Steel Slag as Cementitious Material: Feasibility Study

Construction and Building Materials 204 (2019) 458–467
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Experimental methods to determine the feasibility of steel slags as
supplementary cementitious materials
Ying Wang a,b, Prannoy Suraneni a,⇑
a
b
Department of Civil, Architectural, and Environmental Engineering, University of Miami, Coral Gables, FL 33146, USA
School of Civil and Resources Engineering, University of Science and Technology Beijing, 100083, China
h i g h l i g h t s
Hydraulic and pozzolanic behavior of basic oxygen furnace slag (BOFS), ladle furnace slag (LFS), and ground granulated blast furnace slag (GGBFS) are
studied.
Multiple tests are performed to determine the feasibility of these slags as SCMs.
GGBFS shows the best performance; BOFS and LFS should not be used as SCMs as-is.
The combined use of test methods is more accurate.
a r t i c l e
i n f o
Article history:
Received 12 November 2018
Received in revised form 11 January 2019
Accepted 27 January 2019
Keywords:
Steel slag
Supplementary cementitious materials
Isothermal calorimetry
Thermogravimetric analysis
a b s t r a c t
Limited applications for the large amounts of steel slags produced globally exist. In this study, the
hydraulic and pozzolanic behavior of basic oxygen furnace slag (BOFS) and ladle furnace slag (LFS) are
studied and contrasted with the behavior of ground granulated blast furnace slag (GGBFS). Pozzolanic
test, isothermal calorimetry, thermogravimetric analysis, and compressive strength test are performed
to determine the feasibility of these slags as supplementary cementitious materials (SCMs). The pozzolanic test is performed by measuring the heat release and calcium hydroxide consumption of slagcalcium hydroxide blends at 50 °C and pH 13.5. The other tests are performed on cementitious pastes
with 20% slag replacement by mass at 23 °C and 50 °C. GGBFS shows the best performance among all
slags in the tests, followed by BOFS, followed by LFS. It is suggested that BOFS and LFS should not be used
as SCMs as-is. The poor performance of LFS is due to flash setting, likely caused due to the rapid reaction
of C12A7, which causes hydration retardation and poor strength gain. The BOFS shows moderate performance as it has low hydraulic activity, likely explained by its low amorphous content and the presence of
low amount of reactive crystalline phases. An important finding from this study is that in order to state
whether materials could be used as SCMs or not, the combined use of pozzolanic test, isothermal
calorimetry, and compressive strength test is more accurate than the use of only one test method.
While the results obtained may here not be broadly generalizable to other materials due to variations
in slag chemistry, the approach suggested may be generalizable to obtain insights into whether any powder may be used as SCM.
Ó 2019 Elsevier Ltd. All rights reserved.
1. Introduction
Slagging agents are commonly used in crude iron and crude
steel manufacture to strip impurities from the iron ore in the blast
furnace and from the crude iron and scrap steel in the steel furnace. The slagging agents combine with the impurities to form iron
and steel slags, which are separated from the metals. The slags are
⇑ Corresponding author.
E-mail address: [email protected] (P. Suraneni).
https://doi.org/10.1016/j.conbuildmat.2019.01.196
0950-0618/Ó 2019 Elsevier Ltd. All rights reserved.
cooled and processed to form aggregates or powders, which can
potentially be used in the construction industry. Different slag
types may be obtained depending on the type of furnace and the
cooling method employed. Exact data are unavailable on slag production, but estimates indicate that worldwide iron slag and steel
slag outputs were approximately 330 million tons and 200 million
tons, respectively (2017 data) [1]. Numerous applications for aircooled iron slag exist, including as aggregates in concrete, asphaltic
paving, fill; and as a feed for cement kilns [2–11]. Ground granulated blast furnace slag (GGBFS) is used as a partial replacement
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Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
for Portland cement in concrete mixtures or in blended cements
[12–14]. GGBFS is highly amorphous and is a hydraulic material.
Concrete containing GGBFS usually has longer setting times and
lower early strength but shows higher later strength, denser
microstructure and better durability compared with Portland
cement concrete [10]. In contrast to iron slags, uses for steel slags
such as basic oxygen furnace slag (BOFS), electric arc furnace slag
(EAFS), and ladle furnace slag (LFS), are somewhat more limited.
These slags typically contain free lime (CaO) that can result in an
expansion that must be dealt with through appropriate means to
ensure its use in construction [2–22]. Steel slags may also contain
free MgO, which is deleterious as it may hydrate, expand, and
cause reduction in strength at later ages. BOFS can contain significant free CaO and free MgO, while these contents are typically
lower for EAFS and LFS [23]. The use of such slags as supplementary cementitious materials (SCMs) requires a detailed understanding of the slag properties due to their special production
processes and chemistry [6]. BOFS has been suggested to be somewhat reactive due to its composition and concrete containing BOFS
has greater corrosion resistance than that containing conventional
Portland cement [8,24–26]. LFS shows cementitious behavior,
especially in the presence of alkaline activators [14,27], but when
mixed with water, strength gain may be poor, due to hydration
to crystalline aluminate phases, which may cause rapid setting in
addition to conversion issues (similar conversion processes to
those seen in calcium aluminate cements may result due to the
presence of calcium aluminate phases) [27]. While steel slags
may have crystalline calcium silicate phases, they may not necessarily be reactive [28]. The reactivity of EAFS has been reported
to be rather weak [29]. Steel slags have also been used in masonry
mortars and self-compacting concrete providing mixtures with
adequate workability, shrinkage, strength, and durability [8]. As
studies on steel slags are somewhat limited, better understanding
and utilization of steel slags are of great significance in light of
shortages in conventional SCMs and in reducing landfilled waste.
In order to determine whether steel slags could be used as
SCMs, information about their pozzolanic and hydraulic behavior
must be obtained. This is typically done using the strength activity
index test [15,22] or other similar tests that monitor strength gain
[30–32], however, numerous factors apart from pozzolanic and
hydraulic behavior affect strength gain [9]. For example, extremely
finely ground limestone can pass a strength test, even though it is
more appropriately classified as a filler rather than as a SCM. It is
important to differentiate fillers and SCMs, because replacement
levels are limited for fillers and fillers may not provide later-age
contributions to durability that SCMs provide. Other pozzolanic
tests include those that monitor calcium hydroxide consumption,
such as Chappelle and Frattini tests [32,33], however, it is not clear
if such methods are appropriate for slags, which are more hydraulic. A promising alternative to determine the pozzolanic and
hydraulic behavior of SCMs is a test that measures the heat release
and calcium hydroxide consumption of SCMs blended with calcium hydroxide in a high pH solution [33,34]. This test offers significant advantages over conventional pozzolanic tests as it
provides information about both pozzolanic and hydraulic behavior for a wide variety of SCMs. In this study, the pozzolanic and
hydraulic behavior of BOFS and LFS and their effects on the properties of cementitious pastes are quantified and compared with
GGBFS. This is done using four methods: the aforementioned pozzolanic test, isothermal calorimetry, thermogravimetric analysis,
and strength gain [33]. There are two main objectives in this study.
The first objective is to better quantify BOFS and LFS for use as
SCMs in concrete. The second, and more general objective is to
develop an approach using a series of tests which may be used to
determine whether any powder can be used as SCM.
2. Materials and methods
2.1. Materials
Blast furnace slag (BFS), basic oxygen furnace steel slag (BOFS),
flue gas desulfurization gypsum from Xingtai (Hebei province) and
ladle furnace slag (LFS) from Liuzhou (Guangxi province) were
used in this study. An ASTM Type I/II cement was also used in
the study. The bulk oxide composition (as obtained from a calibrated X-ray fluorescence device (FluoroMax-4, Horiba)) and
Blaine fineness of the cement, gypsum and the slags are shown
in Table 1. All slags have considerable contents of Ca, Al, and Mg.
The presence of free CaO and free MgO may cause expansion during the hydration process [23]. The LFS used here is considerably
different from conventional LFS materials (although it should be
pointed out that steel slags show considerable variation in their
chemistries); it has relatively higher amounts of Al and lower
amounts of Si. This is because Al bars were used to extract and purify the steel during the manufacture (information provided by the
manufacturer). As the materials have similar fineness values, differences in reactivity due to differences in fineness are unlikely.
Fig. 1 shows X-ray diffraction (XRD) patterns (Rigaku D/Max-RB
diffractometer, Rigaku, Cu Ka anode) for the three slags. As quantitative XRD was not performed, evaluations about amorphous
and crystalline nature have some uncertainty associated with
them. The GGBFS is mostly amorphous (based on the amorphous
hump and only one crystalline peak) but shows the presence of
crystalline Fukalite (Ca4Si2O6(CO3)(OH)2). Due to its amorphous
nature and its chemical composition with large amounts of CaO,
SiO2, and Al2O3 (Table 1 and Fig. 1), it is expected that the GGBFS
would behave as an SCM, consistent with literature [12–14]. The
BOFS appears to be far less amorphous than the GGBFS, as an obvious amorphous hump is not detected [15]. It shows the presence of
numerous crystalline phases, including b-C2S, c-C2S, C3S (monoclinic), CaO (free CaO), MgO (free MgO), and RO phase (a continuous solid solution composed of divalent metal oxides, such as FeO,
MgO, MnO, CaO, etc.). The Fe2O3 content is high (25% by mass),
although no significant amount of metallic iron was observed. As
C3S and C2S (note that in cement chemistry notation C = CaO,
S = SiO2, and A = Al2O3) react with water and result in strength gain
in portland cement, the BOFS could potentially be used as an SCM
(although classically SCMs are amorphous aluminosilicates and not
crystalline) [9]. The LFS appears to have amorphous content lesser
than GGBFS but greater than BOFS (semi-quantitatively based on
the sizes of the amorphous humps [15]). Strong peaks of the crystalline phase Mayenite (Ca12Al14O33, C12A7) are detected in the XRD
Table 1
Oxide composition (mass %) and Blaine fineness (m2/kg) of the raw materials.
Material
CaO
SiO2
Al2O3
MgO
Fe2O3
Na2O
K2O
SO3
Blaine fineness
GGBFS
BOFS
LFS
Cement
Gypsum
46.54
41.03
55.86
64.33
48.13
29.78
16.15
0.73
20.46
2.62
12.18
4.31
36.91
4.68
0.96
6.00
3.64
3.40
1.17
1.48
1.22
27.03
1.16
3.94
0.48
0.45
0.20
0.00
0.16
0.08
0.41
0.17
0.01
0.35
0.21
0.36
0.16
0.40
2.47
44.03
420
400
400
399
373
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Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
Fig. 1. X-ray diffraction (XRD) patterns for three slags.
pattern, in addition to peaks of CaO and MgO. As it seems to have a
moderate amorphous content, the LFS is also a potential SCM.
C12A7 is a component of calcium aluminate cement (CAC) and other
rapid-set cements; it reacts extremely rapidly with water in the
absence of gypsum [27,35–37]. In the absence of gypsum, some
have suggested that the initial hydration products of C12A7 will
convert from metastable C2AH8 to stable C3AH6, causing an
increase in porosity and a consequent decrease in strength [27].
BOFS and LFS show minor peaks of free CaO and free MgO, which
may be deleterious and cause reduction in strength. The phases
and amorphous contents of the BOFS and LFS are broadly consistent with literature [4,6,8,10,14,27].
2.2. Experimental methods
Reagent grade calcium hydroxide and potassium hydroxide
were used to prepare the model systems for the pozzolanic test
[33]. The model system consisted of a 3 to 1 mass ratio of calcium
hydroxide and SCM mixed with solutions of 0.5 M potassium
hydroxide, at a liquid-to-solid ratio of 0.9. A total of 40 g of material was mixed in a plastic container using a spatula for 4 min.
Immediately after mixing, 6–7 g of material was transferred into
a glass ampoule, which was then placed in an isothermal calorimeter (TAM Air, TA Instruments) that had been preconditioned at
50 °C. Due to temperature differentials, it typically took 45 min
for the signal to stabilize, and this part of the signal was not
recorded. For highly reactive SCMs, this may result in a slight
underestimation of the total heat release. The heat flow from the
samples was recorded for 240 h, after which the samples were
removed from the calorimeter.
Approximately 50 mg of material was taken from the sample
and thermogravimetric analysis (Q50, TA Instruments) (TGA) was
performed on it to determine calcium hydroxide contents using
the tangential method [38]. The analysis was performed by heating
from 23 to 600 °C at a rate of 10 °C/minute in a nitrogen purged
atmosphere. The calcium hydroxide consumption is based on a
comparison of the final and initial amounts of calcium hydroxide
normalized to the mass of SCM in the model systems.
Cementitious pastes were made using a 20% SCM mass replacement level and a water-to-cementitious ratio (w/cm) of 0.40. This
was done for the three slags and a control cement paste without
any slag. A total of 40 g of material was hand mixed in a plastic
container using a spatula for 4 min. These samples were hand
mixed in order to better compare the results with the pozzolanic
test. Immediately after mixing, 6–7 g of material was transferred
into a glass calorimeter ampoule, which was then placed in an
isothermal calorimeter (TAM Air, TA Instruments) that had been
preconditioned at two temperatures 23 ± 0.05 °C and 50 ± 0.05 °C.
The heat flow from the samples was recorded for 168 h, after
which time the samples were removed from the calorimeter. For
samples at 50 °C, approximately 50 mg of material was taken from
the sample and thermogravimetric analysis (Q50, TA Instruments)
was performed on it to determine calcium hydroxide contents
using the tangential method [38]. The analysis was performed by
heating from 23 to 600 °C, at a rate of 10 °C/minute in a nitrogen
purged atmosphere.
Compressive strength testing was also performed on the
cementitious pastes. Pastes with the same mixture design as earlier were mixed using a paddle mixer. The mixing procedure is
according to ASTM C305-14 [39]. These pastes were cast into
5 cm cubic molds and cured in a moist chamber with a relative
humidity greater than 95% for one day. Cubes were then
demoulded and then cured inside a solution of simulated pore
solution at 23 ± 1 °C. The composition of the simulated pore solution was 0.2 M NaOH + 0.6 M KOH + 0.03 M Ca(OH)2 [40–42]. At
1, 7, 28, and 91 days, three cubes were tested in compression
and presented strength results are averages of the three replicates.
After samples were tested for compressive strength, small pieces
from the core of the samples were ground, and thermogravimetric
analysis (Q50, TA Instruments) was performed on it to determine
calcium hydroxide contents in a manner similar to that outlined
in the previous paragraph.
Pastes containing LFS at w/cm 0.40 showed rapid setting behavior, perhaps due to the C12A7 reaction [37,43]. For compressive
strength and thermogravimetric analysis, the LFS pastes were
tested at a w/cm of 0.50 and hand mixed, similar to the hand mixing adopted for the pozzolanic testing. It was not possible to easily
mix pastes with LFS at w/cm 0.40 or mix using a paddle mixer for
compressive strength tests because of rapid setting, which did not
allow for the casting of paste cubes. Therefore, results presented in
this work for compressive strength and thermogravimetric analysis are for the LFS at w/cm 0.50. While the use of different w/cm
values may present a possible issue, isothermal calorimetry and
thermogravimetric analysis results of the LFS pastes at w/cm
0.40 and 0.50 were compared and showed little effect of the w/
cm on the paste behavior, as discussed later in the text. In order
to investigate the rapid setting of the LFS, pastes with w/cm 0.40
were mixed with additional gypsum of 6% and 12%. Isothermal
calorimetry till 7 days and thermogravimetric analysis (at 7 days)
at 23 °C was carried out in a manner similar to that described earlier. A summary of the testing that was performed is shown in
Table 2.
3. Results and discussion
3.1. Pozzolanic test
Fig. 2 shows the heat release plotted against the calcium
hydroxide consumption for the slags. In addition, SCM classification and corresponding regions in the curve for these classifications from previous results are also shown in the graph for
comparison [34]. Both isothermal calorimetry and thermogravimetric analysis results had less than 5% coefficient of variability
in replicate testing. The slags show relatively low calcium hydroxide consumption, ranging from 11.6 to 32.2 g/100 g SCM. The
slags ordered by increasing order of calcium hydroxide consumption are LFS < BOFS < GGBFS. The heat release values range from
217.5 to 496.8 J/g SCM, and slags ordered by increasing order of
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Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
Table 2
Summary of testing regime.
Test
Mixture design
Materials
Conditions
Ages
Pozzolanic test
Slag: CH = 1:3, pH 13.5
All slags
50 °C, sealed cured
Isothermal calorimetry
Isothermal for 10 days, TGA at
10 days
For 7 days
23 °C, sealed cured
50 °C, sealed cured
20% SCM, w/cm = 0.40; w/cm = 0.50
23 °C, cured in pore
1, 7, 28, 91 days
for LFS
solution
50 °C, sealed cured
7 days
23 °C, cured in pore
1, 7, 28, 91 days
solution
Additional testing performed on LFS is isothermal calorimetry till 7 days and TGA at 7 days at 23 °C for pastes with w/cm 0.40, w/cm 0.50, and
w/cm 0.40 with 6% and 12% additional gypsum. Additional testing performed on BOFS is TGA at 7 and 28 days at 23 °C and 50 °C for BOFS
pastes mixed with water at a water-to-solids ratio of 0.40.
TGA
Compressive strength
Additional tests
20% SCM, w/cm = 0.40
Cement pastes with slags and control
paste (no slag)
Fig. 2. Pozzolanic test results for the tested slags. Classification results (empty
circles and labels) from prior work [33,34] are also provided for comparison.
heat release are BOFS < LFS < GGBFS. The calcium hydroxide consumption of the LFS is negative, meaning that LFS generates calcium hydroxide in the pozzolanic test, possibly because of the
reaction of the free lime present in the slag. GGBFS has the highest
calcium hydroxide consumption, which could be because of its
higher amorphous SiO2 + Al2O3 content. Heat release for GGBFS
and LFS are similar and significantly greater than BOFS. This may
be because of the greater amounts of amorphous phases in the
GGBFS and LFS compared to the BOFS [15]. While the BOFS contains reactive crystalline phases such as C3S, it may be that the
more reactive phases are present in lower quantities, which can
be confirmed using quantitative XRD. Based on the classification
of SCM reactivity [33,34], all the slags are hydraulic materials;
BFS and LFS are more reactive and BOFS is less reactive.
3.2. Isothermal calorimetry
Fig. 3 shows the heat flow (mW/g cementitious material) and
cumulative heat release (J/g cementitious material) for the slags
and the control cement at 23 ± 0.05 °C. The variation in ultimate
heat release for replicate samples was less than 3%. The LFS shows
a significant retardation of the hydration; its main hydration peak
is at approximately 28 h, while the other materials have a main
peak at approximately 9 h. The order of the heat release from
greatest to least at 168 h is LFS > Cement > GGBFS > BOFS. While
the LFS strongly retards early hydration, at later ages, it has the
Fig. 3. The (a) heat flow and (b) cumulative heat release of cementitious pastes at
23 °C.
greatest heat release. This is likely because of the large amount
of the C12A7 in the LFS, which causes a sulfate imbalance [44–
48]. The reaction of the C12A7 releases a significant amount of aluminum ions, which may be adsorbed on calcium silicate surfaces,
thereby retarding silicate dissolution and cement hydration
[47,49–51]. These results suggest that BOFS could potentially be
used as SCM, as its heat flow and heat release are in general similar
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Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
to BFS (which is in turn similar to the cement). The use of GGBFS
and BOFS reduces the total heat release somewhat and could
potentially be used to solve the issue of thermal cracking due to
the different thermal stresses in mass concrete structures [52].
GGBFS is more reactive than BOFS, therefore, the heat release of
the GGBFS is closer to the cement than the BOFS. As the hydration
continues, the heat flow of the GGBFS gradually exceeds that of the
cement, so the heat release gradually approaches the cement. This
is because of hydration and pozzolanic reactions of the GGBFS with
alkali hydroxide and calcium hydroxide [13]. It is not obvious to
identify SCMs using isothermal calorimetry using just heat flow
and heat release, though more complicated measures including
isolation of slag contribution to heat release may help in such identification [53].
Fig. 4 shows the heat flow (mW/g cementitious material) and
heat release (J/g cementitious material) for the slags and the control cement at 50 ± 0.05 °C. The LFS shows a significant retardation
of the hydration; its main hydration peak is at approximately 13 h,
while the other materials have a main peak at approximately 3 h.
The order of the heat release from greatest to least at 168 h is
GGBFS > LFS > Cement > BOFS. Compared with results at 23 °C, all
slags experienced significant thermal activation (compared to the
cement), as the heat release of GGBFS and LFS exceeded that of
the cement paste, and the heat release of BOFS was almost equal
to the cement paste. At 50 °C, the hydration kinetics of all pastes
Fig. 4. The (a) heat flow and (b) cumulative heat release of cementitious pastes at
50 °C.
are accelerated, as reflected in lower time to reach the maximum
heat release. The maximum heat release values of BF, BOFS, LFS,
and cement paste at 50 °C are increased by 26%, 20%, 11% and
9%, respectively, when compared to values at 23 °C. The maximum
heat flow of GGBFS, BOFS, and cement are approximately four
times that at 23 °C, while the heat flow of LFS is two times that
at 23 °C.
3.3. Thermogravimetric analysis (TGA)
The calcium hydroxide evolution of the paste samples at 23 °C
is shown in Fig. 5(a). In general, for tested samples, calcium
hydroxide content had less than 3% coefficient of variability in
replicate testing. For the cement paste, the calcium hydroxide content increases sharply from 1 to 7 days, and then somewhat levels
off. GGBFS initially shows a greater calcium hydroxide content
than the cement paste, likely due to filler effect [54–56]. At later
ages, GGBFS shows lower calcium hydroxide amounts than
cement, due to dilution and calcium hydroxide consumption by
the slag [33,34]. BOFS shows slightly higher calcium hydroxide
amounts than cement at early ages (due to the filler effect), and
at later ages, shows a calcium hydroxide content between the
GGBFS and the cement. This may be expected from the pozzolanic
Fig. 5. (a) Calcium hydroxide content and (b) calcium hydroxide consumption of
cementitious pastes.
Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
test (Fig. 2), as the BOFS has very little calcium hydroxide consumption. The LFS initially shows very low calcium hydroxide content (3.96 g/100 g paste at 1 day). This is not due to calcium
hydroxide consumption, but due to the retardation of cement
hydration (Fig. 3). At later ages, LFS and BOFS show similar calcium
hydroxide values, which is expected from the pozzolanic test
(Fig. 2).
The difference between the calcium hydroxide amounts in the
cement pastes (no SCM) and the cementitious paste (with SCM)
divided by the amount of the SCM gives the calcium hydroxide
consumption for each SCM in cementitious pastes. When considering the contribution of the cement in generating calcium hydroxide
amount in the cementitious paste, the filler effect should be taken
into account [54–56]. Here, based on the previous results [51,57], a
filler effect of 5% (an increase in the degree of hydration of cement
by 5%) is assumed. Fig. 5(b) shows the calcium hydroxide consumption at different ages for the tested materials. GGBFS has an
increasing calcium hydroxide consumption during the testing
duration, although the rate reduces as time increases. The calcium
hydroxide consumption of BOFS decreases and becomes more negative as time increases, indicating that BOFS is likely producing calcium hydroxide in cementitious pastes with SCM. This may be
because of the reaction of the free lime in the BOFS (Fig. 1). The calcium hydroxide consumption of LFS is very large at 1 day, which is
not an actual consumption, but due to retardation of the cement
hydration. At later ages, the LFS has a negative calcium hydroxide
consumption, suggesting that, similar to BOFS, it is producing calcium hydroxide under these conditions. As mentioned earlier, in
this set of tests, the w/cm value for LFS pastes is 0.50, which is different from the other pastes, 0.40.
Fig. 6(a) shows the effect of temperature on calcium hydroxide
contents in the pastes at 7 days. It is noted that these samples are
not under the same curing conditions as pastes at 23 °C are cured
in saturated pore solutions whereas pastes at 50 °C are sealed
cured. As the temperature increases, the calcium hydroxide content of the cement paste increases. However, the calcium hydroxide content of the BOFS and LFS increases only slightly. The calcium
hydroxide of the GGBFS remains almost unchanged. The calcium
hydroxide consumption, as defined earlier, is shown in Fig. 6(b).
As the temperature increases, the calcium hydroxide consumption
of the GGBFS increases, or, in other words, the GGBFS consumes
more calcium hydroxide produced by the cement. Similar observations have been made in literature [52]. The BOFS shows slightly
greater calcium hydroxide consumption as temperature increases.
The exact reason for this is unclear and may be due to a complex
interplay between the acceleration of cement and BOFS hydration
as the temperature increases. As the temperature increases, the LFS
shows significantly greater calcium hydroxide consumption, likely
because the cement hydration is not as retarded at a higher
temperature.
463
Fig.6. (a) Calcium hydroxide content and (b) calcium hydroxide consumption of
cementitious pastes at 7 days.
3.4. Strength gain
Fig. 7 shows the compressive strength of the different pastes
cured at room temperature. In general, compressive strength
results had less than 5% coefficient of variability in replicate testing. At 1 day, cement paste had the highest compressive strength
(21.2 MPa). The strength of GGBFS was slightly lower than the
cement paste (18.4 MPa), and the strength of the BOFS was lower
than the GGBFS (14.2 MPa). The strength of the LFS paste was
extremely low (2.2 MPa). As hydration continued, the strength of
GGBFS and cement paste samples continued to increase, and the
GGBFS strength exceeded the cement paste strength at 7 days.
BOFS showed a considerable increase in strength till 7 days. The
strength of the BOFS paste also increased at later ages, but ultimate
values were significantly lower than cement paste and GGBFS.
Fig. 7. The compressive strength evolution of cementitious pastes.
Ultimate strength (91 days) values of GGBFS (69.6 MPa) were
approximately 10% greater than the cement paste (63.2 MPa) and
30% greater than the BOFS (48.5 MPa). The LFS pastes increased
in strength from 1 to 7 days, and the strength increased only slowly
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Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
after 7 days. However, the strength of these pastes was always significantly below 15 MPa [15]. The strength gain of the GGBFS is
explained due to its hydraulic nature, due to the high amorphous
content (which is likely a calcium aluminosilicate glass). This
phase dissolves and reacts with the high pH pore solution and with
calcium hydroxide to form C-S-H and other hydration products,
thereby increasing the compressive strength [4,13,15,22,31]. The
results of XRD show that the amorphous content in the BOFS is
lower than in the GGBFS. The BOFS seems to have lower hydraulic
nature than GGBFS, either because of a lower amorphous content
or because of a lower content of reactive crystalline materials; this
low hydraulic activity is confirmed by the pozzolanic test (Fig. 2)
[22]. If the BOFS is unreactive or only slightly reactive, then the
strength of the paste should be approximately 20% lower than
the control cement paste. However, at 28 and 91 days, the reduction is between 23 and 30%, suggesting that another mechanism
may also be responsible for the poor strength gain behavior of
the BOFS paste. This could be due to the reaction of free CaO and
free MgO, which are expansive and cause strength loss. TGA performed on BOFS paste mixed with water at a liquid–solid ratio of
0.40 at 7 and 28 days showed small amounts of calcium hydroxide
at 50 °C but not at 23 °C. Based on this, a maximum of 2.6% of free
CaO in the BOFS was estimated, which could additionally have contributed to the poor performance of the BOFS paste. No magnesium
hydroxide was detected, suggesting that the free MgO content is
negligible. A number of studies have shown that certain BOFS
may also form a coating that is different in composition from the
interior of the slag particle, which may hinder reactivity and the
strength gain of BOFS pastes, which may be an additional contributor to the poor performance [58,59]. The poor strength gain
behavior of the LFS is likely due to the presence of large amounts
of C12A7, which causes flash set, sulfate imbalance, and retarded
cement hydration [27,35–37,60]. Potential conversion of C2AH8
to C3AH6, which is known in LFS-water mixtures, may also contribute to the strength loss [27]. It seems that under these conditions, the strength gain does not ultimately recover.
The relative strength gain of the pastes with slag is calculated as
the ratio of the compressive strength of the cementitious pastes
and the compressive strength of the cement paste [15,22]. Some
have suggested a relationship between relative strength gain and
slag basicity [15,22]. The latter is calculated as the sum of the mass
fractions of CaO and MgO divided by the sum of the mass fraction
of SiO2 and Al2O3. In this study, an obvious relationship between
relative strength gain and basicity was not observed, which disagrees with previous research [13,15,22]. This discrepancy is
explained by the fact that different slags have different amounts
of amorphous phases and here only bulk chemical composition is
known. In addition, some of the tested slags cause sulfate imbalance, which changes the strength gain behavior. As the strength
gain behavior of the BOFS and LFS is poor, it is not suggested to
use these materials as-is as SCMs.
Fig. 8 shows the relationship between the compressive
strength of the pastes and the calcium hydroxide consumption
at each corresponding age for pastes ages cured at 23 °C. While
there is significant scatter in the data, the paste strength gains
show a roughly positive correlation with the calcium hydroxide
consumption, that is, the more calcium hydroxide the slags consume, the greater the strength increases, although the relationship is not linear. In this analysis, the LFS data point to the far
right with artificially high calcium hydroxide consumption is
excluded. This result can be explained by reaction of amorphous
phases in the slag with calcium hydroxide resulting in C-S-H gel,
which increases the paste strength. The results obtained agree
with literature on pozzolanic action of other SCMs [31,32,61]
and reduction of calcium hydroxide in cementitious pastes with
fly ash [62].
Fig. 8. Relationship between compressive strength and calcium hydroxide
consumption.
Relationships between strength and other parameters may also
be considered. Fig. 9(a) shows the relationship between heat
release from pozzolanic test and compressive strength of cementitious pastes, at ages of 1 and 7 days. As the heat release increases,
the compressive strength of all samples increases; however, all
materials show different relationships, suggesting that a unified
relationship between heat release in the pozzolanic test and compressive strength is not possible for the tested materials. Or, in
other words, the pozzolanic test heat release is not an accurate
indicator of the compressive strength, when considering all slags
tested here. Others have shown that the heat release in pozzolanic
tests may be correlated with later age strength gain [30–32] (for
example at 91 days, as the pozzolanic testing is accelerated). However, it is clear when comparing the GGBFS and LFS strength gains
that such a relationship is not possible in our case. Fig. 9(b) shows
the relationship between heat release from isothermal calorimetry
and compressive strength of cementitious pastes, at ages of 1 and
7 days. The relationship seen for LFS is extremely different from
the relationships seen for BOFS, GGBFS, and cement paste. If the
point to the far right is excluded, then a roughly linear relationship
between strength gain and heat release could be concluded, however, the scatter in the data suggests that the one may not be able
to predict compressive strength development only by using heat
release for these slags [31].
3.5. Effect of gypsum on LFS pastes
The LFS pastes showed rapid set, likely due to C12A7 hydration,
which resulted in a retardation of the cement hydration and a drastic reduction in the strength gain [43,50,51]. In order to investigate
a potential solution, additional gypsum was added to LFS pastes at
23 °C [45,47,63]. In addition, the effect of w/cm on the hydration of
LFS pastes was also investigated at w/cm 0.40 and 0.50. TGA was
carried out on these pastes at the end of the testing duration (at
7 days) to investigate changes in the hydrate assemblage due to
the addition of gypsum.
Fig. 10 shows heat flow and heat release for the LFS with varying w/cm and gypsum additions. The w/cm does not have a strong
effect on the hydration kinetics, although it does increase the
cumulative heat release. The addition of gypsum results in a reduction of the peak time, with the addition of 12% gypsum reducing
the peak time by approximately 20 h. As the gypsum content
Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
Fig. 9. Relationship between heat release from (a) Pozzolanic test and (b)
Isothermal calorimetry and compressive strength of cementitious pastes.
increases, the aluminate (sulfate depletion) peak is delayed and the
silicate peak is accelerated, which is a classic sign of sulfate imbalance [35,45,47]. The heat flow curve suggests that the peak that is
being seen in the LFS pastes without gypsum is the silicate peak,
which is delayed because of the early occurrence of the aluminate
peak [44–46,48]. Without gypsum, C12A7 reacts almost immediately upon mixing, so the peak is not detected. With 12% gypsum
addition, the aluminate and silicate peaks are almost overlapping,
and suggest that the sulfate imbalance induced due to the C12A7
may be resolved by the addition of 12% or greater gypsum
amounts. Compressive strength testing of the mixture with 12%
gypsum at 1 day did not show a significant improvement
(<1 MPa difference in strength), suggesting that even greater
amounts of gypsum may be needed to address the sulfate imbalance. However, for actual applications, it may not always be feasible in terms of cost or convenience to add such a large amount of
gypsum.
Fig. 11 shows the derivative weight curve from the TGA. The
peak around 140 °C belongs to calcium aluminate hydrates, which
is reduced as the gypsum addition increases [44,64]. Peaks due to
ettringite are also observed at approximately 100 °C in the pastes
with gypsum but not in the pastes without gypsum. Other changes
in the response or changes due to w/cm seem minor.
465
Fig. 10. The (a) heat flow and (b) cumulative heat release of LFS pastes with
additional gypsum.
Fig. 11. Derivative weight curve of LFS pastes with additional gypsum using TGA.
466
Y. Wang, P. Suraneni / Construction and Building Materials 204 (2019) 458–467
Table 3
Comparison of the findings from different methods regarding the feasibility of a
material as SCM.
Test method
GGBFS
BOFS
LFS
Pozzolanic test
Isothermal calorimetry
SCM
Not clear
SCM, less reactive
Not clear
TGA
Compressive strength
SCM
SCM
Not clear
SCM, less reactive
SCM
Retardation of
hydration
Not clear
Not an SCM
3.6. Comparison of test methods
Table 3 shows a comparison of the implications that the different test methods provide regarding the feasibility of a material as
SCM. All methods provide some information about whether a
material can be used as SCM or not. By combining the results, it
is concluded that BOFS and LFS should not be used as SCMs as-is.
While the BOFS could potentially be used as an SCM, its reactivity
seems to be quite low, and the use of 20% BOFS replacement leads
to a reduction in strength of between 23 and 30%. The following
may be noted about the different test methods:
1. The pozzolanic test provides information about hydraulic and
pozzolanic behavior of tested materials; however, interpretation may be complex for materials such as LFS.
2. It is not obvious to identify SCMs from isothermal calorimetry;
however, the test reveals materials which have a negative effect
on the hydration (such as LFS).
3. TGA identifies materials as SCMs provided the calcium hydroxide consumption is large enough, however, this may not be
apparent at early ages.
4. Compressive strength testing identifies materials as SCMs;
however, it may also misidentify filler materials as SCMs if they
are fine enough.
Using only one method to determine the feasibility of materials
as SCMs may lead to misleading results, as also suggested by Fig. 9.
Therefore, an important finding from this study is that an accurate
manner in which materials may be identified as SCMs would be the
combined use of test methods. This may be done by using the pozzolanic test, isothermal calorimetry, and compressive test results.
The pozzolanic test provides information about whether the materials are pozzolanic or hydraulic and the extent of pozzolanic or
hydraulic behavior [33]. If the interpretation is not obvious or
materials appear less reactive, then a compressive strength test
should be run to ensure that adequate compressive strength is
achieved. The isothermal calorimetry is an additional check to
ensure that the material does not have a strong negative effect
on the cement hydration. The results from these tests may be used
together with compositional information (XRF and XRD) and fineness information (Blaine fineness or laser diffraction) to study a
wide variety of materials for their feasibility as SCMs.
4. Conclusions
A comprehensive testing regime was carried out to evaluate
whether steel slags (BOFS and LFS) could be used as SCMs. The
main conclusions from the study are:
1. The pozzolanic test suggests that all tested slags are hydraulic;
GGBFS and LFS are more reactive than BOFS.
2. Isothermal calorimetry shows that LFS strongly retards cement
hydration, likely due to C12A7 reaction. This leads to sulfate
imbalance which may be corrected through the use of additional gypsum.
3. TGA shows that GGBFS consumes calcium hydroxide. The other
slags do not consume calcium hydroxide or produce calcium
hydroxide.
4. Compressive strength testing reveals that GGBFS shows the best
performance, exceeding the strength of the cement paste at
later ages. The BOFS shows a moderate strength gain, however,
ultimate strengths are considerably lower than that with
cement paste. LFS shows negligible strengths, due to C12A7
reaction.
5. The results from this testing suggest that LFS and BOFS should
not be used as SCMs as-is; however, modifications such as the
addition of gypsum may be beneficial.
6. The combined use of pozzolanic test, isothermal calorimetry,
and compressive strength test may be used to accurately identify whether materials may be used as SCMs.
Conflicts of interest
The authors have no conflicts of interest to declare.
Acknowledgements
Sivakumar Ramanathan, Nima Hosseinzadeh Nanehkaran, and
Michael Croly (all at the University of Miami) are thanked for help
with performing experiments detailed here.
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