Efficiency of high energy over conventional milling of
granulated blast furnace slag powder to improve
mechanical performance of slag cement paste
Ahmed Bouaziz, Rabah Hamzaoui, Sofiane Guessasma, Ridha Lakhal, Djamel
Achoura, Nordine Leklou
To cite this version:
Ahmed Bouaziz, Rabah Hamzaoui, Sofiane Guessasma, Ridha Lakhal, Djamel Achoura, et al.. Efficiency of high energy over conventional milling of granulated blast furnace slag powder to improve
mechanical performance of slag cement paste. Powder Technology, Elsevier, 2017, 308, pp.37-46.
10.1016/j.powtec.2016.12.014. hal-01602598
HAL Id: hal-01602598
https://hal.archives-ouvertes.fr/hal-01602598
Submitted on 13 Dec 2017
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Efficiency of high energy over conventional milling of granulated blast
furnace slag powder to improve mechanical performance of slag
cement paste
Ahmed Bouaziz a, Rabah Hamzaoui b, Sofiane Guessasma c,⁎, Ridha Lakhal d, Djamel Achoura d, Nordine Leklou e
a
Civil engineering research laboratory, University of Biskra, Biskra, Algeria
Université Paris-Est, Institut de Recherche en Constructibilité, ESTP, 28 avenue du Président Wilson, 94234 Cachan, France
c
INRA, Research Unit BIA UR1268, Rue Geraudiere, F-44316 Nantes, France
d
Materials and Environment laboratory, University of Annaba, Algeria
e
LUNAM, Université de Nantes – IUT Saint-Nazaire, GeM, CNRS UMR 6183, Research, Institute in Civil Engineering and Mechanics, France
b
This work aims at bridging the efficiency of ball milling of granulated blast furnace slag (GBFS) to the structural
and mechanical properties of slag cement pastes. Both conventional and high energy milling of GBFS are considered with a milling duration varied between 1 and 10 h. X-ray diffraction, infra-red spectroscopy, granulometry
analysis and scanning electron microscopy are used to draw the main lines of structural and morphological
changes occurring during milling. Cement pastes formulated using 45% of GBFS in substitution are characterized.
Workability, X-ray diffraction analysis, differential scanning calorimetry and compressive testing are performed
to analyse main structural changes and reactions driven by the presence of milled GBFS as well as its direct consequence on the mechanical strength of slag cement pastes. Slag milling indicates the superior efficiency of highenergy milling, which allows a maximum slag finesse of 1.79 m2/g after 3 h of milling. Major structural changes
occur during the first 3 h of high energy milling while conventional milling does not induce any remarkable
trend. These changes concern amorphisation of the bulk structure in addition to the fracturing and agglomeration
of slag particles. Workability of slag cement pastes is remarkably improved when using 1 h of high-energy slag
milling. This result is consistent with slag finesse trend with respect to milling time and with the improvement
of GBFS reactivity. The substitution of 45% of cement (CEM I 52.5) by GBFS is only beneficial at the condition of
performing high-energy milling for at least 1 h.
Keywords:
Granulated blast furnace slag
High-energy ball milling
Cement paste
Compressive strength
Structural characterisation
(GGBFS) in cement. More than a dozen of good reasons are exposed
there such as good workability, improved strength and durability, resistance to sulphate attack, etc. Slag beneficial role on environment is reported by different contributors [3–5]. The use of slag as substitute is
expected to reduce the environmental footprint of clinker manufacturing with a rate as large as five times less of CO2 emission. This kind of
substitution contributes to sustainable cement production since the
construction sector is recognized as a large source of carbon emissions
[6].
Different recycling routes of slag appear in the literature like the hydrothermal hot pressing explored by Yanagisawa et al. [7]. Their study
shows that both pressure and water content are major influential process variables on tensile strength and bulk density of slag compacts
compared to the reaction time.
Ozbay et al. [3] expose some critical factors, which act as leverage of
slag hydraulic activity and heat of hydration. These are related to the
chemistry of the slag, its degree of finesse and substitution ratio. The
chemistry of the slag can be adjusted using activators such as NaOH.
1. Introduction
In extractive metallurgy, blast furnace slag (BFS) forms as a result of
melting of iron oxides (raw ore, pellet, sinter) in blast furnace. Besides
its role during iron smelting for assisting impurity removal from the
ore, slag is used as an industrial by-product in several applications. In
civil engineering, slag invaded two major markets, namely cement production and road construction. In particular, the market uptake justified
major research direction for slag as a cement additive. For instance, Bellman and Stark [1] report the beneficial effect of slag on mechanical performance of slag mortar at an early age subject to appropriate activation.
In chapter one of a book dedicated to by-products, Siddique [2] promotes the benefits of using Ground Granulated Blast Furnace Slag
⁎ Corresponding author.
E-mail addresses: ad.bouaziz@gmail.com (A. Bouaziz), rhamzaoui@estp-paris.eu
(R. Hamzaoui), sofiane.guessasma@inra.fr (S. Guessasma), ridhalakhal@yahoo.fr
(R. Lakhal), achoudj@yahoo.fr (D. Achoura), nordine.leklou@univ-nantes.fr (N. Leklou).
1
Activators play a central role in producing CSH phases. Their concentrations modulate the degree of reactivity of slag as shown by Hilbig and
Buchwald [8]. Complex hydration mechanisms take place during slag
activation. These are responsible for the cleavage of the slag network
and production of several mineral phases [9]. The understanding of
the hydration mechanisms is important to anticipate reactivity of slag
with cementitious materials. Soft X-ray transmission microscopy
shows that development of hydration products in terms of morphology
and growth rate is affected by the nature of activation suspensions [10].
According to the BFS cooling process, several types of slag can be
structurally distinguished. Slag may appear as amorphous, crystalline
or some combination of both [7,10–14]. If cooling is slow such as in
free air, slag is grey, porous and exhibits a crystallized structure. However, if cooling is fast, such as when using water quenching [11], slag becomes vitrified.
Slag is referred as granulated slag when subject to grinding according to the standard NF IN 15167-1 [15]. Granulated blast furnace slag
(GBFS) has a morphological form of clear-colour sand. GBFS shares the
same main chemical compounds (mainly SiO2, CaO, Al2O3, MgO) [2,11,
16–20] as ordinary Portland cement [20–23]. GBFS has a direct utilization as additive in cement with rates varying from 6% up to 95% for
CEM II and CEM III according to the norm NF EN 197-1 [22,23].
Mechanical strength improvement of slag concrete is not subject to
major debate but a large discrepancy is observed for reported gains
[11,24]. Jelidi et al. [11] show that only minor gain in compressive
strength (b4%) is expected for slag concrete after 28 days. Concrete mixtures with GGBS allow only 15% of gain in long term (one year) compressive strength according to the work of Seleem et al. [24].
The gain in mechanical performance is a challenging issue since the
compressive strength of cement containing N 95% of clinker (CEM I) is
nearly 53 MPa after 28 curing days [22,25]. Compared to other cement
types containing high rates of slag such as CEM II (6%–35%) or CEM III
(36%–95%), compressive strength decreases by 19% to 38%.
The challenge of increasing mechanical performance of slag cementitious materials is addressed in different ways. El-didamony et al. [20]
considered different combinations of Portland cement, GBFS (ground
granulated blast furnace slag) and drinking water sludge in the formulation of cement pastes. Compressive strength as large as 90 MPa is
achieved for those pastes containing higher contents (N24%) of GBFS
after 30 days of curing. The strategy of using additional phases to further
increase the strength does not often pay. Seelem et al. [24] show that
multi-phase mixtures involving GGBS (ground granulated blast furnace
slag) is not suitable to for long term strength degradation under seawater attack. Their results indicate that GGBS alone is the most efficient additive against strength deterioration with only 3% of strength decrease
after a year of exposure to sea water environment. Bellman and Stark
[1] show that overcoming low early stage compressive strength of mortar is possible if hydration of slag is better improved. The authors show
that the use of different salts resulting in the increase of pH improves
the compressive strength of slag mortar by N 166% after 2 days of curing.
The gain in strength represents 47% at an age of 28 days. Abdalqader et
al. [26] considered alkali activation of fly ash/slag to improve the performance of cement using sodium carbonate. The best compressive
strength trend with curing time is achieved when 10% of Na2CO2 is
added to GGBS with 80 MPa as the highest score after 90 days.
GGBS reactivity with water under alkali environment as well as with
calcium hydroxide determines the strength trend at early age upon the
formation of hydration products [2]. Modulation of this reactivity using
activation is certainly a promising pathway for improving strength of
slag cementitious materials. Numerous ways of activation are reported
in the literature, most of them rely on alkali activation [8,9]. Salman et
al. [27] studied the effect of Na-silicate, KOH and NaOH activation of
steel slag mortar. They achieved substantial rate of improvement of
early age compressive strength (after 3 days of steam curing) with
Na-silicate-NaOH compared to K-silicate-KOH activation. Despite the
statement of the authors on the relatively moderate gain in strength
(maximum strength 43 MPa) at long term curing (90 days), the reading
of their results still indicate beneficial effect of alkali activation.
Both calcium sulphate addition [1] and chemical activation are also
privileged ways to use efficiently slag in cementitious materials [15].
Dimitrova et al. [13] studied slag activation towards metal ions using
thermal treatment. They demonstrate that copper removal is positively
correlated to the slag heating temperature. Grinding techniques are also
used as mechanical activators [28–33]. These techniques increase slag
finesse, its specific surface area and thus they are expected to significantly affect pozzolanic and hydration kinetics. Wan et al. [28] used different types of grinding including ball, airflow and vibromill to relate
GGBS finesse to strength of slag mortar. Highest finesse scores (i.e.,
smallest average diameter and largest surface area) are attributed to
vibromill processing. The authors show that this grinding process results in largest compressive and flexural strengths of slag mortars.
Kriskova et al. [29] studied the effect of bead milling on mechanical
activation of steel slag. Besides the substantial increase in surface area
and amorphisation of slag, their study show that slag mortars exhibit
different curing trends depending on the type of slag. However, mechanical activation demonstrates insufficient gain in both compressive
and flexural strengths. Kumar et al. [30] introduced surface activation
criterion in addition to specific surface area as a factor influencing mechanical activation efficiency of slag. The authors show that surface activation potential is nonlinearly correlated to milling time for
maximum durations of 60 min [30]. Significant modulation of the hydration kinetics is obtained from non-reacted to complete hydrated
slag. The result of such activation on mechanical performance of cementitious materials is, unfortunately, not considered by the authors.
Allahverdi and Mahinroosta [31] attempted such study in the case of
high phosphorous slag cement. The authors achieve compressive
strengths as large as 120 MPa for slag mortar at an age of 180 days.
This substantial improvement of compressive strength can be related
to the large milling durations attempted by the authors (up to 18 h). It
appears also from the results of the authors that the intensity of the
milling reflected by the finesse is monotonously correlated to mechanical strength of slag mortar. Bougara et al. [32] combined similar finesse
(between 250 and 420 m2/kg) of granulated blast furnace slag with alkali activation (using NaOH and KOH). Compressive strength results
achieved by the authors are below 50 MPa at an age of 90 days and
for slag content up to 50%. The authors considered also the effect of curing temperature between 20 °C and 60 °C [32]. They noticed the reduction of compressive strength with the increase of curing temperature
even if they recommend the use of 40 °C as a curing temperature for
an optimal strength increase trend. Behim and Clastres [33] correlated
El Hadjar slag content (0–100%) and finesse (2500–4000 cm2/g) to the
compressive strength of slag mortars. The largest compressive strengths
are related to the largest finesse and smallest content of slag (10%).
Strength scores b56 MPa are achieved for the best formulations after
90 days of hydration.
With classical or conventional grinding slag finesse does not exceed
5000 cm2/g after 24 h of milling [32,33]. High energy ball milling allows
superior results (up to 9700 cm2/g in [29] and 8000 m2/g in [30]).
The analysis of the literature work on mechanical activation of slag
cementitious materials shows that sparse information is provided
about the structural justification of mechanical strength trends of slag
cementitious materials. With no intention to diminish the quality and
significant outcome of the research work attempted by several contributors, the focus of some of them on morphological analysis is obvious.
This is assuredly related to the direct consequence of grinding on particle size and specific surface area analysis [28,32]. Some other contributors addressed the result of slag grinding process through various
instrumental techniques such as XRD, calorimetry and infra-red (FTIR)
analysis but bounded their interest to slag rather than slag in cementitious materials [29]. And some others were able to correlate the result
of low energy grinding to structural analysis (XRD, calorimetry, TEM,
FTIR) [30,31]. In this work, we propose to address this gap in structural
2
analysis of high energy ball milling activation of granulated blast furnace slag as well as its consequence on mechanical performance of
slag cement pastes. The mechanical activation is considered here
through high energy planetary ball mill thanks to friction mode process
described in an earlier work [34]. The outcome of high energy milling is
compared to classical milling of GBFS to highlight main differences. Substitution of cement CEM I by GBFS with a ratio as large as 45% is
attempted. Knowing that CEM I contains large content of clinker (up
to 95%), this substitution is intended to formulate more sustainable cement. This work addresses the intensity of pozzolanic reactions, which
are expected to be modulated by grinding parameters of GBFS.
2. Materials and methods
The major raw materials are cement CEMI 52.5 CP1 NF EN 197-1
from Calcia and granulated blast furnace slag provided by ArcelorMittal
Algeria, SPA. The chemical composition of milled granulated blast furnace slag (GBFS) is performed using X-ray fluorescence spectroscopy.
These experiments are carried out using the S2 RANGER instrument
from Bruker and results are shown in Table 1. The chemical composition
is also monitored during slag milling. This milling is carried out using
both high energy and classical ball milling machines. High energy milling is based on planetary high-energy ball mill RETSCH PM400 from
Retsch Company whereas conventional milling is based on Eberhard
Bauer apparatus operating at a speed of 50 rpm. The milling is performed using steel balls having a diameter between 20 mm and
50 mm in diameter for duration of 10 h. The capacity of the milling conventional equipment is 0.05 m3, which allows processing 2 kg of slag in
a single shot. High energy milling apparatus is composed of four vials
mounted on a planar disc. The rotation of the disc is adjusted to
400 rpm. Vials rotation is in the opposite direction with a speed of
800 rpm. The milling time t is varied in the range (1–9) hours. Grinding
energy is transmitted to the slag using steel balls of 30 mm in diameter.
The steel vial capacity is 500 ml. The weight of powder samples is adjusted to 200 g per vial whereas the ball-to-powder weight ratio is 4.
Sealing is operated to avoid air contamination during milling process.
Metal contamination is also managed by imposing a delay of 30 min
after each hour of milling. This contamination emerges from iron or
chromium release from the balls, vials or both.
Formulation of the reference cement paste is performed using mixture of cement (C) and water (W) with W/C content ratio of 0.3. Slag cement pastes are prepared with a substitution ratio of 45 wt.%. Mixture of
cement (C), grinding slag (GS) and water (W) is prepared using a constant proportion throughout the formulations with a ratio W/L of 0.3
where L = C + GS.
Cement paste mixtures are poured into right-prism moulds of dimensions 4 × 4 × 16 cm3. Samples are unmould after 24 h, cut into 3
cubes of 4 × 4 × 4 cm3 and stored in water vats at temperature of 20 °C.
X-ray investigation is performed using Bruker D2 phaser diffractometer with a continuous scanning mode and Cu Kα radiation (λ =
0.1541 nm). The lines are measured in the 2θ range (5–100)° by an increment of 0.02° for 15 s. The software used for building the X-ray diffraction diagrams is DIFFRAC.EVA with ICDD PDF2. Sampling from the
same vial is performed to achieve reliable X-ray diffraction diagrams
as function of milling time. Sampling is repeated four times for the
slag powders and two times for cement pastes. The change in lattice parameters of calcium hydroxide (portlandite) Ca(OH)2 is calculated from
the fitting of X-ray patterns using Winnel software. This fitting considers the shift of the high angle diffraction line for all X-ray patterns
using Bragg's law. The crystallite size D is calculated using the XRD profile analysis determined by Williamson–Hall method [35]. More details
concerning calculation method are available in [36]. IR spectroscopy is
performed at room temperature using Thermo Scientific Nicolet IS10 instrument equipped with Smart iTR (with Diamond Plate) accessory. The
spectra are processed using Omnic software.
Granulometer Laser LS 230 is used to determine particle size distributions within the range (0.04 μm–2000 μm). Brunauer–Emmett–Teller
(BET) adsorption method and Gemini VII 2390 Micrometrics instrument are considered to measure the specific surface area of milled
slag. Due to limits of the apparatus, unmilled slag is sieved to achieve
particle size smaller than 300 μm. Analysis of specific surface area is performed using static volumetric technique working on the principle of
gas pressure balance. The internal volume and the temperature surrounding both tubes are maintained at identical conditions. The morphology of unmilled and milled powders are characterized using
scanning electron microscopy (SEM) EVO40 (Carl Zeiss®) with a Bruker
energy-dispersive spectrometer (EDS). The enthalpy change ΔH is determined using Differential Scanning Calorimetry based on NETZSCH
DSC 04 F1 Phoenix equipment. Samples of about 20–25 mg are sealed
in aluminum crucibles and heated at a rate of 20 °C/min under a nitrogen flow rate of 20 ml/min in the sample chamber and 70 ml/min rate
in the furnace chamber. The total heat is measured in a wide temperature range (25–600) °C.
Mechanical testing is carried out using 3R 250 kN equipment. Compression testing is performed on cement paste samples of dimensions
4 × 4 × 4 cm3 at different curing times (7, 28 and 120) days. For each
milling condition, 3 replicates are considered.
Workability of fresh slag cement paste is investigated using
flowtable test according to the NF EN 1015-3 norm [37]. The flow test
uses a standard conical frustum shape with a diameter of 10 cm. The
mould is centered on the flow table and filled in a two stage process
with fresh cement paste. It is lifted vertically and temped 15 times during 15 s. The relative change in diameter of the cement paste is monitored to achieve flow or consistency evolution є (%) following the
expression:
ε ð%Þ ¼ ðd−d0 Þ=d0
ð1Þ
where d is the diameter of the cement paste after 15 times tamping,
d0 = 10 cm is the initial diameter.
3. Results and discussion
3.1. Morphology and structural properties of milled slag
Table 1 shows the results of chemical composition monitoring of
GBFS as function of milling duration using high-energy ball milling.
Also are reported the composition of the unmilled and the milled
GBFS using the conventional milling. Table 1 confirms no traces of iron
or chromium contamination. The composition of the as-received slag
Table 1
Chemical composition of GBFS for all milling conditions.
t (hour)
CaO
SiO2
MgO
Al2O3
MnO
SO3
BaO
Fe2O3
K2O
SrO
Cl
TiO2
0
1
3
6
9
10a
48.04
47.59
47.73
48.42
47.24
48.15
30.8
29.84
30.70
30.17
29.62
30.14
6.58
6.6
4.6
5.9
6.90
6.52
5.57
5.81
5.51
5.70
5.85
5.91
3.16
2.59
2.31
2.64
2.44
3.01
1.82
1.72
2.08
1.53
1.63
1.92
0.7
0.66
0.49
0.69
0.64
1.05
1.01
1.08
1.03
1.04
1.01
1.09
0.9
0.9
0.88
0.77
0.86
0.91
0.38
0.29
0.33
0.30
0.27
0.24
0.168
0.15
0.13
0.12
0.11
0.14
0.158
0.28
0.26
0.30
0.23
0.20
a
Conventional milling.
3
contains a typical high rate of lime (CaO), a large content of silica (SiO2),
a low content of alumina (Al2O3) and magnesia (MgO). These are the
most representative species of the studied slag as they explain 94% of
the composition. The unmilled slag is also typical to what is reported
in the literature about blast furnace slag aggregate [2,11,16–20]. Only
slight changes in the oxide contents are not to be mentioned.
The effect of milling on slag composition is rather minor. Major oxides composing the slag for which the content is larger than 5% do not
exhibit significant change (almost 10% of variation) if we exclude magnesia (MgO) content which shifts from 6.58% down to 4.6% after 3 h of
milling (30% of variation). Furthermore, manganese oxide (MnO) content change oscillates between 16% (content of 2.64% at t = 6 h) and
27% (content of 2.31% at t = 3 h) with no consistency of the trend
with the milling time.
Fig. 1 compares XDR patterns of processed GBFS using both conventional and high energy ball milling. The spectra relative to high energy
milling are differentiated using the milling time (t). XRD pattern corresponding to t = 0 h refers to the pattern of the as-received GBFS. This
latter pattern shows diffuse scattering signature of the amorphous
GBFS structure identified from the broad diffuse hump peak in the interval 2θ between 20° and 40°. This pattern is reported in the literature as a
typical characteristic of the vitreous structure of GBFS due to the presence of glassy fractions [17,19,33,38]. The XRD pattern of the as-received GBFS shows the presence of vaterite mineral form of CaCO3
identified from the peaks (112) and (008) at 2θ = 27.1° and 42.8°, respectively. XRD pattern of GBFS similar to the one studied here issued
from (same provenance than our GBFS),
Behim et al. [33] report XRD (CoKα radiation) spectra of El-Hajar
GBFS similar to the one studied here. The authors identify the two diffuse hump peaks at approximatively 2θ = 20°–48° and 2θ = 46°–68°.
The same authors [33] report crystallized minerals like calcite, iron
and traces of gehlenite (C2AS) or akermanite (C2MS2) within the amorphous bulk. XRD patterns of GBFS from Taiyuan steel company in China
[9] issued using CuKα irradiation places the broad diffuse hump peak in
the region of 2θ between 20° and 38°. Zhang et al. [9] shows the presence of four kinds of mineral phases, namely akermanite, gehlenite, calcium silicate, and merwinite.
Structural modifications inferred to conventional milling can be read
from Fig. 1. We observe a shift by 3° of the diffuse hump peak to the
range 2θ (23°–35°). This shift is accompanied by the appearance of the
mineral calcite containing the most thermodynamically stable form of
CaCO3. Calcite is identified by the peak (104) at 2θ = 29.4°. As for
high-energy ball milling, complete amorphisation of the bulk material
is achieved after the first hour of milling. Both conventional and highenergy milling have comparable diffuse scattering signature. The difference between the two milling processes comes from the large milling
duration (10h) required by conventional milling to achieve the same
amorphisation degree resulting from only 1 h of high energy ball
milling.
The superior result of high-energy milling is explained by the intense nature of the repeated welding and fracturing of the powder mixture. During high energy milling, severe plastic deformation process of
the powder particles takes place stimulated by ball-particle-ball and
ball–particle vial wall interactions [39,40]. These results compare fairly
with the general trend of amorphisation reported by Kriskova et al.
[29] on slag bead milling for 6 h. The authors [29] observe also differences between minerals in terms of degree of amorphisation.
Fig. 2 compares infrared spectra of GBFS using both ways of milling.
Assignment of peak bands of the as-received GBFS is provided by the
Fig. 1. XRD patterns of GBFS using conventional and high energy millings.
Fig. 2. FT-IR spectra of GBFS using conventional and high energy millings.
4
(a)
spectral search feature implemented in the Thermo Scientific Omnic series software. The search performed with a sensitivity of 80% delivers
the assignments summarized in Table 2. These are also highlighted by
dash lines in Fig. 2. All OH groups identified in the band width 4000–
2500 cm−1 for the as-received slag show up for all milling conditions
but with slight shifts and smaller intensities. In the range 2500–
2000 cm− 1, C\\O and H\\O\\H stretching modes are present in the
raw material. These also resist both conventional and high energy milling except at the position 2017 cm−1. This band peak disappears after
both types of milling. In the range 2000–500 cm−1, the band position
at 1419 cm− 1 is assigned to the symmetric stretching mode of the
O\\C\\O bond (Table 2). This stretching bond is identified at a different
position (1450 cm−1) by Abdalqader et al. [26]. Conventional milling
shifts the peak form the band position 1419 cm−1 to 1437 cm−1 with
a decreasing intensity. This shift can be attributed to the presence of different form of CaCO3 as discussed from the XRD patterns (Fig. 1). The
same band peak disappears after the first hour of high energy ball milling due to the complete amorphisation of the bulk material.
Si\\O stretching band is identified in the as-received GBFS by two
peaks at 874 cm−1 and 1004 cm−1. These peaks are replaced by one
peak at 911 cm− 1 highlighted using a bold dash line after milling
(both conventional and high-energy ball milling). Abdalqader et al.
[26] attribute the shoulder at 875 cm−1 to the asymmetric stretching
of AlO4 groups. A minor sharp peak at the position 713 cm−1 is assigned
to Ca\\O stretching band. This peak is shifted to smaller wavenumbers
and flattens upon both millings. The last identified band peak at position
662 cm−1 corresponds to Si\\O\\Si bending vibration modes. Due to its
relative weak intensity, this peak is not highlighted in Fig. 2. The associated bending mode resists milling for all attempted durations.
Particle size distribution analysis of GBFS is presented in Fig. 3 for all
milling conditions. Fig. 3a exhibits the main modifications occurring on
the particle size profiles depending on milling type and duration. All
particle size distributions are plotted in cumulative form. Due to the
large-size particles composing the as-received slag powder,
granulometry is determined using classical sieving method. The median
size value D50 of the as-received slag is close to 1.1 mm. Granulometry
of 85% of the distribution is below a size of 2 mm. If laser diffraction is
used to obtain the initial particle size of the as-received slag, this leads
certainly to underestimation of the outcome of the milling process on
particle size reduction. Salman et al. [27] used laser granulometry to
characterize the size dispersion of stainless steel refining slag. They
achieve a median value of 45 μm and point out the limit of the technique
to appropriately address shape effects. The reading of their SEM
micrograph shows also possible underestimation of the starting
granulometry.
In the present study, a size shift of about two orders of magnitude
appears as a result of milling (Fig. 3a). Granulometry achieved using
conventional milling is close to the one obtained at larger high energy
milling durations (t N 3 h). However, the kinetics of particle size shrinkage related to high energy milling shows that milling types are far from
being equivalent. Indeed, significant particle size reduction is achieved
after the completion of the first hour of high energy milling. The further
increase in particle size for larger times (t N 1 h) demonstrates an evidence of particle agglomeration. This kinetics is further described later
(b)
Average particle size (µm)
Size class:
Assignment
662
713
874, 1004
1419
2017, 2160
2312
3667, 3814, 3851
Si\
\O\
\Si bending vibration modes
Ca\
\O stretching band
Si\
\O stretching band
O\
\C\
\O asymmetric stretching vibration bonds
H\
\O\
\H stretching
C\
\O stretching vibration
OH groups
D100
100
10
1
0
2
4
6
8
10
12
Milling time (h)
Fig. 3. (a) Particle size cumulative distribution as function of milling conditions. (b)
Analysis of average particle size versus milling time for different particle size classes.
using SEM images. Quantitative analysis of size class trends as function
of milling time shows the following (Fig. 3b). Evidence of agglomeration
is supported by the D15 size class. Fine particles (D15) shows a steep increase in size up to 6 h of high energy milling with an average size of
2.5 μm reached at t = 6 h. The D15 trend surpasses the achieved particle
size using conventional milling. However, the analysis of the final milling duration shows that there is some transfer to the upper class (D60),
which results in a smaller particle size. The same trend is noticed for
D60 but with a smaller rate. Here, the transfer to upper classes due to
agglomeration is limited compared to the previous case. The largest
D60 value remains smaller compared to the average size achieved by
conventional milling. The same trend is noticed for D85 with a slower
rate of particle size increase with respect to milling duration. Finally,
the D100 trend is characterized by a strong competition between particle fracturing and agglomeration. In particular, particle shrinkage continues up to an optimal milling time of 3 h. The monotonous positive
trend noticed for t N 3 h translates the upper hand of agglomeration
over particle shrinkage.
In order to keep going on fracturing of small particles (b30 μm), 1 h
or less of high energy milling is optimal. Fracturing of larger particles
can be considered up to 3 h but a cost of accepting agglomeration of
smaller ones. Kumar et al. [30] considered attrition milling of slag up
to 1 h. The authors show a continuous shift of particle size distribution
Table 2
Assignments of FT-IR peaks corresponding to the as-received slag.
Wave number (cm−1)
D15
D60
D85
Conventional milling
5
of slag towards small classes with a median value reaching 3.7 μm at the
final milling time.
Results of slag milling reported by Kriskova et al. [29] show no evidence of agglomeration for large milling durations (between 2 and
6 h). With regards to the achieved specific surface area in their results,
bead milling used in their study can be considered as intensive. This result contrasts with the present study. The main difference between their
results and ours may be explained by the presence of process control
agents (ethanol), which is supposed to enhance fracturing
phenomenon.
Fig. 4 shows the evolution of GBFS morphology as function of milling
duration. The as-received slag exhibits an airy irregular shape with a
smooth surface. Although, it is difficult to measure characteristic dimensions from SEM, the surface density of porosities seems to be close to 4%
(Fig. 4a). The porosity size varies between 3 and 88 μm with an average
size of 10 μm. Result of conventional milling (Fig. 4b) shows significant
particle size dispersion. In addition, particle morphology modification is
evident upon milling. Larger particles maintain, to some extent, the irregular shape, whereas smaller ones appear more globular and sticky.
The shape factor (i.e., ratio between the smallest and largest intercept
length) is 0.51 ± 0.07 for large particles. The same structural attribute
measured for small particles is close to 0.91 ± 0.17. No evidence of porosity is found within both small and large particles. The observed
change in morphology reflects the result of repeated welding, fracturing, and rewilding of particles [40]. High-energy milling performed for
1 h (Fig. 4c) leads to the same particle size reduction observed for conventional milling but differences in particle morphology and degree of
finesse are striking. We observe a denser and homogeneous population
of fine particles that decorate larger ones. The smaller particles tend to
weld together and the larger particles to fracture. Further increase of
high energy milling (Fig. 4c–f) accentuates particle rewelding to such
a state where the milled powder appears as a connected cohesive network. The development of the particle network can be explained by
an increase of the local temperature, which enhances cohesive forces
between aggregates [41].
The balance between fracturing and welding seems to be achieved
within the first hour of milling (Fig. 4c) as no evident of reduction in
size of large particle is perceived for larger milling times. Wan et al.
[28] confirm morphological changes from irregular to spherical shapes
through SEM observations of milled slag. The authors claim also the possibility to control better the particle size dispersity using airflow mill
and show different outcomes depending on the type of milling process.
To better capture the outcome of the present milling process, specific
surface area results are exposed in Fig. 5. The specific surface area of the
as-received slag is 0.29 m2/g. Most reported results in the literature
places specific surface area levels for GBFS between 0.37 m2/g and
0.48 m2/g [2]. Conventional milling increases specific surface area by
38%. This is a much lesser extent than the result of the first hour of
high energy milling (434%). In addition, the increase of specific surface
area reaches its maximum value (1.8 m2/g) after 3 h of high-energy
Fig. 4. SEM micrographs showing the evolution of slag particle morphology from the (a) as-received to the (b) conventional and (c) high energy milling for milling time (t) of (c) 1 h, (d)
3 h, (e) 6 h and (f) 9 h.
6
surface area is pointed out by Wan et al. [32]. In addition, Nath and
Sarker. [42] noticed that the slump of concrete and flow of mortar decreased with the increase of slag content in the mixtures.
Specific surface area (m²/g)
2.0
1.6
3.3. Structural and thermal behavior of slag cement paste
1.2
X-ray diffraction patterns of slag cement pastes performed are
shown in Fig. 7 for all milling conditions. These patterns are informative
of hydration reaction for hardened paste at 120 curing days. The reference paste cement CEM I contains calcium hydroxide (portlandite)
Ca(OH)2 of a hexagonal structure characterized by different plan
peaks P(001), P(100), P(101), P(102), P(110), P(111), P(201), P(112).
We have also evidence of calcium carbonate (calcite) CaCO3 presence
with rhombohedral structure characterized by different plan peaks
C(104), C(113), C(202). In addition, calcium silicate hydrate (C\\S\\H)
is detected with Ca2SiO43H2O monoclinic structure identified by S(100),
S(112), S(016), S(-507), S(-321), S(-122) and also ettringite with
Ca6Al2(SO4)3(OH)1226H2O hexagonal structure identified by E(101),
E(220), E(304), E(322) and E(332). Former studies indicate that CSH
gel is generally identified as an amorphous structure [43,44] and
Ettreingite is detectible after long curing time [45]. Paste cement CEM
I modified by 45% of slag milled using conventional route exhibits the
same collection of peaks of nearly the same intensities. In addition, the
presence of peak S1(211) is attributed to a second C\\S\\H with
Ca2SiO4H2O orthorhombic structure. Other characteristic peaks
A(102), A(522), A(260) are informative of the presence of C\\A\\H
with Ca2(AlO2)3(OH)(H2 O) orthorhombic structure.
Modified cement pastes with high energy milled slag shows evidence of C\\S\\H peak S1(211) irrespective of milling duration (Fig.
7). The remarkable effect of high energy milling can be attributed to
the decreasing intensity, enlargement and slight shift of portlandite
peaks. The decaying portalandite peaks can be attributed to pozzolanic
reactions between activated slag and calcium hydroxide. The hydration
High energy milling
0.8
Conventional milling
0.4
0.0
0
2
4
6
8
10
Milling time (h)
Fig. 5. Specific surface area of GGBFS for both conventional and high energy milling.
milling. The decreasing branch observed for longer milling time confirms the tendency of agglomeration. This result is supported by the
analysis of granulometry (Fig. 3), and more particularly the trend exhibited by the D100 size class (Fig. 3b).
Reported results on specific surface area evolution as function of
milling are numerous. Wan et al. [28] have found a lower range of specific surface area values between 0.51 m2/g and 0.685 m2/g for different
milling processes. Kriskova et al. [29] have found higher levels of specific
surface area in the range (0.7 m2/g–9.7 m2/g) after 6 h of milling.
3.2. Workability of cement-slag pastes
Slag cement pastes prepared using 45% of slag in substitution result
in the workability tendencies shown in Fig. 6. One can read the similarity between consistency scores and specific surface area results shown
in Fig. 5. The reference CEM I paste exhibits the lowest consistency
score (ε). The use of conventional milling has an unperceivable effect
on cement paste consistency. One hour of high energy is sufficient to increase the consistency of the slag cement paste by 43% to its maximum
level. A more stable steady-state consistency is observed for longer milling times. These observations match the workability tendencies reported on cementitious materials modified using slag [2,3,28,42]. For
instance, the same positive correlation between workability and specific
105
Flow test ε (%)
100
95
90
High energy milling
85
80
Conventional milling
75
70
65
0
2
4
6
8
10
12
Milling time (h)
Fig. 6. Workability results of slag cement pastes based on flow table experiments.
Fig. 7. XRD patterns of slag cement pastes as function of milling time.
7
C2 S þ 2H→0:5C3 S2 H3 þ 0:5CH
ð1Þ
C3 S þ 3H→0:5C3 S2 H3 þ 1:5CH
ð2Þ
Heat flow (mW/mg)
process of Portland cement can be summarized as follows [46,47]:
whereas the pozzolanic reaction can be summarized as flows [48,49]:
S þ 1:5 CH→0:5C3 S2 H3
ð3Þ
A þ 4CH þ 9H→C4 AH13
ð4Þ
where C: CaO, S: SiO2, A: Al2O3, H: H2O, CH: Ca(OH)2.
As observed from the expressions (3) and (4), the active silica (SiO2)
and aluminate (Al2O3) of milled slag react with calcium hydroxide originated from Portland cement hydration giving raise to calcium silicate
hydrate, i.e., C\\S\\H like in expression (3) or calcium aluminate hydrate, i.e., C\\A\\H like in expression (4). Shi and Day [50] have studied
the products of pozzolanic reaction mechanisms in lime + pozzolan cement pastes in the presence of Na2SO4 and CaCl2. Their X-ray diffraction
(XRD) analysis pointed out the large and sharp diffuse band of calcium
silicate hydrate (C\\S\\H) with curing time and the vanishing trend of
Ca(OH)2 after 180 days. We believe that, in our case, the boarding and
slight shift of portlandite peaks can be attributed to the reduction of
crystallite size and inter-reticular distance of portlandite, respectively
[36].
Fig. 8 shows the evolution of lattice parameter (a, c), cell volume
(V = a × a × c) and crystallite size of portlandite for all studied formulations. The lattice parameters (a) and (c) of portlandite structure given
by ICDD file are a = 0.3590 nm and c = 0.4916 nm, respectively. The
same parameters determined from the reading of XRD results show
slight changes with a = 0.3604 ± 0.0005 nm and c = 0.4942 ±
0.0005 nm. The lattice parameter (a) decreases with the increase of
high energy milling time up to 3 h before stabilization. This result contrasts with the effect of conventional milling, where negligible variation
of lattice parameter (a) is observed. The lattice parameter (c) follows
the opposite trend versus milling time with the same stabilization
after 3 h of high energy milling. Here also, the effect of conventional
milling is minor. The evolution of the cell (V) and the crystallite size
(D) confirm the critical effect of the three first hours of high energy milling, more particularly for the crystallite size, D. Indeed, the crystallite
size decreases from its reference condition (D = 79.4 ± 2.5 nm) to its
steady-state value (D = 17 ± 1.5 nm), which represents about 79% as
an amount of variation. In contrast to this, conventional milling has no
particular effect on both crystallite size and cell volume.
Fig. 9 shows DSC results of slag cement pastes for all formulations involving slag milling time. Heat flow curves show the existence of some
endothermic peaks attributed to the decomposition of water bonding in
a
V
-1
0.56
70
c
D
60
50
0.52
40
0.48
30
0.44
20
0.40
10
Conventional milling
0
2
4
6
8
10
High energy milling
t=1h
t=3h
t=6h
t=9h
-1.0
200
300
400
500
600
Temperature (°C)
Fig. 9. DSC curves of slag cement pastes as function of milling time.
two particular regions. The first region of decomposition is bounded between 100 °C and 350 °C. It is a characteristic of adsorbed water and it
may be related to decomposition of ettringite, dehydration of calciumsilicate hydrate (C\\S\\H) gel and calcium-aluminate hydrate. This
statement is based on identified phases from the XRD analysis (Fig. 8).
The second stage of decomposition is in the range of 400 °C and
550 °C. This region is characteristic of crystalized water and corresponds
to the decomposition of Ca(OH)2. The confrontation of these observations with the concerned literature show that the range between
50 °C and 300 °C is generally attributed to decomposition of ettringite,
C\\S\\H, C\\A\\H and monosulfoaluminate, whereas the range from
400 °C to 500 °C is attributed to decomposition of calcium hydroxide
[51–55].
To follow the enthalpy change (ΔH) associated with the decomposition of both adsorbed and crystalized water, an estimate on the areas
under the endothermic peaks in the temperature range (100 °C–
250 °C) and (400 °C to 550 °C) is provided. Table 3 summarizes (ΔH)
values as function of milling time. For cement paste formulations involving high energy milling (0 b t b 10 h), we observe that ΔH associated to the decomposition of crystallized water follows a two-stage
evolution with a first increasing stage up to 3 h and then a steadystate value. The decreasing tendency of ΔH associated to crystallized
water can be related to the pozzolanic reactions between milled slag
and calcium hydroxide. This explanation is supported by DRX results.
The former trend is not ultimately observed for ΔH associated to
absorbed water. In this case, a slight increase after 3 h of high energy
milling can be attributed to secondary C\\S\\H formation from pozzolanic reactions between milled slag and Ca(OH)2. Undetectable changes
in both ΔH values associated to crystallized and absorbed waters for
those cement paste formulations involving slag milling using conventional route.
Table 3
Enthalpy change (ΔH) of both adsorbed and crystalized water for all slag cement paste formulations as function of slag milling time.
0
0.36
-0.5
100
Crystallite size D (nm)
0.60
0.0
-1.5
80
3
a, c (nm), V (x10 nm )
0.64
Reference cement paste
t = 10 h (Conventional milling)
12
Milling time (h)
Fig. 8. Evolution of the lattice parameters (a), (c), cell volume (V) and crystallite size (D) of
portlandite as function of milling time for both conventional and high energy milling.
Milling time, t (h)
ΔH adsorbed water (J/g)
ΔH crystalized water (J/g)
0
10a
1
3
6
9
80.5
79.4
76.1
87.1
86.1
82.1
115.2 ± 3
104.5 ± 3
68.7 ± 2.8
55.7 ± 2.5
54.8 ± 2.7
56.5 ± 2.8
a
8
Conventional milling.
±
±
±
±
±
±
2
1.7
2.1
2.4
1.8
1.7
Compression strength gain (%)
Conventional milling
High-energy milling
10
t=3h
t=6h
t=1h
5
0
Mechanical results provide an insight on the substitution of cement by
milled slag. Improvement of both short and long term compressive
strength of slag cement paste is possible with formulations involving
45% of cement CEM I substitution with high energy milled slag.
t=9h
Acknowledgments and funding information
t = 10 h
PHEBM3h
-5
PHEBM6h
PHEBM9h
We thank Margarita Walferdein from IRC/ESTP, Cachan, France, for
her technical assistance concerning structure and mechanical
measurements.
CEMI+ 45% milled slag
-10
Curing (day)
7
28
120
-15
-20
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Fig. 10. Compression strength gain of slag cement pastes as function of milling time.
3.4. Mechanical behavior of cement-slag pastes
Fig. 10 shows the compression strength gain for slag paste cements
for all milling conditions as function of slag milling time. The gain is
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4. Conclusions
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rates of slag finesse. High energy milling efficiency is limited by the agglomeration of slag particles, which is observed for milling durations superior to 3 h. This study concludes that the most relevant milling
durations are within the hour unit. The decrease of the crystallite size
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observed for conventional milling. Mechanical activation of slag in cement paste is demonstrated through the presence of C\\S\\H and the
decaying profile of portlandite. High energy milling has a direct effect
on the pozzolanic reactions that take place in the slag cement paste.
Table 4
Compression strength of slag cement pastes as function of milling and curing conditions.
Formulation Slag milling time
(h)
Compressive strength (MPa)
Curing,
7 days
Curing,
28 days
Curing,
120 days
CEMI000b
PCOM10H
PHEBM1H
PHEBM3H
PHEBM6H
PHEBM9H
52
40
47
52
54
53
55
46
54
57
59
55
57
49
60
63
62
56
a
b
No slag
10a
1
3
6
9
Conventional milling.
Reference.
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