Construction and Building Materials 24 (2010) 1134–1140
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Construction and Building Materials
journal homepage: www.elsevier.com/locate/conbuildmat
Hydration properties of basic oxygen furnace steel slag
Wang Qiang, Yan Peiyu *
Department of Civil Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
a r t i c l e
i n f o
Article history:
Received 18 September 2009
Received in revised form 12 December 2009
Accepted 16 December 2009
Available online 13 January 2010
Keywords:
Activity
Hydration
Morphologies
Products
Steel slag
a b s t r a c t
Basic oxygen furnace steel slag is the most common steel slag in China. In this study, the hydration properties of this kind of steel slag were investigated. Steel slag was ground separately to 458 m2/kg as well as
506 m2/kg. Different hydration conditions were set by changing the temperature or pH value. Hydration
exothermic rate was measured within 4 days. Non-evaporable water content, hydration products and
hardened paste morphologies were investigated at 1, 3, 7, 28 and 90 days. The results showed that the
hydration process of steel slag was similar with that of cement. However, its hydration rate was much
lower than cement. The hydration rate of steel slag at the early age could be accelerated by raising the
fineness of particles, curing temperature or alkalinity of solution. However, raising the pH value of solution had little efficiency for the later hydration of steel slag and raising curing temperature even had negative influence on its later hydration. CSH gel and Ca(OH)2 were the main hydration products of steel slag.
A part of C3S and C2S crystal in steel slag had very low activity and unhydrated after 90 days. RO phase
was almost inert. The interface between the particles of RO phase and CSH gel was a weak region in the
system.
Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction
Steel slag is a by-product from the processing of iron to steel,
the amount of which emitted takes about ten percent of the steel
production. The raw materials and smelting process determine
the properties of steel slag. Its chemical composition consists of
CaO 45–60%, SiO2 10–15%, Al2O3 1–5%, F2O3 3–9%, MgO 3–13%,
FeO 7–20%, and P2O5 1–4% [1]. Chemical composition is an important parameter to determine the hydraulic activity of steel slag.
The alkalinity A = CaO/(SiO2 + P2O5), proposed by Mason [2], can
be used to evaluate the hydration activity of steel slag. If alkalinity
>1.8, the steel slag should be considered as cementitious material
[3].
The mineral compositions of basic oxygen furnace steel slag include Olivine, merwinite, dicalcium silicate (C2S), tricalcium silicate (C3S), tetracalcium aluminoferrite (C4AF), dicalcium ferrite
(C2F), CaO–FeO–MnO–MgO solid solution (RO phase) and freeCaO [4,5], of which C2S, C3S and RO phase are the main phases.
The presence of C2S, C3S, C4AF and C2F endorses steel slag cementitious properties. However, the activity of C2S and C3S in steel slag
is much lower than those in Portland cement because the rate of
cooling process is quite low.
China is a big country for manufacturing steel and its output accounts for one fifth of the total output all over the world. Some of
* Corresponding author. Tel.: +86 13501215836; fax: +86 01062784982.
E-mail address: yanpy@tsinghua.edu.cn (P.Y. Yan).
0950-0618/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.conbuildmat.2009.12.028
steel slag has been used as backfill in subgrade engineering and
foundation engineering, but it is not a high efficient way. The utilization ratio of steel slag in China is lower than ten percent. Most
part of steel slag has been discharged as waste that contaminates
the environment. Application of steel slag in concrete is an efficient
way to improve the utilization ratio.
Some investigations were performed for using steel slag as
aggregate in concrete [6,7]. Researches have confirmed that concrete prepared with steel slag aggregate showed better durability
and physical properties than concrete prepared with crushed
limestone aggregate. If the specific surface area of steel slag is
raised near or over that of cement, steel slag can be used as
an additive in cement production or as concrete mineral admixture such as fly ash or blast furnace slag. Steel slag cement could
get satisfactory physical properties [4,8–11] and durability [12].
Steel slag cement demands less water and has longer setting
time than the reference pure cement [4]. Steel slag could improve mechanical properties of concrete with suitable adding
rate [13,14].
In order to use steel slag as a supplementary cementitious
material during the manufacture of cement or concrete, its hydration properties should be known. During the hydration process of
complex binder, which is composed of cement and steel slag, the
hydration of steel slag is influenced by the heat and alkaline condition caused by the prior hydration of cement [4,15]. In this study,
the hydration of steel slag under different temperature and alkaline condition was investigated.
Q. Wang, P.Y. Yan / Construction and Building Materials 24 (2010) 1134–1140
1135
Fig. 1. X-ray diffraction of steel slag used.
2. Experimental
2.1. Raw materials
Steel slag, coming from scrap smelting in basic oxygen furnace, was supplied by
Tangshan Steel and Iron Company (Tangshan, PR China). Its mineralogical phases,
which are determined by XRD analysis, are given in Fig. 1. Its chemical compositions are listed in Table 1. According to the calculation method of alkalinity proposed by Mason [2], the alkalinity of the sample is 2.37.
Theoretically, the activity of steel slag can be excited by mechanical grinding.
However, increasing the specific surface area of steel slag means more consumption
of energy. Based on the comprehensive consideration of the activity and economy,
the suitable specific surface area of steel slag is about 450 m2/kg. Two samples of
steel slag were prepared, one with specific surface area of 458 m2/kg, as the reference sample (CS), and the other with specific surface area of 506 m2/kg (FS). The results of particle size distributions of these two samples tested by a laser scattering
analyzer are given in Fig. 2.
2.2. Methods
The steel slag pastes were prepared by mixing steel slag with water or sodium
hydroxide solution at pH value of 13.0. The water/steel slag or solution/steel slag
ratio of the paste was 0.3 (mass ratio). The pastes were cast into the plastic centrifuge tubes and sealed strictly right after being stirred uniformly. Then the pastes
were divided into three parts for research: the first part was cured at temperature
of 20 ± 1 °C till the testing age (curing condition 1), the second part was first cured
at temperature of 40 ± 1 °C for 14 days and then at temperature of 20 ± 1 °C for the
remaining ages (curing condition 2), and the third part was first cured at temperature of 65 ± 1 °C for 14 days and then at temperature of 20 ± 1 °C for the remaining
ages (curing condition 3). At the ages of 1, 3, 7, 28 and 90 days, the hydration was
stopped by soaking the samples in acetone.
The rate of heat evolution during the hydration of steel slag was measured by a
Toni7338 isothermal calorimeter with an accuracy of 0.2 J/g under a constant temperature of 25 °C.
Non-evaporable water of the hydrated pastes was determined to evaluate the
degree of hydration. The pulverized samples were first put into an oven and heated
at 65 °C for 24 h, and then transferred into a muffle furnace and heated at 1000 °C
Fig. 2. Particle size distributions of two steel slag samples.
for 2.5 h. The non-evaporable water content was obtained as the difference in mass
between the sample heated at 65 °C and 1000 °C normalized by the mass after heating 65 °C, and correcting for the loss on ignition of unhydrated steel slag.
The hydration products of steel slag were mineralogically determined by X-ray
diffraction. XRD measurements were conducted with a TTR ||| diffractometer using
nickel-filtered CuKa1 radiation (=1.5405 Å), 50 kV voltage and 200 mA current.
The microstructures of hydration products of steel slag were observed by SEM
and the distributions of elements in the products were detected by EDX.
3. Results and discussion
3.1. Hydration rate of steel slag
Fig. 3 shows the exothermic rate during the hydration of steel
slag at a water/steel slag ratio of 0.30. Though the hydration rates
of CS and FS are different, the hydration processes of them are the
same, which could be divided into five stages. An exothermic peak
was formed soon after the steel slag was mixed with water due to
the release of surface energy of steel slag particles (stage 1). This
peak, whose value was 30–40 J g 1 h 1, could be called the first
exothermic peak. In order to reflect the whole process more
clearly, the maximum value of ordinate was set very small. Thus,
the first exothermic peaks of two steel slag samples were invisible
in the figure. After the first exothermic peak, the exothermic rate
decreased rapidly. The concentration of Ca2+ needed a period to
reach saturation state before further hydration of steel slag, and
this period was called dormant period (stage 2). Then the hydration process entered the acceleration period due to the hydration
of some relative highly active phases such as C2S, C3S, and the second exothermic peak formed (stage 3). With the reduction of active
Fig. 3. Heat evolution during the hydration of steel slag with different fineness.
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Q. Wang, P.Y. Yan / Construction and Building Materials 24 (2010) 1134–1140
accelerate the hydration of some C2S and C3S [16]. So there was
not a dormant period before the hydration of C2S and C3S, which
Table 1
Chemical composition of steel slag w/%.
SiO2
Al2O3
FeO + Fe2O3
CaO
MgO
SO3
Na2Oeq
Loss on
ignition
17.09
4.53
23.86
40.46
10.46
0
0.42
0.91
Note: Na2Oeq = Na2O + 0.658K2O.
components, the hydration process entered the deceleration period
(stage 4). Finally the exothermic rate decreased to a very low level
(nearly 0 J g 1 h 1), and the hydration process entered the steady
period (stage 5).
From above analysis, it can be seen that the hydration process of
steel slag is very similar with that of cement. C2S and C3S are the
main active components of steel slag but their activity is very
low because they crystallize in a slowly cooling process. What’s
more, their content is much lower than those in cement. Thus,
the hydration rate of steel slag is much lower than that of cement.
So, steel slag can be regarded as a cementitious material with low
activity.
As shown in Fig. 3, the second exothermic peak of FS formed
earlier than CS, and its exothermic rate was higher than that of
CS in 96 h. So it could be found that the finer the steel slag is the
higher hydration rate at the early age is. Mechanical grinding can
make the mineral crystal lattice dislocated and increase the fineness of steel slag. Therefore, it will increase the contact area between particles and water, which promotes the hydration
activity of particles. As shown in Fig. 2, the FS sample had more fine
particles and less coarse particles than the CS sample. As a result,
the FS sample had more highly active particles than the CS sample.
It is a practicable way to accelerate the hydration rate of steel slag
by increasing the specific surface area of particles.
The influence of increasing the initial pH value by mixing steel
slag with NaOH solution on hydration rate of steel slag was studied. Though the hydration rates of CS and FS are different, their
hydration processes are similar. It can be deduced that the influence of pH condition on the hydration process of CS and FS is similar. So only sample CS was chosen as the reference sample. The pH
value of NaOH solution was 13.0 and the solution/steel slag ratio
was 0.3. As shown in Fig. 4, CS_NaOH represents the sample activated by NaOH solution. The exothermic rate of CS_NaOH declined
much more slowly after the first exothermic peak than that of CS.
This is because OH is helpful to disintegrate vitreous and thus
Fig. 4. Heat evolution during the hydration of steel slag under different pH
conditions.
Fig. 5. Non-evaporable water content of steel slag pastes cured at temperature of
20 ± 1 °C.
Fig. 6. Non-evaporable water content of CS under different curing temperature
conditions.
Fig. 7. Non-evaporable water content of FS under different curing temperature
conditions.
Q. Wang, P.Y. Yan / Construction and Building Materials 24 (2010) 1134–1140
1137
the exothermic rate of CS and CS_NaOH, it could be seen that the
hydration rate of steel slag was accelerated especially after 48 h
by increasing the pH value of hydration condition.
3.2. Non-evaporable water (wn)
Fig. 8. Non-evaporable water content of CS under different curing alkaline
conditions.
Fig. 5 shows wn contents of two different steel slag pastes cured
at temperature of 20 ± 1 °C until the age of 90 days. The wn content
of FS sample was higher than that of CS sample at the same hydration age, and the gap became larger with the curing age grows. This
is an indication that the hydration rate of FS, either at the early age
or at the later age, is higher than that of CS.
Generally, it is known that raising curing temperature would increase reaction degree of cement at early age, however it may have
negative influence on the later hydration. Similar phenomenon for
steel slag was also observed in this study. The effect of curing temperature on the wn contents are shown in Figs. 6 and 7. It is clear
from both the figures that the higher the temperature was, the
higher the wn content was at the early age (to 7 days). Fig. 6 shows
that the gaps of wn contents of different curing temperature were
the largest at the age of 7 days and got smaller with hydration
age grew. Therefore, high curing temperature is not beneficial to
the hydration rate of steel slag at the later age. The results in
Fig. 7 support the above conclusion further. As seen in Fig. 7, the
wn content of FS pastes cured in condition 3 was the lowest and
the wn contents of the other two conditions were close to each
other at the age of 90 days.
The wn contents of steel slag pastes, with or without NaOH-activating, are shown in Figs. 8 and 9. CS_NaOH and FS_NaOH represent the samples with NaOH-activating. Both figures reflect that
the gap of wn contents between samples with or without NaOHactivating extended until 28 days, which indicates that the hydration rate was accelerated by increasing the pH value of hydration
condition at this stage. However the effect of NaOH-activating decreased after 28 days. The activation efficiency for FS was higher
than CS at the early age. However the NaOH-activating efficiency
was inferior to high temperature-activating for both FS and CS at
early age.
Fig. 9. Non-evaporable water content of FS under different curing alkaline
conditions.
3.3. Hydration products
means there would not be an obvious acceleration period. Therefore, the second exothermic peak could not form. By comparing
The main difference between CS sample and FS sample is the
fineness. The chemical and mineral compositions of the two
Fig. 10. XRD patterns of hydration products of FS.
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samples are the same. The fineness of particles has influence on the
hydration rate, but it has little influence on the species of hydration
products. Therefore, only FS sample was investigated in this
section.
Fig. 10 shows the X-ray diffraction patterns of FS pastes (water/
steel slag = 0.3, curing condition 1) hydrated for 1, 3, 28 and
90 days. With the growth of curing age, the peaks of most main
components of steel slag such as C3S and C2S became weak except
that the peaks of Fe3O4 and RO phase remained almost unchanged.
Although Ca(OH)2 should be present with the hydration of C3S and
C2S, it could not be clearly identified at the early age due to the low
hydration rate. However, Ca(OH)2 was identified after 28 days. The
Fig. 11. SEM morphologies and EDX analysis of hydration products of FS at the age of 28 days, (a) SEM picture, (b) EDX result of point 1, (c) EDX result of point 2.
Fig. 12. SEM morphologies and EDX analysis of hydration products of FS at the age of 90 days. (a) SEM picture, (b) EDX result.
Q. Wang, P.Y. Yan / Construction and Building Materials 24 (2010) 1134–1140
1139
(water/steel slag = 0.3, curing condition 3) at 1 day age. It could
be observed that there were many products in the system, which
indicates that high temperature curing is very efficient to accelerate the early hydration.
Fig. 14 shows XRD patterns of hydration products of steel slag
under different curing condition at the age of 90 days. Sample A
represents paste cured under condition 3 (water/steel slag = 0.3),
and sample B represents paste cured under condition 1 (NaOH
solution/steel slag = 0.3). Through comparing Figs. 14 and 10, it
could be seen that neither did high temperature activation nor
NaOH solution activation have significant influence on the longage hydration products of steel slag.
4. Conclusions
Fig. 13. SEM morphologies of hydration products of FS at the age of 1 day under
curing condition 3.
peaks of C3S and C2S could be still well distinguished after 90 days,
which indicates that some C3S and C2S with low activation remained in the system.
Fig. 11a shows the microstructure of hydration products of FS
pastes (water/steel slag = 0.3, curing condition 1) at the age of
28 days. As seen in this figure, Gel was the main product of steel
slag, which could not be identified by XRD because of its amorphous state. The EDX result in Fig. 11b reveals that the gel was a
kind of impure CSH containing Al, Mg, Fe and so on. Some unhydrated particles were wrapped by the gel, which would decrease
the firmness of the microstructure. The EDX result in Fig. 11c reveals that the main elements of this particle were Fe, Mg, and O.
Apparently, this particle is RO phase.
Fig. 12a shows the microstructure of hydration products at the
age of 90 days. Due to sustainable hydration of cementitious compositions, the whole structure of hardened paste was much denser
than that at the age of 28 days. The unhydrated particle was RO
phase based on the EDX analysis (Fig. 12b). The RO phase was almost inert. The interface between gel and RO phase is a weak region of the system, in which it is easy to form cracks as shown in
Fig. 12a.
The pastes, cured in condition 1, either with or without NaOHactivation, were in plastic state within 3 days. However the pastes
cured in condition 3 hardened quite quickly and turned to solid
state within 1 day. Fig. 13 shows the microstructures of FS paste
1. The hydration process of steel slag, which can be divided into
five stages, is very similar with that of cement. Steel slag can
be regarded as a cementitious material with low activity.
2. The hydration rate of steel slag, either at the early age or at the
later age, can be improved by increasing specific surface area of
slag through mechanical grinding.
3. Increasing the pH value of hydration condition can promote the
hydration rate of steel slag within 28 days, however the effectiveness decreases thereafter.
4. Raising the curing temperature will accelerate the hydration of
steel slag at the early age, however it will slow down the hydration rate at the later age, and this phenomenon is more obvious
for fine steel slag.
5. C3S and C2S are the main active components of steel slag. Their
hydration products, CSH gel and Ca(OH)2, are the most important hydration products of steel slag. A part of C3S and C2S have
very low activity and they remain undydrated or just take part
in very weak reaction even after 90 days. RO phase is almost
inert. It doesn’t take action even under the high temperature
activation or NaOH-activation in this study. The interface
between the particles of RO phase and CSH gel is a weak region
of the system. The long-age hydration products of steel slag
changed little under high temperature activation or NaOH solution activation.
Acknowledgments
Authors would like to acknowledge the financial support of The
Key Technologies R&D Program of China Grant No. 2006BAF02A24
Fig. 14. XRD patterns of hydration products of FS under different curing condition.
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Q. Wang, P.Y. Yan / Construction and Building Materials 24 (2010) 1134–1140
and National Basic Research Program of China
2009CB623106.
Grant No.
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