin solidied sludge appeared to be a relatively complex
process.It depended on their chemical speciation, association to
the respec-tive organic complexes, adsorption on specic binding
sites of min-eral surface and physical retention in a porous
structure [18], etc.In respect to Pb, the increased solubility in
the S/S pastes withthe addition of aluminate, as seen in Fig. 7a,
was most likely dueto the competition between aluminate and Pb for
the active com-ponents from PC, which inuenced the ionic
incorporation/com-plexation into the crystalline network of
hydrated products. Thatwas a possible explanation that the blended
additives were lesseffective for lead xing than PC (Fig. 7a).
Furthermore a similarleaching behavior could be observed for Cr, Cd
and Ni, as illustratedin Fig. 7bd. The results also showed that all
mortars had lowerleaching of heavy metals, clearly below the legal
limits in China.For the retention of Cr and Cd, XRD analysis (Fig.
8) indicated thepresence of Fe (CrO ) (H O) and Al (OH) (CrO ) 36H
O,
A4/610 (c) cured under laboratory conditions for 28 days.
ng Materials 30 (2012) 675681 6792 4 3 2 3 13 11 4 4 2
accordingly supporting the co-precipitation mechanism proposedby
Theodoratos et al. [21] and Quina et al. [19]. Meanwhile
theencapsulation into hydrates phases [14] (e.g.
Ca6Al2Cr3O1832H2Oand Ca4Al2CrO1012H2O) partially corresponded to
the leachingcontrol of Cr. The conguration of Cd in S/S pastes
however, possi-bly because of the low crystallization, was not
detected. In addi-tion, the complexation with organics and
silicates from thedecalcication of CSH might also affect the
leachability of Crand Cd, as suggested by Cappuyns and Swennen [5]
and Chenet al. [4]. Referring to Ni, surface adsorption reaction,
however,was generally considered as the most concernful controlling
factor.
Nevertheless leachate levels of approximately 2.0 mg/L or be-low
were observed, although the leaching of these elements in-creased
with decreasing pH. The relatively low extract amountfor acidic pH
made them not critical ones when solidied sludgewas disposed in a
sanitary landll, in spite of the low legal limits(Pb: 5 mg/L; Cr:
15 mg/L; Cd: 1 mg/L; Ni 5 mg/L) in China (GB5085.3-2007).
Evidently, the addition of aluminate as a modier
-
uildi680 G. Zhen et al. / Construction and Bfor PC during the
S/S process of sludge was not markedly detrimen-tal to the leaching
control of heavy metals.
4. Conclusions
The cement-based S/S technique incorporating
aluminate12CaO7Al2O3 was studied for the treatment of
organic-highsludge. The addition of aluminate signicantly enhanced
the ce-ment-based S/S performance. The solid matrix manufactured
withthe aluminate/cement ratio of 4/6 and binder addition of 10
wt.%presented 28 day-strength of 157.2 kPa, whereas
approximately25.1 kPa obtained for the cement only-sludge mix. XRD,
TG-DSC
Fig. 7. Leached amount (mg/L) of Pb (a), Cr (
Fig. 8. X-ray diffractograms of the S/S pastes between 2h = 5.2
and 46.8.ng Materials 30 (2012) 675681and SEM analysis revealed
that the presence of aluminate counter-acted the interference from
organic matters and facilitated the for-mation of crystalline
phases such as ettringite C6AS3H32, calciteCaCO3 and quartz, which
were able to ll the pores of the solidiedpastes and lead to denser
microstructure making a contribution tothe superior strength
development.
In addition, assessment of environmental compatibility of thenal
products indicated that the addition of aluminate slightly fellthe
resistance of solidied mortars to acidic environments,
never-theless the concentration of heavy metals in the leachates
were be-low the certain legal limits (GB 5085.3-2007) set in
China.
Acknowledgements
This research has been nanced by the Science and
TechnologyCommission of Shanghai Municipality with Contract No. 08
DZ1202802 and 09DZ 1204105.
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G. Zhen et al. / Construction and Building Materials 30 (2012)
675681 681
Hydration process of the aluminate 12CaO?7Al2O3-1 Introduction2
Materials and methods2.1 Characterization of the sludge2.2
Binders2.3 Preparation of specimens2.4 Analytical methods2.4.1
Unconfined compressive strength (UCS)2.4.2 X-ray diffraction (XRD)
analysis2.4.3 Scanning electron microscopy (SEM) analysis2.4.4
Thermal analysis2.4.5 Acid neutralization capacity (ANC)
3 Results and discussion3.1 Confined compressive strength3.2
X-ray diffraction analysis3.3 Thermal analysis3.4 Scanning electron
microscopy (SEM) analysis3.5 Acid neutralization capacity3.6
Leaching behavior of heavy metals in solidified sludge exposed to
nitric acid
4 ConclusionsAcknowledgementsReferences
