Cement and Concrete Composites 66 (2016) 10e23
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Cement and Concrete Composites
journal homepage: www.elsevier.com/locate/cemconcomp
Chemical activation of hybrid binders based on siliceous fly ash
and Portland cement
Salaheddine Alahrache a, Frank Winnefeld a, *, Jean-Baptiste Champenois b,
Frank Hesselbarth c, Barbara Lothenbach a
a
b
c
Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Concrete and Construction Chemistry, Dübendorf, Switzerland
Saint-Gobain Recherche, Aubervilliers, France
Saint-Gobain Weber, Datteln, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 5 June 2015
Received in revised form
25 September 2015
Accepted 2 November 2015
Available online 6 November 2015
The hydration of Portland cement (PC) blended with a high amount of a siliceous fly ash (70% fly ash, 30%
PC) has been examined. The fly ash contributes significantly to the long-term strength development,
when compared to a reference sample with quartz powder. However the long setting time and the poor
early strength prevent the use of such binders. Therefore the effect of different activators (sodium carbonate, potassium sodium silicate, potassium citrate and sodium oxalate) on the setting, the hydration
kinetics and the strength development of the fly ash-PC blend has been investigated.
The addition of the activators increases the pH and decreases thus the calcium concentrations in the
pore solution, which leads to a faster reaction of alite and thus to early setting and increased early
strength. On the long term, the high alkali concentrations lower the compressive strength and lead to a
(partial) destabilization of ettringite.
Sodium oxalate and potassium sodium silicate accelerate both the setting of the fly ash-PC blend and
increase the early compressive strength. Furthermore, they show better compressive strengths at later
ages compared to the other activators. Based on these findings, they can be considered as the most
suitable accelerators among the investigated activators.
© 2015 Elsevier Ltd. All rights reserved.
Keywords:
Fly ash
Chemical activation
Hydration kinetics
Hybrid binders
Hydration degree
Thermodynamic modeling
1. Introduction
Coal is the biggest single source of energy for electricity production; coal-fired power plants provide over 42% of global electricity supply [1]. As a by-product large quantities of fly ash are
generated. In 2004, the United States generated 70.8 million tons of
coal fly ash and 60% of this ash was disposed into ground or ocean
[2]. Europe produced 41 million tons of coal fly ash in 2007, and the
re-utilization rate was only 47% [2]. Approximately 33% of the total
fly ash is used in Europe as cement raw material, as constituent of
blended cements and as mineral addition for the production of
concrete [3]. The production of Portland cement consumes a lot of
natural resources and energy and emits greenhouse gases, mainly
CO2. In this context, using fly ash as a partial substitute for Portland
cement (PC) in blended cements, dry mix mortars or concrete is a
frequent solution for upcycling the fly ash and decreasing the use of
* Corresponding author.
E-mail address: frank.winnefeld@empa.ch (F. Winnefeld).
http://dx.doi.org/10.1016/j.cemconcomp.2015.11.003
0958-9465/© 2015 Elsevier Ltd. All rights reserved.
Portland cement clinker and therefore the consumption of energy
and the production of CO2.
However, there are limits for the use of fly ash in blended cements or concrete set by the respective standards and building
codes. A possibility for the use of high quantities of fly ash in
building materials is their application in alkali activated binders
[4e10]. However, generally high additions of strong alkaline activators are needed in such systems. In its simplest form, the activator is a highly concentrated alkali hydroxide (up to 14 M) such as
sodium hydroxide or potassium hydroxide [11e13] or alkali silicate
solution [14e16]. The strongly alkaline solution promotes the reaction of fly ash and the formation of alumosilicate gels such as
Na2OeAl2O3eSiO2eH2O gel (N-A-S-H) [17e19]. The high alkalinity
required in this type of activation might cause health issues,
especially when such binders are used in dry mix mortar formulations (e.g. tile adhesives), where the workers may get into direct
contact with the highly alkaline material. A mixture of fly ash with
Portland cement avoids the use of alkaline solutions. In this case,
the PC hydration provides the early compressive strength and the
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Ca(OH)2 formed by the hydration of PC reacts later with the fly ash.
The main hydration products of such blends are generally CeSeH,
portlandite, ettringite and AFm phases [20e25].
As fly ash reacts only slowly in such systems, activation especially for blends with high fly ash contents is needed, preferably
avoiding highly alkaline conditions. For fly ash blended PC, the effect of different such “mild” activators have previously been rendez-Jime
nez et al. [23] found that the hydration of a
ported. Ferna
blend of 80 mass-% fly ash and 20 mass-% PC in the presence of 4
mass-% Na2CO3 or 4 mass-% Na2SO4 generates a mix of C-(A)-S-H
and (C)eN-A-S-H gels. In addition, calcite is formed in the case of
Na2CO3 and ettringite in the case of Na2SO4 activator. Na2SO4
activator accelerates also the early alite hydration. The use of other
activators like CaCl2 [26,27] and alkali and alkali-earth sulfates
[28,29] has been reported as well. Besides, the alkalinity of the pore
solutions is increased and the initial pozzolanic reaction of the fly
ash is accelerated. Recently, activators with carboxylate functions
(e.g. citrates, lactates) were patented for fly ash based binders free
of PC [30e33], which seem to be particularly efficient with class C
fly ashes.
However there is a lack of detailed studies of activators with
limited alkalinity and of their efficiency in accelerating very high
volume fly ash hybrid binders based on low calcium fly ash and
Portland cement. In this article, a detailed study of the interaction
of a siliceous fly ash-PC blend (70/30 by mass) with chemical activators (potassium sodium silicate ¼ (K,Na)2SiO3; sodium
carbonate ¼ Na2CO3; sodium oxalate ¼ Na2Oxalate; potassium
citrate ¼ K3Citrate) is presented. The influence of the selected activators on the hydration of a fly ash-PC blend was investigated by
calorimetric, chemical shrinkage and compressive strength measurements. The chemical composition of the pore solutions was
analyzed by ion chromatography at different hydration times, and
the nature of the hydrate phases was examined by X-ray diffraction.
The reactivity of the fly ash in a simulated PC environment
(pozzolanic test using a slurry of Ca(OH)2 þ CaCO3 þ KOH according
to [22]) in the presence of the different activators was studied.
Thermodynamic modeling was used to calculate the reaction degree of the fly ash as derived from the pozzolanic test and the
effective saturation indices of the relevant hydrate phases using the
pore solution composition.
A possible application of these high fly ash blended cements
would be the use in dry mix mortars (e.g. tile adhesives) where a
high compressive strength is not necessary. However, setting time
and strength development should be rapid enough to allow an
application at usual open times for a conventional (not rapidhardening) tile adhesive.
2. Materials and methods
2.1. Raw materials
Portland cement (CEM I 52.5 N acc. to EN 197e1) and a siliceous
fly ash (type V according to EN 197-1) were used as raw materials.
Their chemical and mineralogical compositions obtained by X-ray
fluorescence (XRF) and quantitative X-ray diffraction analyses
(QXRD) as well as the particle characteristics of the fly ash are
compiled in Table 1.
As inert reference material, a quartz powder with similar particle size distribution as the fly ash was used (see Appendix). The
quartz showed no significant pozzolanic reactivity at 20 C up to 91
days of hydration (see Appendix).
Laboratory grade chemicals were used as activators. Their purity
is as follows: (K,Na)2SiO3 (composition K2O ¼ 19.6 mass-%,
Na2O ¼ 9.2 mass-%, SiO2 ¼ 59.1 mass-%, H2O ¼ 12.1 mass-%),
Na2CO3 (>99.95%), Na2Oxalate (>99.5%) and K3Citrate (>99.5%).
11
Na2CO3 and Na2Oxalate were applied as fine powders (all
particles < 63 mm), the K3Citrate used had a particle size mainly
between 125 and 250 mm, and (K,Na)2SiO3 had a particle size distribution of 25% particles above 200 mm and 25% particles below
63 mm. The selection of the activators was based on a literature
survey followed by a previous screening test regarding setting
times.
2.2. Preparation of cement pastes
Cement pastes containing 70 mass-% of FA and 30 mass-% of PC
or 70 mass-% of quartz powder and 30 mass-% of PC were blended
with a IKA Ultra-Turrax T50 mixer at 360 rpm for 30 s and then at
600 rpm for another 30 s using a water to binder ratio (w/b) of 0.40.
The activators were added as dry powder to the fly ash-PC blend in
the following proportions: 2 mass-% (K,Na)2SiO3, 3 mass-% Na2CO3,
8 mass-% Na2Oxalate and 3 mass-% K3Citrate referred to total
binder. These amounts of activators were selected based on preliminary tests of setting time. In the case of (K,Na)2SiO3 the dosage
refers to the dry matter. All experiments were carried out at 20 C.
2.3. Setting time
The setting time measurements were carried out with a Vicat
needle apparatus according to EN 196e3 at a relative humidity of
70%.
2.4. Conduction calorimetry
The development of the hydration heat flow of the samples was
investigated using a TAM Air isothermal calorimeter (TA Instruments). About 7 g of freshly mixed paste were weighed into a
flask, which was then capped and placed into the calorimeter. The
duration of the measurements was 7 days. The cumulative heat of
hydration was determined by integration of the heat flow curve
between 1 h and 7 days.
2.5. Chemical shrinkage
Chemical shrinkage was measured according to Procedure A
described in ASTM C 1608e07. About 5 g of freshly mixed paste
were inserted in a glass vial, and de-aired and demineralized water
was added. A graduated pipette with a total volume of 1.00 ml and
an accuracy of 0.01 ml was inserted. Paraffin oil was added in the
pipette to avoid water evaporation. The vials were immersed in a
temperature-controlled water bath at 20 C. Measurements were
performed by reading the watereoil interface level of the pipettes
at regular intervals during 28 days on three replicate specimens.
This method can be used to assess the reactivity of supplementary
cementitious materials such as fly ash [22] or slag [34].
2.6. Compressive strength
Compressive strength was measured on mortars in accordance
to EN 196e1 with a w/b ratio of 0.50. The filled molds were stored
at 20 C and a relative humidity of >95%. The prisms were
demoulded after 1 day and cured subsequently at 20 C in sealed
bags. Compressive strength was tested after 1, 3, 7, 28 and 91 days.
2.7. Pozzolanic reactivity test
In order to determine the reactivity of the fly ash in the presence
of the activators, the consumption of portlandite due to the reaction
of the fly ash (or quartz) in a simulated PC environment (i.e. a slurry
of Ca(OH)2 þ CaCO3 þ KOH) was determined according to [22]. A
12
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Table 1
Characteristics of the fly ash (FA) and the Portland cement (PC) used in this study expressed in mass percent.
XRF analysis
Mineralogical phase composition
Average composition
FA
PC
FA
PC
FA glassy phase 1
55.4
23.5
Mullite
9.5
Alite
64.6
SiO2
59.4
SiO2
Al2O3
22.2
3.9
Quartz
7.5
Belite
19.9
Al2O3
19.8
Fe2O3
8.1
1.4
Hematite
0.9
Aluminate
7.8
Fe2O3
6.8
0.02
0.01
Magnetite
1.8
Ferrite
1.6
Cr2O3
<0.1
Cr2O3
MnO
0.09
0.05
Periclase
0.6
Gypsum
0.4
MnO
0.1
TiO2
0.9
0.2
Anhydrite
0.2
Hemihydrate
0.8
TiO2
1.1
P2O5
0.7
0.2
Glassy phase
77.8 Anhydrite
2.9
P2O5
0.9
CaO
4.3
64.8
TOC 2
1.8
Calcite
1.6
CaO
5.4
MgO
1.9
0.7
Quartz
0.4
MgO
2.1
K2O
2.2
0.7
K2O
2.9
Na2O
1.1
0.04
Na2O
1.3
0.2
3.0
SO3
0.1
SO3
SrO
0.2
n.d. 3
BaO
0.2
n.d. 3
Particle characteristics of the FA 6
Density (g/cm3) d10 (µm)
d50 (µm) d90 (µm)
L.O.I. 4
2.3
1.5
C5
2.0
n.d. 3
2.23
2.4
21
147
1
The average composition of the fly ash glass phase was calculated by subtracting the composition of the
crystalline phases from the total composition of the fly ash as determined by XRF.
2
TOC = total organic carbon, refers mainly to unburnt carbon
3
n.d. = not determined
4
L.O.I = loss on ignition
5
total carbon content determined by combustion analysis
6
The particle size distribution by laser diffraction (Malvern Mastersizer X, dispersion in isopropanol with
ultrasound) is given in the Appendix.
blend of 7.50 g of fly ash (or quartz as reference), 7.88 g of portlandite and 1.13 g of calcite was mixed with 20.25 g of 0.3 M KOH
solution without or with activators. The samples were cast in 60 ml
polyethylene flasks, sealed and stored at 20 C under agitation with
a shaker until the time of testing (after 7, 28 and 91 days). The
hydration of the pastes was stopped before being analyzed by
solvent exchange with isopropanol and subsequent washing of the
residue with diethyl ether. A short drying for 5 min at 40 C was
applied to evaporate the remaining diethyl ether. The solid phase
was analyzed by thermogravimetry (Mettler Toledo TGA/SDTA
851). A sample of approximately 40 mg was heated with 20 C/min
from 30 C to 980 C under nitrogen atmosphere. The amount of
portlandite was determined from the weight loss between 375 and
525 C using the tangential method and referred to the mass of the
paste ignited at 900 C [21,22]. The decrease of portlandite content
with hydration time can be used as characteristic value for the
pozzolanic reaction and for the reaction degree of the fly ash.
However, the partial consumption of the Ca(OH)2 by the activator
(e.g. the sodium oxalate removes a part of the calcium hydroxide by
the precipitation of calcium oxalate) should be taken into consideration. The reaction degree of the fly ash is calculated by thermodynamic modeling referring to the amount of the Ca(OH)2
consumed by the fly ash (see Chapter 2.10).
2.8. X-ray diffraction analyses
For determining the phase compositions of the PC and the fly
ash by Rietveld refinement [35e37] a Panalytical X'Pert Pro MPD
diffractometer in a qe2q configuration with an incident beam
monochromator and CuKa1 radiation was used. The samples were
scanned for 120 min between 5 and 75 2q using the X'Celerator
detector. For the quantification of the amorphous content of the fly
ash, the G-factor method [38e40] was applied using CaF2 as
external standard.
The X-ray diffraction analyses of the hydrated pastes were carried out after a hydration time of 28 days. The hydration was
stopped by solvent exchange method as described previously. A
Panalytical X'Pert Pro MPD diffractometer in a qeq configuration
with CoKa radiation was used. The diffractograms were recorded
using the X'Celerator detector between 5 and 85 2q with a
measurement time of 60 min and evaluated qualitatively. For phase
identification and Rietveld refinement, X'Pert High Score Plus 3.0
software from Panalytical was used.
2.9. Pore solution analyses
After 6 h, 1, 7 and 28 days of hydration, the pore solutions were
extracted by the steel die method [41] using pressures up to 255 MPa.
The collected solutions were filtered with nylon filters with a mesh
size of 0.45 mm. The concentrations of Na, K, Ca, Al, Si and sulfate were
measured by ion chromatography (Dionex DP ICS-3000). Each solution was diluted by a factor 5, 10 and/or 100 and measured in
duplicate. The hydroxide concentrations of the pore solutions were
determined in undiluted samples with a combined pH electrode
calibrated against KOH solutions of known concentrations.
2.10. Thermodynamic modeling
Thermodynamic modeling was used to study the chemical and
mineralogical changes associated with the addition of the activators to the fly ash-PC blends. The calculations were performed using
the Gibbs free energy minimisation software GEMS [42,43] using
the Nagra/PSI Thermodynamic database [44] expanded with additional data for solids relevant for cementitious systems [45e47] and
for oxalate and citrate [48e50]; a summary of the thermodynamic
data used for the organics is given in Table 2. The alkali uptake by
CeSeH was taken into account using a distribution ratio Rd of
0.42 ml/g [51,52] for both Na and K. The aluminum uptake in
CeSeH was neglected in the calculations as it is generally low in PC
dominated cements and as thermodynamic models for aluminum
uptake by high Ca/Si CeSeH are not yet available for the GEMS
cement database.
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
13
Table 2
Summary of the thermodynamic data used for citrate and oxalate. Cit ¼ OOC-CH2-C(OH)COOeCH2eCOO, Ox ¼ OOC-COO.
Species
Aqueous
HCit2H2Cit1H3Cit0
MgCitMgHCit0
MgH2Citþ
CaCitCaHCit0
CaH2Citþ
KCit2K2CitNaCit2Na2CitAlHCitþ
AlCit0
AlCitOHSolids
H3Cit(cr)
Ca3Cit2(H2O)4(cr)
Aqueous
HOxH2Ox
MgOx(aq)
Mg(Ox)22
CaOx(aq)
2Ca(Ox)2
Solids
CaOx$H2O(cr)
CaOx$2H2O(cr)
CaOx$3H2O(cr)
Reaction
Reference
log KS0
HCit2 4 Cit3 þ Hþ
H2Cit 4 HCit2 þ Hþ
H3Cit04 H2CitMgCit 4 Mg2þ þ Cit3MgHCit0 4 Mg2þ þ HCit2MgH2Citþ 4 Mg2þ þ H2CitCaCit 4 Ca2þ þ Cit3CaHCit0 4 Ca2þ þ HCit2CaH2Citþ 4 Ca2þ þ H2CitKCit2 4 Kþ þ Cit3
K2Cit 4 2Kþ þ Cit3NaCit2 4 Naþ þ Cit3Na2Cit 4 2Naþ þ Cit3AlHCitþ 4 Al3þ þ HCit2AlCit0 4 Al3þ þ Cit3AlCitOH þ Hþ 4 AlCit0
6.36
4.78
3.13
4.81
2.60
1.31
4.80
2.92
1.53
1.03
1.39
1.00
1.81
4.70
8.00
3.40
[48]
[48]
[48]
[48]
[48]
[48]
[48]
[48]
[48]
[49]
[49]
[49]
[49]
[50]
[50]
[50]
1.33
17.9
[48]
[48]
HOx 4 Ox2 þ Hþ
H2Ox 4 HOx þ Hþ
MgOx 4 Mg2þ þ Ox22þ
Mg(Ox)2
þ 2 Ox22 4 Mg
CaOx(aq) 4 Ca2þ þ Ox22Ca(Ox)2
2 4 CaOx(aq) þ Ox
4.25
1.40
3.56
5.17
3.19
0.83
[48]
[48]
[48]
[48]
[48]
[48]
CaOx$H2O(cr) 4 Ca2þ þ H2O(l) þ Ox2CaOx$2H2O(cr) 4 Ca2þ þ 2H2O(l) þ Ox2CaOx$3H2O(cr) 4 Ca2þ þ 3H2O(l) þ Ox2-
8.73
8.30
8.19
[48]
[48]
[48]
H3Cit(H2O) (cr) 4 H3Cit0 þ H2O
Ca3Cit2(H2O)4(cr) 4 3Ca2þ þ 4H2O þ 2Cit3-
GEMS was used to calculate the activities of the aqueous species
in the pore solutions using the extended DebyeeHückel equation
[53]. The saturation indices (SI) of the solid phases were calculated
by the equation: SI ¼ log(IAP/KS0) with IAP ¼ ion activity product
calculated from the measured concentrations and KS0 ¼ solubility
product of the solid. As the use of SI can be misleading when
comparing phases which dissociate into a different number of ions,
effective SI were calculated by dividing the SI by the number of ions
participating in the reactions to form the solids, see Ref. [54] for
details. A negative effective saturation index corresponds to
undersaturation, a positive value corresponds to oversaturation.
GEMS was also applied to calculate the hydration degree of the
fly ash based on its consumption of Ca(OH)2 in the pozzolanic tests.
The fraction of fly ash reacted was obtained based on the comparison of the observed consumption of portlandite with the expected consumption of portlandite per g fly ash using mass balance
calculations carried out in GEMS. In the cases of Na2CO3, Na2Oxalate
and K3Citrate addition the consumption of portlandite by the precipitation of the respective calcium salts is considered.
Moreover, the effect of increasing amounts of the added activators on the hydrate assemblage after 28 days of the fly ash-PC
blend was determined by GEMS, assuming complete hydration of
the PC. For the fly ash reaction degree after 28 days a value of 7%
was chosen which is in agreement to previous studies [55e57] and
the results obtained in the present study for the plain PC-fly ash
blend.
3. Results and discussions
3.1. Fly ash e PC blend without activator
The plain fly ash-PC blend is used as a reference to understand
the effect of the different activators. In addition, a quartz reference
is used to study the effect of the fly ash on PC hydration.
3.1.1. Hydration kinetics
The fly ash-PC blend shows an initial setting after 9 h (Table 3).
This relatively late setting is in agreement with the low content of
Portland cement. The calorimetric measurement shows two significant heat flow maxima during the first 30 h corresponding
mainly to the PC reaction (Fig. 1a).
The first peak after around 16 h is attributed to alite reaction
while the second peak after around 20 h corresponds to the
depletion of calcium sulfate and the renewed reaction of aluminate
[58]. Some authors have reported that the fly ash increased the
reactivity of PC as the higher effective water to cement ratio in the
fly ash blended cement creates more space for the growth of the PC
hydrates [59e61] and as the additional nucleation sites on the fly
ash particles surface promote the hydration of PC [62,63]. The
quartz reference shows a much faster heat development as previously shown e.g. by Deschner et al. [22], indicating that fly ash
surfaces are less efficient regarding acceleration of PC hydration
compared to quartz surfaces, that the presence of easily soluble
solids associated with the fly ash might retard the PC reaction or
that the aluminum released from the fly ash might delay alite
dissolution [64]. The retarding effect of the fly ash compared to
quartz is only visible during the first day of hydration. After 24 h,
both mixtures show the same cumulative heat (Fig. 1b) and the
Table 3
Initial and final setting time of the fly ash - PC blend (70/30 by mass) without and
with activators.
Activator
Initial setting time (h)
Final setting time (h)
without activator
2 mass-% (K,Na)2SiO3
3 mass-% Na2CO3
8 mass-% Na2Oxalate
3 mass-% K3Citrate
9.0
1.5
2.5
5.3
3.3
10.0
4.5
5.3
6.0
11.3
14
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Fig. 2. a) Chemical shrinkage of the fly ash-PC blends without and in the presence of
the different activators, b) Evolution of the fly ash degree of reaction calculated by
GEMS using the data from the pozzolanic test as a function of hydration reaction time
in the presence of the different activators.
Fig. 1. Conduction calorimetry of the fly ash-PC and of the quartz-PC blends (70/30 by
mass) without and with (K,Na)2SiO3 and Na2CO3, respectively, (a) specific heat flow, (b)
cumulative heat normalized to the solid content.
same compressive strength (Table 3).
The cumulative heat (Fig. 1b) and chemical shrinkage (Fig. 2a) of
the fly ash blended PC increase with time. The early chemical
shrinkage is associated to PC hydration, while the fly ash contributes rather to the late chemical shrinkage. Thus also the compressive strengths of the fly ash-PC and the quartz-PC blends are
comparable during the first 7 days of curing, as mainly the PC has
reacted (Table 3). The compressive strength of the quartz reference
shows only a slight strength gain between 7 and 91 days and reaches a value of 13 MPa after 91 days. The compressive strength of
the fly ash continues to increase with time and reaches 35 MPa after
91 days, indicating that the fly ash contributes positively to the
long-term development of compressive strength.
3.1.2. Reactivity of fly ash
The reactivity of the fly ash is studied in a simulated PC environment by a pozzolanic test, i.e. by measuring the portlandite
consumption in samples containing Ca(OH)2 and CaCO3 in 0.3 M
KOH solution to mimic the pH conditions of a Portland cement. The
TGA analyses of the fly ash samples after 7, 28 and 91 days of reaction show peaks in the derivative weight loss curve indicating the
presence of ettringite (the aluminate and the sulfate originate from
the fly ash), hemi- and monocarbonate and portlandite at around
105 C, 125e175 C and 470 C, respectively (see Appendix); a broad
peak from 50 to 400 C indicates the presence of CeSeH.
The portlandite content decreases with time due to the reaction
of the fly ash, while no such change is observed in the quartz
sample. Fig. 2b shows the fly ash reaction degree as obtained by
comparison of the observed consumption of portlandite with the
expected consumption of portlandite based on mass balance calculations using GEMS. The reaction of the fly ash increases with
time and reaches approximately 20% after 91 days.
3.1.3. Hydration products
The hydration of the fly ash blended with PC at 20 C results in
the formation of monocarbonate, hemicarbonate, ettringite and
portlandite as shown by the X-ray diffraction pattern of the plain fly
ash-PC paste after 28 days of curing (Fig. 3a). The TGA data confirms
this finding (Fig. 3b). CeSeH cannot be detected by XRD due to its
ill-crystalline/amorphous nature, but its presence can be derived
from the TGA data. The observed phases are in agreement with
previous observations in similar systems [23].
3.1.4. Pore solution analyses and saturation indices
The reaction of fly ash blended with Portland cement leads to
the formation of a low Ca/Si CeSeH, to an increase of the silicon
and to a decrease of calcium concentrations in the pore solution
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
15
Fig. 3. X-ray diffraction patterns (a) and TGA thermograms (b) of the fly ash e PC blend without and in the presence of (K,Na)2SiO3 and Na2CO3, respectively, after 28 days of
hydration at 20 C (A ¼ alite, B ¼ belite, C ¼ calcite, E ¼ ettringite, Hc ¼ hemicarbonate, M ¼ mullite, Mc ¼ monocarbonate, P ¼ portlandite, Q ¼ quartz).
[21,22] as shown in Fig. 4 for the sample without activator. The pH
values increase up to 7 days of hydration due to dissolution of alkalis from the anhydrous compounds and due to a reduction of the
amount of pore solution with ongoing hydration. After 28 days the
pH decreases in agreement with e.g. [22], indicating the pozzolanic
reaction of the fly ash. The full data set of the measured pore solutions is given in the Appendix.
The pore solutions are initially saturated with respect to CeSeH
and Ca(OH)2 and oversaturated with respect to ettringite (Table 5).
After 1 day, when the calcium sulfate has been consumed, the solutions are saturated with respect to ettringite and become, due to
the reaction of fly ash, slowly undersaturated with respect to portlandite, which is consistent with the observed decrease of portlandite with time as found in the pozzolanic test (see Appendix).
3.1.5. Hydration mechanism of the fly ash-PC blend
During the first 7 days, the hydration of PC dominates the reaction of the blend with water and is mainly responsible for setting,
heat release and early compressive strength. The fly ash reacts more
slowly leading to the consumption of Ca(OH)2 and the formation of
additional CeSeH, ettringite and AFm phases. The reaction of the
fly ash slowly but clearly increases the compressive strength at later
ages. In the case of the quartz, where no (or very little) pozzolanic
reaction occurs, compressive strength increases only slightly
beyond 7 days as shown in Table 4.
3.2. Activation by (K,Na)2SiO3
3.2.1. Hydration kinetics
Adding 2 mass-% of (K,Na)2SiO3 to the fly ash-PC blend decreases the initial setting time from 9 h to 4.5 h (Table 3). The PC
hydration is accelerated; the main hydration peak in calorimetry
(Fig. 1a) is shifted to earlier time compared to the sample without
activator indicating a strong acceleration of the PC reaction. The
peak shape is also modified significantly (no separate calcium
sulfate depletion peak is visible). The cumulative heat at early age
(48 h) is higher than in the case of the plain paste (Fig. 1b). The
similarity between the calorimetric signals (shape and position) of
the PC blended with fly ash and with quartz, respectively, shows
that (K,Na)2SiO3 accelerates mainly the reaction of the PC.
A similar chemical shrinkage is observed both in the absence
and presence of (K,Na)2SiO3 (Fig. 2a), while the compressive
strength after 1 day is enhanced in the presence of (K,Na)2SiO3
(Table 4). However, at later hydration times, the presence of
16
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Fig. 4. Evolution of (a) Si concentration, (b) Ca concentration and (c) pH of the pore solution of the fly ash-PC blends in the presence of different activators. Accuracy is ±10% for the
ion concentrations and ±0.05 for pH.
Table 4
Compressive strength development of the fly ash-PC blend (70/30 by mass) without and with activators compared to the quartz reference.
Activator
Compressive strength (MPa)
1d
quartz reference
without activator
2 mass-% (K,Na)2SiO3
3 mass-% Na2CO3
8 mass-% Na2Oxalate
3 mass-% K3Citrate
3.2
3.3
4.8
3.6
4.6
0.5
3d
±
±
±
±
±
±
0.1
0.1
0.5
0.2
0.1
0.1
8.7
8.7
8.4
6.8
4.4
0.6
7d
±
±
±
±
±
±
0.3
0.3
0.1
0.2
0.2
0.1
9.9
12.7
10.0
7.4
10.3
0.8
28 d
±
±
±
±
±
±
0.4
0.3
0.4
0.4
0.1
0.1
13.1
22.0
19.0
14.8
18.0
9.3
91 d
±
±
±
±
±
±
0.2
0.4
0.6
0.1
0.3
1.8
13.4
34.5
28.0
18.9
25.0
29.1
±
±
±
±
±
±
0.2
0.6
0.5
0.2
0.1
0.1
3.2.2. Reactivity of fly ash
The pozzolanic test shows that (K,Na)2SiO3 accelerates the fly
ash reaction mainly between 3 and 28 days. Both the samples with
and without (K,Na)2SiO3 reach a comparable reaction degree after
91 days of approximately 20% (Fig. 2b).
Thermodynamic calculations were used to clarify the effect of
adding (K,Na)2SiO3 as shown in Fig. 5. They indicate that the
addition of (K,Na)2SiO3 increases the total amount of CeSeH and
decreases the amount of portlandite due to the reaction of portlandite with (K,Na)2SiO3 to form CeSeH in agreement with the
differences observed by XRD (Fig. 3a). The calculations indicate that
at higher additions of (K,Na)2SiO3 the amount of ettringite is expected to decrease due to the very high pH and higher ionic
strength resulting in a decrease of the activity of water, which destabilizes the water-rich ettringite in comparison to AFm phases as
e.g. shown by Albert et al. [66]. Similar to our observation, Donatello et al. [23] reported also a destabilization of ettringite upon
the addition of Na2SO4. Hydrotalcite and hydrogarnet, which are
predicted by the modeling without and with the addition of the
activator, could not clearly be identified experimentally.
3.2.3. Hydration products
The same crystalline hydration products were detected by X-ray
diffraction and thermogravimetry in the fly ash blended PC with
and without (K,Na)2SiO3 (Fig. 3a&b). However, the amount of
portlandite is significantly lower in the sample with (K,Na)2SiO3.
3.2.4. Pore solution analyses and effective saturation indices
Already after 6 h, the initial silicon concentration of 490 mM
(referring to the silicon provided by (K,Na)2SiO3) is lowered to
<1 mM (Fig. 4a) indicating the early formation of CeSeH. This is in
agreement with the calorimetry main hydration peak that is
(K,Na)2SiO3 decreases the compressive strength compared to the
sample without activator (Table 4). The higher early compressive
strength with (K,Na)2SiO3, is probably related to additional CeSeH
formed due to the addition of silicates and the faster hydration,
while the lower late strength could be related to the presence of
high alkali concentrations. Sant et al. [65] observed that the addition of KOH and NaOH solutions to Portland cement pastes
decreased the compressive strength and increased porosity at
comparable degree of clinker reaction.
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Table 5
Calculated effective saturation indices of selected relevant hydrate phases of the
studied binders. Undersaturation: <e0.5; saturation: 0.5 to 0.5; oversaturation:
>0.5.
Without activator
€tl. Ms.
Time days Portl. am. SiO2 CeS
Stra
Ettr.
eH
0.25
0.16 6.62
0.08 0.26 0.36 0.73
1
0.04 6.30
0.02 0.32 0.01 0.07
7
0.05 6.10
0.04 0.42 0.10 0.01
28
0.31 5.03
0.04 0.08 0.16 0.15
Time days Portl. am. SiO2 CeS
eH
0.25
0.13 5.36
0.05
1
0.02 6.02
0.02
7
0.04 5.99
0.01
28
0.32 4.92
0.02
Time days Portl. am. SiO2 CeS
eH
0.25
0.21 4.72
0.13
1
0.33 5.32
0.14
7
0.47 4.70
0.11
28
0.39 4.35
0.05
Time days Portl. am. SiO2 CeS
eH
0.25
0.60 3.57
0.07
1
0.44 4.76
0.10
7
0.55 4.35
0.09
28
0.64 3.87
0.03
Time days Portl. am. SiO2 CeS
eH
0.25
0.32 5.33
0.60
1
0.09 5.69
0.23
7
0.02 5.68
0.15
28
0.11 5.55
0.29
(K,Na)2SiO3
€tl. Ms.
Stra
Ettr.
Gypsum
0.02
1.44
1.43
1.85
Gypsum
0.34 0.13 0.59 0.11
0.46 0.06 0.39 0.42
0.41 0.12 0.04 1.53
0.27 0.27 0.22 1.80
Na2CO3
€tl. Ms.
Stra
Ettr.
Gypsum Calcite Mc.
0.10 0.00 0.21 0.89
0.60 0.30 0.09 1.19
0.34 0.28 0.11 1.34
0.17 0.17 0.03 1.14
Na2Oxalate
€tl. Ms.
Stra
0.41
0.58
0.34
0.48
0.51
0.46
0.44
0.56
K3Citrate
€tl. Ms.
Stra
Ettr.
Gypsum
0.37
0.34
0.39
0.45
1.63
1.63
1.87
1.77
Ettr.
Gypsum
0.59
0.12
0.55 0.14
0.54 0.09
0.01 0.11
0.30 0.45 0.59 0.73
0.00 0.13 0.16 1.43
0.24 0.08 0.07 1.74
0.21 0.00 0.06 1.46
€tl. ¼ str€
Portl. ¼ portlandite, am. SiO2 ¼ amorphous silica, Stra
atlingite,
Ms. ¼ monosulfate, Ettr. ¼ ettringite, Mc. ¼ monocarbonate.
observed already after 6 h (Fig. 1a). The calcium ions from the alite
reaction react with the silicate ions leading to CeSeH formation
and a decrease of the silicon concentration in the pore solution with
time. The pH is approximately 0.3 units higher (Fig. 4c) and the
calcium concentration lower (Fig. 4b) than in the sample without
activator.
Fig. 5. Modeled hydrate phases and pH of the fly ash-PC blend after 28 days of hydration as a function of the amount of the added (K,Na)2SiO3 (the hydration degrees of
PC and fly ash are considered to be 100% and 7%, respectively). The vertical dotted line
indicates the studied composition.
17
The solution is saturated with respect to CeSeH and portlandite
initially and becomes undersaturated with respect to portlandite
with time (Table 5). The ettringite oversaturation decreases along
time (Table 5).
3.2.5. Assumed mechanism of (K,Na)2SiO3 addition
Less portlandite in the solid phase (Fig. 3) and lower silicon and
calcium concentrations (Fig. 4a and b) in the pore solution were
observed in the presence of (K,Na)2SiO3 compared to the plain
system. The high pH (Fig. 4c) and the low calcium concentrations
(Fig. 4b) accelerate the early alite dissolution as visible in the strong
acceleration of the main peak observed by calorimetry (Fig. 1a) and
the higher one day strength (Table 4). Nicoleau et al. [67] showed
that alite reaction is faster in the presence of low calcium concentrations as observed here. Correspondingly, Kumar et al. [68]
observed that the addition of NaOH and KOH accelerates the alite
reaction as the high pH lowers the calcium concentrations. The
faster reaction of alite and the formation of additional CeSeH due
to the reaction of the added silica with the portlandite result in the
early setting of the system (Table 3) and the relatively early high
compressive strength (Table 4). The relatively lower compressive
strength at later ages (Table 4) is consistent with observations in PC
systems, where the addition of NaOH and KOH has decreased the
compressive strength [65]. Furthermore, (K,Na)2SiO3 accelerates
the fly ash reaction (Fig. 2b).
3.3. Activation by Na2CO3
3.3.1. Hydration kinetics
The addition of sodium carbonate (3 mass-%) reduces the initial
and final setting time of the fly ash blended PC (Table 3).
The hydration of PC is accelerated; the first hydration maximum
in the heat flow curve occurs after 5 h (Fig. 1a) and coincides with
final setting. A second, more intense heat flow maximum can be
recognized after about 14 h. The cumulative heat (Fig. 1b) and the
chemical shrinkage (Fig. 2a) are increased compared to the sample
without activator.
The addition of Na2CO3 has little effect on the 1 day strength but
decreases significantly the compressive strength at later ages
(Table 4).
3.3.2. Reactivity of fly ash
The reactivity of the fly ash in the PC simulated medium in the
presence of Na2CO3 is similar to the samples without activator
(Fig. 2b).
3.3.3. Hydration products
The XRD data indicate the presence of less portlandite when
Na2CO3 has been added (Fig. 3a). In addition, only low amounts of
ettringite are present after 28 days, while the formation of monocarbonate is favored in the presence of the activator (Fig. 3b). In
agreement with the experimental observations, thermodynamic
modeling (Fig. 6) indicates that the addition of Na2CO3 results in the
presence of less portlandite and a significant pH increase from 13.3
to 14 as shown in Fig. 4c (note that the calculated pH values are
somewhat higher than the measured pH values as the alkali uptake
model underestimates the alkali uptake by CeSeH). The modeling
predicts the stabilization of monocarbonate on the expenses of
ettringite due to the increase of pH and carbonate concentration,
although to a somewhat lower extent than observed experimentally. The increased uptake of aluminum in CSH at high
aluminum concentrations is not captured in the model. In addition,
the inaccuracy of the thermodynamic data used has also to be
considered as relatively small changes (±1 log unit) in the solubility
products of ettringite and monocarbonate lead to the
18
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Fig. 6. Modeled hydrate phases and pH of the fly ash-PC blend after 28 days of hydration as a function of the amount of the added Na2CO3 (the hydration degrees of PC
and fly ash are considered to be 100% and 7%, respectively). The vertical dotted line
indicates the studied composition.
destabilization of ettringite at around 4 wt.% Na2CO3.
3.3.4. Pore solution analyses and effective saturation indices
A relatively high pH is observed in the case of Na2CO3 activator
(Fig. 4c), which results from the reaction (Equ. 1).
Na2CO3 þ Ca(OH)2 / CaCO3 þ 2Naþ þ 2OH
(1)
The higher pH leads to lower calcium concentrations (z0.1 mM)
than observed in the other samples (Fig. 4b) and as stated before
the early alite dissolution is accelerated at low calcium concentrations as visible in the acceleration of the main calorimetry peak in
Fig. 1a.
The calculated effective saturation indices (Table 5) indicate that
the solutions are only initially oversaturated with respect to
ettringite, but become undersaturated with time, consistent with
the presence of little ettringite. The destabilization of ettringite is
also visible in the relatively high sulfate concentrations of
z200 mM compared to <1 mM in the paste without activator (see
Appendix). The solutions remain oversaturated with respect to
calcite during the first week and are always undersaturated with
respect to portlandite as shown in Table 5.
3.3.5. Assumed mechanism of Na2CO3 addition
The early reaction of the Portland cement is somewhat accelerated due to the higher pH (Fig. 4c) and the lower calcium concentrations (Fig. 4b) resulting in an early set (Table 3). In contrast
the reactivity of the fly ash is not significantly affected (Fig. 2b). The
pH is increased due to the reaction of sodium carbonate with
portlandite, resulting in the decrease of the amount of portlandite
compared to samples without Na2CO3. The high pH values and the
high carbonate concentrations in the pore solution favor the formation of monocarboaluminate instead of ettringite (Fig. 3b). The
compressive strength measured after 7 days and longer (Table 4) is
significantly lower than observed for the sample without activator
and for the (K,Na)2SiO3 activator. Again, this is attributed to the pH
as an increase of pH has been observed to decrease compressive
strength in Portland cement pastes [65].
3.4. Activation by Na2Oxalate
3.4.1. Hydration kinetics
The presence of sodium oxalate accelerates the setting of the fly
Fig. 7. Conduction calorimetry of the fly ash-PC and of the quartz-PC blends (70/30 by
mass) without and with Na2Oxalate and K3Citrate, respectively, (a) specific heat flow,
(b) cumulative heat normalized to the solid content.
ash blended cement (Table 3), and the reaction of the PC in the
blend is significantly accelerated as observed by calorimetry
(Fig. 7a). The reaction of the PC mainly occurs already between 4
and 8 h and within shorter timespan than observed for the other
activators. Again, this acceleration of the PC reaction is probably
due to the relatively high pH values (Fig. 4c) and low calcium
(Fig. 4b) concentrations that accelerate the PC reaction. Thus also
the cumulative heat is during the first 24 h higher than for the
sample without activator (Fig. 7b). After 1 day and longer, however,
little difference is observed. Also the chemical shrinkage is not
significantly affected by the presence of sodium oxalate (Fig. 2a),
indicating a similar degree of PC and fly ash reaction as in the
samples without activator.
The compressive strength after one day is higher with activator
than without due to the fast PC reaction (Table 4). After 7, 28 and 91
days lower compressive strength values are observed as also for
Na2CO3 and (K,Na)2SiO3.
The pozzolanity test indicates that sodium oxalate significantly
increases the reactivity of the fly ash as shown in Fig. 2b. This acceleration could be related to the very distinct pH increase due to
the high amount of sodium oxalate used and/or to a specific
interaction of oxalate with the fly ash. In additional experiments,
where the effect sodium oxalate on glass dissolution was studied
under constant pH conditions, no signifcant influence on glass
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
dissolution was observed, confirming that the observed acceleration in the presence of sodium oxalate is based on the increased pH
values.
Kundin et al. [69] reported the inhibition of glass dissolution at
pH values < 5 for diopside glass in the presence of oxalic acid due to
formation of stable calciumemagnesium oxalate complexes on the
glass surface. Oelkers et al. [70] reported that there is no effect on
basaltic glass dissolution rates of oxalate ion at pH between 1 and 2
or pH between 7 and 11. To our knowledge, there is no previous
information about the effect of oxalate on the glass dissolution at
pH around 13.5 (the pH values of the pore solution of the fly ash-PC
blend in the presence of Na2Oxalate (Fig. 4c) and of the
Ca(OH)2 þ CaCO3 þ KOH medium used in the pozzolanic test).
3.4.2. Hydration products
Like in the case of Na2CO3, the addition of sodium oxalate results
in the presence of very little portlandite after 28 days as shown in
Fig. 8a&b. The formation of Ca-oxalate (CaC2O4$H2O) is observed
(Equ. 2) leading to a pH increase.
Na2C2O4 þ Ca(OH)2 þ H2O / CaC2O4$H2O þ 2Naþ þ 2OH
19
(2)
No ettringite is observed by XRD (Fig. 8a), which is confirmed by
TGA (Fig. 8b). TGA indicates that besides monocarbonate (weight
loss at about 150e170 C) a second AFm phase might be present
(weight loss at about 190e200 C). It can be suggested that this refers to a “hemi-oxalate” 3CaO$Al2O3$0.5Ca(OH)2$0.5CaC2O4$9H2O
€llmann [71], which would fit to this TGA dehydration
described by Po
pattern. However it could not clearly be identified in the XRD data, as
its main reflections coincide with those of monocarbonate.
The absence of ettringite may again be related to the high pH
(Fig. 4c) and ionic strength (see Appendix) in the pore solution and
thus low activity of water leading to a destabilization of ettringite or
to the high Al-uptake by CSH.
The results of the thermodynamic modeling regarding the
effect of sodium oxalate in Fig. 9 confirm the experimental
findings. The addition of sodium oxalate lowers the amount of
portlandite strongly while Ca-oxalate (CaC2O4$H2O) is formed. As
there are no thermodynamic data available yet for
Fig. 8. X-ray diffraction patterns (a) and TGA thermograms (b) of the fly ash e PC blend without and in the presence of Na2Oxalate and K2Citrate, respectively, after 28 days of
hydration at 20 C (A ¼ alite, B ¼ belite, C ¼ CeSeH, CO ¼ calcium oxalate monohydrate, E ¼ ettringite, Hc ¼ hemicarbonate, M ¼ mullite, Mc ¼ monocarbonate,
OA ¼ 3CaO$Al2O3$0.5Ca(OH)2$0.5CaC2O4$9H2O, P ¼ portlandite, Q ¼ quartz).
20
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Fig. 9. Modeled hydrate phases and pH of the fly ash-PC blend after 28 days of hydration as a function of the amount of the added Na2Oxalate (the hydration degrees of
PC and fly ash are considered to be 100% and 7%, respectively). The vertical dotted line
indicates the studied composition.
3CaO$Al2O3$0.5Ca(OH)2$0.5CaC2O4$9H2O, it could not be
included in the modeling. The increase of the pH and ionic
strength destabilizes ettringite (although somewhat less than
observed experimentally). As a relatively high amount of sodium
oxalate has been added (8 mass-% corresponding to 1.64 mol/L),
the increase of pH and ionic strength is more distinct than for the
other additives which leads to a stronger destabilization of
ettringite.
3.4.3. Pore solution analyses and effective saturation indices
The silica content (Fig. 4a) and the pH (Fig. 4c) of the pore solution in the case of Na2Oxalate are particularly high due to the high
quantity used, while the calcium concentration is low (Fig. 4b) due
to the consumption of calcium by the precipitation of calcium
oxalate.
The solutions are always undersaturated with respect to portlandite (Table 5) and slightly undersaturated with respect to
ettringite, consistent with the XRD and TGA results shown in Fig. 8.
Note that an undersaturation with respect to a solid does not
necessarily mean that none of this solid can be observed, undersaturation indicates only that a phase dissolves under these conditions, a process which can be very slow in dense cement matrices.
The solutions are near saturation with respect to CeSeH, the main
hydrate observed by TGA.
of potassium citrate accelerates the initial setting (Table 3). Potassium citrate has been reported on the one hand to be an efficient
activator for class C fly ashes [30e33]. On the other hand, however,
citrate is known to retard the hydration of PC [72,73]. In fact, no PC
main hydration peak could be observed by calorimetry in the
presence of 3 mass-% potassium citrate within the first 7 days
(Fig. 7a). The slow reactivity of the PC results also in the low
chemical shrinkage observed up to 30 days (Fig. 2a) and in a very
poor strength up to 28 days (Table 4). Only the sample after 91 days
shows a compressive strength comparable to the other samples.
Potassium citrate retards also the fly ash reaction in the PC
simulated medium (Fig. 2b). While citric acid increases glass reactivity at low pH value [74], the glass dissolution rate under alkaline
condition decreases in the presence of citrate [75]. Although the
mechanisms responsible for the observed decrease of glass dissolution are not clear, it can be speculated that the formation of citrate
complexes at the glass surface could be an explanation for the
observed decrease of reaction.
3.5.2. Hydration products
After 28 days, a similar amount of portlandite is observed by
XRD and TGA in the presence of potassium citrate (Fig. 8) compared
to the reference paste without activator.
Thermodynamic modeling (Fig. 10) of the effect of potassium
citrate on the hydrated fly ash-PC blend shows that adding potassium citrate is expected to lead to the formation of a small quantity
of calcium citrate (Ca3(C6O7H5)2$4H2O), which could not be identified by XRD probably due to a too low content, but else does not
change significantly the kind or amount of the hydration products.
A slight decrease of the amount of ettringite and portlandite and an
increase of the amount of AFm are calculated due to the increase in
ionic strength.
3.5.3. Pore solution analyses and effective saturation indices
The presence of 240 mM potassium citrate leads to relatively
high alkali concentrations (see Appendix) and pH values (Fig. 4c) in
the pore solution. The solutions are saturated with respect to
CeSeH, portlandite and ettringite (Table 5).
3.5.4. Assumed mechanism of potassium citrate addition
Potassium citrate in high concentrations retards strongly the
reaction of PC (Fig. 8a) and of the fly ash (Fig. 2b) as also visible by
3.4.4. Assumed mechanism of sodium oxalate addition
Sodium oxalate reacts with calcium to form calcium oxalate
(Fig. 8) which lowers the amount of portlandite strongly. The reaction lowers also calcium concentrations (Fig. 4b) and increases
the pH (Fig. 4c) of the pore solution which accelerates PC reaction
(Fig. 7a) resulting in a fast set (Table 3) and a relatively high 1 day
strength (Table 4). There is also evidence for the formation of an
oxalate-containing AFm phase. After longer hydration times, a
lower compressive strength compared to the reference without
activator is observed, although sodium oxalate promotes the fly ash
dissolution (Fig. 2b). The high pH leads to a destabilization of
ettringite as also reported by [23] in the case of Na2SO4 addition.
3.5. Activation by potassium citrate
3.5.1. Hydration kinetics
As observed for the other accelerators, the addition of 3 mass-%
Fig. 10. Modeled hydrate phases and pH of the fly ash-PC blend after 28 days of hydration as a function of the amount of the added K3Citrate (the hydration degrees of PC
and fly ash are considered to be 100% and 7%, respectively). The vertical dotted line
indicates the studied composition.
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
the low chemical shrinkage observed (Fig. 2a) and by the very slow
development of compressive strength (Table 4). It is possible that
the formation of citrate complexes at the surface of the fly ash and
clinker phases could be an explanation for the observed decrease of
reaction.
21
Sodium oxalate and (K,Na)2SiO3 accelerate both the setting of
the fly ash-PC blend and increase the early compressive strength of
the fly ash-PC blend. Furthermore, they show better compressive
strengths at later ages compared to the other activators. Based on
these findings, they can be considered as the most suitable activators for the fly ash-PC blend among the investigated activators.
4. Conclusion
Acknowledgments
The hydration of a fly ash-PC blend (70/30 by mass) without and
with the addition of activators was examined. The activators (potassium sodium silicate, sodium carbonate, sodium oxalate and
potassium citrate) where chosen in a way that (i) they increase pH
to stimulate fly ash dissolution (ii) they e with the exception of
potassium citrate - accelerate the PC reaction (setting, early
compressive strength) and (iii) that highly alkaline conditions are
avoided due to health issues as the binders should be suitable for
dry mix mortars such as tile adhesives.
In the system without activators the fly ash reacts and the reaction contributes significantly to the developement of compressive strength after 7 days and longer.
The addition of the activators enhances the compressive
strength at early age (i.e. sodium oxalate and (K,Na)2SiO3) but decreases strength at late age. All activators increase the pH values
and decrease thus the calcium concentrations in the pore solution.
Lower calcium concentrations lead to a faster reaction of alite
[67,68] and thus to early setting and a high early strength. On the
long term, the high alkali concentrations lower the compressive
strength after 91 days and lead to a (partial) destabilization of
ettringite. Thermodynamic modeling indicates that ettringite
destabilization is related to the decrease of water activity due to the
high ionic strength and high pH values in the pore solution due to
the alkaline activators. It can be observed by XRD and TGA that the
amount of ettringite after 28 days is lower with higher measured
alkali and aluminum concentrations, where a higher aluminum
binding by CSH is expected.
The pozzolanic reaction of the fly ash (in a simulated PC medium) is not or only slightly enhanced by Na2CO3. Sodium oxalate
and (K,Na)2SiO3 accelerate the fly ash reaction due to the increased
pH. In additional experiments, where the effect of the different
activators on glass dissolution was studied under constant pH
conditions, no signifcant influence of Na2SiO3, Na2CO3 or sodium
oxalate on glass dissolution was observed, confirming that the
observed acceleration in the presence of sodium oxalate and
(K,Na)2SiO3 is based on the increased pH. In contrast to the other
activators, potassium citrate is found to hinder both the fly ash and
the PC reaction. Either the potassium citrate forms complexes at the
surfaces of fly ash and PC which slows down reaction strongly or
alternatively it could also hinder the formation of the hydration
products (e.g. AFm phases and CeSeH).
The late compressive strength decreases with increasing pH
values observed in the pore solution, confirming earlier observation
in Portland cement pastes [65] that high pH values are negative for
long-term compressive strength. However, it is unclear whether
this strength decrease is related to the very fast initial reaction and
thus to the formation of more dense and less well interlocked hydrate assemblage, whether the properties of CeSeH are negatively
influenced or whether the (partial) destabilization of ettringite is
the reason for this strength decrease at later age.
Saint-Gobain Recherche is acknowledged for financial support.
^ pital, Luigi Brunetti, Boris Ingold
The authors thank Emilie L’Ho
(Empa) and Stephanie Dumont (Saint-Gobain Recherche, France)
for their assistance in the lab, and Christian Müller (Saint-Gobain
Weber AG, Switzerland) for discussions.
Appendix
Fig. A1. Particle size distribution of the fly ash and the quartz used in the study as
determined by laser diffraction.
Fig. A2. Pozzolanic test: Thermogravimetric analysis of the fly ash and the quartz after
storage at 20 C for 7, 28 and 91 days in a KOH solution containing portlandite and
calcite. AFm-phases include hemi- and monocarbonate.
22
S. Alahrache et al. / Cement and Concrete Composites 66 (2016) 10e23
Table A1
Pore solution composition (in mmol/l) of the fly ash - PC blend measured by IC, in the presence of the different activators and at different hydration times (0.25, 1, 7, 28 days).
The concentration of carbonate, oxalate and citrate in the pore solutions was estimated based on charge balance. Ionic strength (IS) (in mmol/l) presented in the last column
was calculated by GEMS. Mg, Fe and other elements with very low concentration were not determined.
Sample
Time/d
Na
K
Ca
SO4
Si
Al
Cl
OH-
No activator
0.25
1
7
28
31
43
62
59
59
70
100
110
29.5
5.4
1.9
0.39
27.8
0.12
0.50
0.40
0.036
0.039
0.041
0.18
0.029
0.042
0.042
0.77
4
1
0
0
47
87
133
93
time/d
Na
K
Ca
SO4
Si
Al
Cl
OH-
0.25
1
7
28
160
160
150
120
230
190
220
210
18.6
3.06
0.87
0.22
185
79
1.8
2.4
0.096
0.061
0.096
0.59
0.019
0.028
0.080
0.35
5
4
1
1
80
163
289
189
time/d
Na
K
Ca
SO4
Si
Al
Cl
OH-
carbonate
0.25
1
7
28
1400
1100
770
740
96
71
83
93
0.51
0.18
0.15
0.24
368
179
99
194
2.71
0.59
1.22
3.75
0.66
0.10
0.55
0.73
4
5
2
2
297
342
271
278
100
150
150
10
time/d
Na
K
Ca
SO4
Si
Al
Cl
OH-
oxalate
0.25
1
7
28
2200
2700
2300
1900
49
70
120
110
0.17
0.085
0.052
0.070
401
165
179
297
65.6
5.01
19.1
35.8
0.41
0.18
1.56
0.51
4
3
2
2
400
407
575
420
300
800
500
300
time/d
Na
K
Ca
SO4
Si
Al
Cl
OH-
citrate
0.25
1
7
28
40
42
44
49
740
660
670
700
8.11
1.26
0.72
1.44
23.1
4.7
2.1
4.2
2.70
0.53
0.53
0.84
0.65
0.67
0.22
0.12
6
5
5
1
454
522
748
458
60
30
200
30
(K,Na)2SiO3 (490 mM)
Na2CO3 (710 mM)
Na2Oxalate (1640 mM)
K3Citrate (240 mM)
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