Received: 7 May 2022 Revised: 8 August 2022 Accepted: 4 September 2022
DOI: 10.1111/jace.18782
RESEARCH ARTICLE
Working mechanism of calcium nitrate as an accelerator for
Portland cement hydration
Tobias Dorn Tamino Hirsch Dietmar Stephan
Institute of Civil Engineering, Group of
Building Materials and Construction
Chemistry, Technische Universität Berlin,
Berlin, Germany
Correspondence
Dietmar Stephan, Institute of Civil
Engineering, Group of Building Materials
and Construction Chemistry, Technische
Universität Berlin, 13355 Berlin, Germany.
Email: [email protected]
Funding information
German Federal Ministry of Education
and Research, Grant/Award Number:
03XP0122A
Abstract
Precast concrete, cold weather concreting, and the emerging technique of
concrete additive manufacturing are applications in which the acceleration of
cement hydration plays a critical role. To allow precise control of early cement
hydration in these applications, a thorough understanding of the working mech-
anisms of cement hydration accelerators is required. This study contributes to
the understanding of the mechanism by which calcium nitrate (Ca(NO3)2)influ-
ences early cement hydration. The influence of Ca(NO3)2on the hydration of an
ordinary Portland cement has been followed by isothermal calorimetry, in situ
X-ray diffraction (XRD), quantitative XRD, compressive strength testing, and
the analysis of the pore solution composition. Further, the initial pore solution,
the initial phase composition, and the phase composition in the fully hydrated
cement have been estimated by thermodynamic calculations to corroborate the
experimentally obtained results. The results indicate that Ca(NO3)2, especially
at the highest analyzed dosage of 5 wt.%, enhances the formation of ettringite
and a nitrate-containing AFm phase. Furthermore, Ca(NO3)2accelerates alite
hydration. Besides the increased Ca concentration in solution, it has been found
that a reduction of the Al concentration in the initial pore solution by Ca(NO3)2
possibly contributes to the accelerating effect of Ca(NO3)2on alite hydration.
KEYWORDS
acceleration, additives, hydration, Portland cement, thermodynamics
1 INTRODUCTION
The rate of ordinary Portland cement (OPC) hydration
can be altered by adding several chemical substances,
which, if they increase the rate of hydration, are referred
to as accelerators. Accelerators can shorten the setting
time (set accelerators), increase the rate of compres-
sive strength development (hardening accelerators), or
affect both, setting and hardening.1Important fields in
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© 2022 The Authors. Journal of the American Ceramic Society published by Wiley Periodicals LLC on behalf of American Ceramic Society.
which accelerators are used include the production of pre-
cast concrete and concreting at cold temperatures, which
tend to significantly prolong setting times.2–4 Recently, as
extrusion-based additive manufacturing (AM) with mortar
and concrete is gaining an increased interest in research
and industrial application, a new field of accelerator
application is emerging.5AM describes an automatically
controlled process of layer-wise material deposition in
which no formwork is used to support the freshly placed
752 wileyonlinelibrary.com/journal/jace J Am Ceram Soc. 2023;106:752–766.
DORN et al. 753
material. Consequently, AM imposes high requirements
on the dimensional stability of the material right after
the deposition.6The addition of accelerators to the mate-
rial within the printing nozzle shortly before the material
deposition might be a good option for obtaining reli-
able AM processes. In such processes, a material with a
sufficiently high flowability to continuously flow within
the automatically operating 3D-printing system could be
applied. Acceleration in the printing nozzle would spark
a sufficiently fast strength gain after deposition for the
material to support its own weight and the weight of subse-
quently deposited material layers.7,8 However, to develop
such AM processes, a precise knowledge of the working
mechanisms of cement hydration accelerators is required.
Calcium nitrate is a frequently applied accelerator.
Ca(NO3)2was introduced as an accelerator for cement
hydration during the 1980s9to replace chloride-containing
accelerators such as calcium chloride (CaCl2), the Cl−ions
of which had been shown to cause damage to the steel rein-
forcement by depassivation of the steel.10,11 Since then, the
effect of Ca(NO3)2on the setting and hardening of OPC
and the underlying mechanism were analyzed in several
studies,12–19 which have been summarized in a recently
published review on cement hydration accelerators by
Dorn et al.20
Regarding the influence of Ca(NO3)2on the aluminate
reaction, Balonis et al.,14 based on thermodynamic cal-
culations as well as experimental evaluations, suggested
that the presence of NO32−ions in solution results in the
formation of nitrate-containing AFm phases (NO3-AFm)
at the expense of monosulfoaluminate (SO4-AFm) or, in
carbonate-containing systems, at the expense of mono-
carboaluminate (CO3-AFm (mono)). NO3-AFm formation
was later confirmed to occur by Franke et al.21 based on
the analysis of ASTM I/II, and ASTM II/V-based cement
pastes accelerated with Ca(NO3)2. In general, AFm phases
are a group of cement hydration products forming platy
hexagonal or pseudohexagonal crystals with the formula
[Ca2Al(OH)6](X)y⋅zH2O in which X represents Cl−,NO
3−,
Al(OH)4−,(CO
32−)0.5,or(SO
42−)0.5.22–24
Moreover, Hill et al.,13 who analyzed the hydration of
Class C fly ash in the presence of Ca(NO3)2, discussed the
formation of nitrate-modified calcium aluminate hydrates.
However, although Hill et al.13 found that the formation
of NO3-AFm competes with the formation of ettringite
(AFt) in Class C fly ash analyzed in their study, Balo-
nis et al.,14 as well as Abdelrazig et al.,12 described an
increased formation of ettringite in the presence of nitrate
in solution for OPCs. According to Balonis et al.,14 SO42−,
available in the SO4-AFm phases, is replaced in the course
of NO3-AFm formation, and the consequently increased
availability of SO42−contributes to the increased ettringite
formation.
Regarding the silicate reaction of OPC, it has been
shown that Ca(NO3)2shortens the time until the main
hydration of alite starts, and it increases the rate of alite
hydration and possibly also that of belite.15–17,25 Justnes
et al.18 suggested that the dissolution of Ca(NO3)2results
in a fast supersaturation of the pore solution with respect
to portlandite (CH), and that the resulting early precip-
itation of CH is responsible for the early onset and the
increased rate of alit hydration. Nicoleau26 argues that
calcium salts allow the homogenous nucleation of C–S–
H by increasing the availability of calcium in solution,
and that this enhanced nucleation is responsible for the
acceleration of alite hydration. In recent years, it has been
regularly discussed that the onset of fast alite hydration in
OPCs is retarded by aluminum in the pore solution.27–33
Consequently, if Ca(NO3)2, by altering the C3Ahydra-
tion, reduces [Al] in the initial pore solution composition,
this might be an aspect of alite hydration acceleration by
Ca(NO3)2that has not yet been fully considered.
The present study aims at evaluating the effect of
Ca(NO3)2in solution on the formation of AFm phases and
ettringite in a hydrating commercial OPC more closely,
as well as on analyzing the development of ion concen-
trations in the early pore solution composition in the
dependence of Ca(NO3)2addition.
Isothermal heat flow calorimetry was used to investi-
gate the influence of Ca(NO3)2on the reactivity of the
analyzedOPCatanygiventimeupto72h.InsituX-ray
diffraction (in-situ XRD) was applied to experimentally
analyze the dissolution of selected cement phases as well
as the formation of crystalline hydration products during
the first 24 h of hydration. At the same time, quantitative
powder XRD (QXRD) was used to quantitatively deter-
mine the phase composition of the hydrated cement in
the dependence of the Ca(NO3)2addition at specific later
times. The development of the pore solution composition
of the OPC in dependence on Ca(NO3)2addition during
the first 60 min of hydration was analyzed by inductively
coupled plasma optical emission spectroscopy (ICP-OES).
The initial pore solution composition, the initial hydration
product formation, and the phase composition of the fully
hydrated cement were, furthermore, modeled by thermo-
dynamic calculations in the dependence of the Ca(NO3)2
addition to corroborate the experimentally determined
results.
2EXPERIMENTAL PROCEDURES
2.1 Materials and methods
A commercial OPC (CEM I 52.5 R) and Ca(NO3)2⋅4H2O
produced by Alfa Aesar (Stock No. 12364, p.a.) have been
754 DORN et al.
used in all experiments. The water contained in the cal-
cium nitrate tetrahydrate was determined by drying to
mass constancy at 110◦C and was subtracted from the
mixing water.
2.2 Mixing proportions and mixing
procedure
Samples are named according to the amount of Ca(NO3)2
that has been added to a specific sample. The number
in the sample nomenclature indicates the added amount
of Ca(NO3)2in wt.% of cement. All experiments, except
compressive strength testing on mortar prisms, have been
performed using cement paste. Most samples have been
prepared with a water to cement ratio (w/c) of 0.5. Sam-
ples prepared for the analysis of pore solution have been
prepared with a w/c of 1. To obtain calorimetric results
that correlate directly to all applied experimental tech-
niques, isothermal heat flow calorimetry was performed
on samples prepared with a w/c of 0.5 and 1.0.
Most samples were mixed for 2 min using a vortex
mixer on the highest level. Samples prepared for compres-
sive strength testing on cement paste cubes were mixed
according to EN 196-3,34 whereas mortar samples prepared
for compressive strength testing on mortar prisms were
mixed according to EN 196-1.35 Ca(NO3)2was added to the
cement together with the mixing water in which it was dis-
solved at least 24 h before the initial contact of cement and
water.
2.3 Isothermal heat flow calorimetry
Isothermal heat flow calorimetry measurements were per-
formed using a calorimeter (TAM Air, TA Instruments,
USA) at 20◦C. Samples were prepared with a w/c of 0.5 (3 g
cement, 1.5 g water) and a w/c of 1.0 (1.72 g cement, 1.72 g
water) to validate the results obtained from pore solution
analysis. Mixing of the samples was performed outside the
device. Each sample was placed in the device immediately
after mixing. The heat flow of each sample was recorded
over 72 h.
2.4 X-ray diffraction (XRD)
All XRD experiments were performed using an XRD
(Empyrean, PANalytical, the Netherlands) in Bragg–
Brentano geometry, equipped with a Cu Kαradiation-
emitting X-ray tube. For the in situ XRD measurements,
∼5 g of cement paste were placed in a temperature-
controlled sample stage set to 20◦C. To protect the samples
from evaporation and carbonation, the samples were cov-
ered with a Kapton film (thickness 7.5 μm) during the
entire measurement. In situ XRD measurements were
performed for a maximum duration of 24 h. Each mea-
surement started 8 min after the initial contact of cement
and water. During the measurement time, a scan in the
range between 6◦and 40◦2ϴwas started every 10 min.
For evaluating the in situ XRD results, reflex intensities
were compared after background subtraction. Based on the
comparison of reflex intensities, a qualitative conclusion
has been drawn on the dissolution of cement phases and
the formation of crystalline hydration products. The in situ
XRD results have not been quantified as the quantification
of in situ XRD results by Rietveld refinement faces sev-
eral problems due to the continuous change of important
parameters such as intensity of the measured background
and crystalline phase content.
To obtain QXRD results, XRD measurements have
been performed on hardened cement pastes. After 1,
7, and 28 days, the hydration was stopped by solvent
exchange using ethanol. To obtain finely ground pow-
ders, which are crucial to obtain reproducible QXRD
results, while avoiding amorphization of particles, the
samples were wet ground after the intended hydration
time in a special mill (McCrone, Retsch, Germany) using
ethanol as grinding liquid. To quantify the amorphous
content, 20 wt.% of rutile (TiO2) was added as an internal
standard to each sample before grinding. No internal
standard was added to the anhydrous cement as amor-
phous content was expected to be too small for a precise
determination.36,37
For the XRD measurements of the powdered samples,
a fixed divergence slit (0.25◦) in the incident beam path
andafixedanti-scatterslit(0.25
◦) in the diffracted beam
path were applied. One scan was performed during 1 h in
the 2ϴrange from 5◦to 65◦. Quantification of the XRD
results was obtained using the Rietveld algorithm in the
open-source XRD and Rietveld refinement software Profex
(version 4.3.4). During Rietveld refinement, the following
parameters were refined: scale factor, sample displace-
ment, background (Chebyshev polynomial function, 11
coefficients), crystallite size, microstrain, selected lattice
parameters, and preferred orientation (spherical harmon-
ics). The reference structures for all phases used in Rietveld
refinements are given in Table 1.
2.5 Compressive strength development
Two sets of experiments regarding the compressive
strength development in the dependence of Ca(NO3)2
addition have been performed. In the first set of experi-
ments, the compressive strength development was tested
DORN et al. 755
TABLE 1 Crystallography Open Database (COD) ID and
reference of all phases used in Rietveld refinements of anhydrous
and hydrated cement samples
Phase COD-ID Reference
Alite 9008366 38
Belite 1546027 39
C3A (cubic) 1000039 40
C3A (orthorhombic) 9014308 41
Brownmillerite 1200009 42
Periclase 1000053 43
Calcite 9000965 44
Arcanite 9007569 45
Gypsum 2300259 46
Bassanite 2105042 47
Anhydrite 5000040 48
Portlandite 1001768 49
Ettringite 9015084 50
CO3-AFm (mono) 1000459 51
CO3-AFm (hemi) 2105252 52
NO3-AFm 2008643 23
Rutile 9015662 53
on 2-cm cubes prepared from cement paste, as described in
Section 2.2. Each compressive strength value presented in
Section 3represents the average result of 10 tested cubes.
In the second set of experiments, the compressive strength
development was tested on mortar prisms according to
EN 196-135 to allow direct comparison of the presented
results with comparable experimental data available in
the literature. The compressive strength was tested after
1, 7, and 28 days of hydration. All samples for compressive
strength testing were removed from their molds 24 h after
preparation. Until testing, samples were stored under
water at 20◦C.
2.6 Pore solution analysis
Pore solutions were analyzed by ICP-OES (IRIS Intrepid II
XSP, Thermo Fisher, USA). Samples for ICP-OES analysis
were prepared at a w/c of 1 to facilitate the extraction of
pore solution via centrifugation.
Pore solution was extracted from samples via centrifu-
gation for 10 min at 1195 g after defined hydration times.
After centrifugation, the pore solution was filtered through
a 450-nm polyester filter to remove any remaining solids.
After filtration, 4 g of the filtered pore solution was diluted
with16gofultrapurewater(σ=0.06 μS/cm). The sam-
ples were acidified with HNO3to pH 2 immediately after
dilution.
FIGURE 1 Particle size distribution of the analyzed cement
determined by laser diffraction analysis
2.7 Thermodynamic calculations
The thermodynamic modeling was performed by
Gibbs energy minimization using the software pack-
age GEMS54,55 in version 3.5 equipped with the databases
PSI-Nagra 12/0756 and Cemdata18.57 This approach was
used to model the phase assemblage at full hydration and
estimate the pseudo-equilibria resulting from the initially
dissolving phases, both depending on the added quantity
of Ca(NO3)2. All calculations were performed for 20◦C.
3RESULTS
3.1 Raw materials characterization
The particle size distribution of the analyzed cement (CEM
I 52.5 R) was measured using laser diffraction analysis
(Mastersizer 2000, Malvern Panalytical, United Kingdom)
and is presented in Figure 1. The analyzed cement has
a specific surface of 4700 ±100 cm2/g, determined by
the Blaine method according to EN 196-6.58 The chemical
composition of the cement was determined by X-ray flu-
orescence (WD-RFA PW 2400, Phillips, the Netherlands)
and is given in Table 2. The mineralogical composi-
tion of the cement was determined through the Rietveld
refinement of XRD results and is presented in Table 3.
3.2 Isothermal heat flow calorimetry
Isothermal heat flow calorimetry is a frequently applied
technique in the analysis of cement hydration and the
effect of cement additives on the hydration as it allows
to follow the heat production rate (heat flow) over time.
756 DORN et al.
TABLE 2 Chemical composition of the analyzed cement determined by X-ray fluorescence (XRF)
Al2O3CaO Fe2O3K2OMgOMnONa
2OP
2O5SiO2SO3TiO2LOI Sum
wt.% 4.9 62.5 3.0 1.2 2.2 0.1 0.3 0.1 19.2 3.3 0.2 2.5 99.1
TABLE 3 Mineral composition of the analyzed cement
determined by the Rietveld refinement of X-ray diffraction (XRD)
data
Phase wt.%
Alite 66
Belite 5
C3A (cubic) 2
C3A (orthorhombic) 5
Brownmillerite 11
Periclase <1
Calcite 4
Arcanite 2
Gypsum <1
Bassanite 1
Anhydrite 3
FIGURE 2 Heat flow and heat development of the tested
cement pastes (w/c =0.5) determined by isothermal heat flow
calorimetry. The heat flow during the first hour of hydration was
excluded from the calculation of the cumulative heat
The results of isothermal heat flow calorimetry (Figure 2)
indicate that Ca(NO3)2dosages <0.8 wt.% of cement only
have a minor influence on cement hydration during the
first 72 h of hydration. Ca(NO3)2dosages ≥1 wt.%, however,
reduce the duration of the induction period. Accordingly,
the onset of the acceleration period and the heat flow max-
imum ascribed to the main hydration of alite is shifted to
earlier times by increasing Ca(NO3)2dosages. For exam-
ple, at a Ca(NO3)2dosage of 5 wt.% of cement, the heat flow
maximum occurs at 276 min and, thus, 270 min earlier than
in the neat reference sample. Further, Ca(NO3)2dosages
>1 wt.% increase heat flow measured during the induction
period. At an addition of 5 wt.% Ca(NO3)2also, the heat
flow measured at the heat flow maximum is increased.
For Ca(NO3)2additions ≥1 wt.%, the cumulative heat of
hydration is slightly increased in the presence of Ca(NO3)2
until ∼12 h of hydration but is not increased after 72 h of
hydration. The results of the heat flow calorimetry mea-
surements performed with w/c of 1 (Figure S1), which
corresponds to the sample composition chosen for pore
solution analysis, show corresponding trends.
3.3 In situ XRD and QXRD
In situ XRD is a technique frequently applied in the anal-
ysis of construction materials to study the consumption
of cement phases and the formation of crystalline hydra-
tion products, such as ettringite and portlandite. The XRD
reflex intensity development of the samples CN_0, CN_2,
and CN_5, measured by in situ XRD over 24 h, is shown
in Figure 3. As identical measurement settings were cho-
sen for each sample, the reflex intensity developments of
the three samples can be compared. The results show a
reduction in the time that passes until the intensity of the
alite reflex decreases, and the CH reflex intensity starts to
rise earlier as the Ca(NO3)2addition was increased. This
indicates an early onset of the alite hydration and an early
formation of CH and C–S–H, respectively, which corre-
lates well with the shift of the heat flow maximum to
earlier times with increasing Ca(NO3)2additions observed
in isothermal calorimetry. The rate of C3A dissolution
appears not to be affected by the presence of Ca(NO3)2in
solution during the first 6–8 h of hydration, which is in-
line with the results of Ye et al.59 The initially observed
gypsum reflex intensities in samples CN_2 and CN_5 are
increased compared to sample CN_0. However, the inten-
sity development of gypsum suggests that the dissolution
of gypsum occurs at an earlier time and at a higher rate
in the presence of increasing amounts of Ca(NO3)2,result-
ing in an earlier gypsum depletion. Depletion of gypsum
occurs after ∼8 h in sample CN_0, after 7 h in sample
CN_2andafter3hinsampleCN_5.Anhydriteisalsocon-
sumed faster in the presence of Ca(NO3)2according to the
reflex intensity development. Parallel to the increased dis-
solution of gypsum and anhydrite, the rate of ettringite
DORN et al. 757
FIGURE 3 Reflex intensity development of several cement phases and hydration products determined by in situ X-ray diffraction (XRD)
analysis of hydrating cement pastes
formation is increased, especially at the high Ca(NO3)2
dosage of 5 wt.%, as indicated by the fast increase of the
ettringite reflex intensities.
Because of the high scan speed and the initially low
crystallinity of AFm phases (including SO4-AFm and
strätlingite as well as NO3-AFm and CO3-AFm (mono and
hemi)), peak intensities of AFm phases are too low to ana-
lyze the development of these phases using in situ XRD
during the first 24 h of hydration.
The quantification of XRD results using the Rietveld
algorithm is a frequently applied technique to obtain
quantitative information on the phase composition of a
hydrated cement sample after a defined period of hydra-
tion. In the present study, the quantification of XRD
results was performed on samples hydrated for 1, 7, and
28 days. The results after 1 day of hydration indicate the
hydrated cement phase composition at the end of the in
situ XRD measurement. Rietveld refinement results of
cement hydrated for 7 and 28 days reflect the cement
phase composition at later stages of hydration. The pat-
tern of sample CN_0 hydrated for 1 day, refined by the
Rietveld algorithm, is shown in Figure 4,representing
the refined patterns leading to the QXRD results summa-
rized in Figure 7. Figure 5shows the complete range of all
diffractograms used for the quantitative determination of
the phase composition, whereas Figure 6focuses on the 2ϴ
range between 5◦and 17◦2ϴ, in which the highest intensity
reflexes of the hydration products of the aluminate reaction
appear.
FIGURE 4 Representative Rietveld refinement pattern of
sample CN_0 hydrated for 1 day. The difference between the
observed and the calculated intensities is plotted below the diagram
At the end of the in situ measurement, after 24 h
of hydration, QXRD results reveal that the remaining
amounts of the clinker minerals alite, belite, C3Aand
C4AF are, within small margins, comparable in all ana-
lyzed samples with a maximum difference between the
individual samples of ∼2wt.%foraliteandbelow1wt.%for
belite, C3AandC
4AF (Figure 7). Thus, although Ca(NO3)2
influences the rate of hydration initially, the progress of
758 DORN et al.
FIGURE 5 Complete diffractograms of cement pastes hydrated for 1, 7, and 28 days in the presence of 0-, 2-, and 5 wt.% Ca(NO3)2.To
facilitate the interpretation, the background has been subtracted. The reflexes of the hydration products ettringite (AFt), portlandite (CH),
alite, and the internal standard rutile (TiO2) are highlighted by droplines
FIGURE 6 Diffractograms of cement pastes hydrated for 1, 7,
and 28 days in the presence of 0-, 2- and 5 wt.% Ca(NO3)2in the
range from 5◦to 17◦2ϴ. The reflexes of the hydration products
ettringite (AFt), NO3-AFm, and CO3-AFm (mono) are highlighted
by droplines
hydration later than 24 h appears to be independent of
Ca(NO3)2addition.
Regarding the crystalline hydration products, the
addition of Ca(NO3)2leads to a significant increase in the
content of ettringite in the hydrated cement. After 24 h
of hydration, 3 wt.% ettringite were determined in the
reference sample, whereas 7 and 9 wt.% were determined
in samples CN_2 and CN_5, respectively. Further, 2 wt.%
NO3-AFm were determined in sample CN_5 after 24 h
of hydration. Moreover, after 7 and 28 days of hydra-
tion, amounts of NO3-AFm >1 wt.% were determined
in sample CN_5 at both ages. At 28 days of hydration,
the amount of CO3-containing AFm phases was reduced
from 4 wt.% in sample CN_0 to 3 wt.% in sample CN_5
(Figure 7).
3.4 Compressive strength development
The evaluation of compressive strength at different hydra-
tion times is a widely applied method to follow the
hardening of cement pastes and mortars. The results of
compressive strength testing are summarized in Figure 8.
The 1-day compressive strength of the 2-cm cubes made
from cement paste is slightly increased in the presence of
1 wt.% Ca(NO3)2from 28.1 to 30.2 N/mm2. However, 5 wt.%
Ca(NO3)2reduces the 1-day compressive strength of the
tested paste cubes to 21 N/mm2. After 7 and 28 days of
hydration, all Ca(NO3)2containing samples show higher
compressive strengths than the reference samples. With
57.3 N/mm2, the highest paste cube compressive strength
has been determined for sample CN_2 after 28 days of
hydration. On average, the 28-day paste cube compressive
strength is increased by ∼13 % in the presence of Ca(NO3)2
compared to reference sample.
Regarding the compressive strength development of
the mortar prisms, similar trends have been observed
as for the 2-cm paste cubes (Figure 8). In the presence
of Ca(NO3)2dosages exceeding 1 wt.%, the 1-day mortar
prism compressive strength is slightly reduced compared
to the reference sample. However, the observed effect of
Ca(NO3)2on the compressive strength development of
mortar prisms is significantly less pronounced as for the
2-cm paste cubes. Especially after 28 days of hydration,
the differences between the tested samples are within the
variance of the results so that no significant influence of
DORN et al. 759
FIGURE 7 Phase composition of cement hydrated for 1, 7, and 28 days in dependence of Ca(NO3)2addition determined by quantitative
X-ray diffraction (QXRD)
FIGURE 8 Compressive strength development of paste cubes
and mortar prisms in dependence of Ca(NO3)2addition
Ca(NO3)2on the 28 days compressive strength could be
determined.
3.5 Pore solution analysis
The analysis of the pore solution by ICP-OES is a method
that, by revealing the ion concentrations in the pore
solution at specific times of hydration, offers valuable
insights into the chemical processes of cement hydration.
As seen in the former sections, the addition of Ca(NO3)2
influences most significantly the early reaction rate. To
FIGURE 9 Ion concentrations in pore solutions extracted by
centrifugation of 10 min after 5, 20, and 50 min of undisturbed
hydration in dependence of Ca(NO3)2addition
investigate this early influence further, pore solution was
collected after 5, 20, and 50 min of undisturbed hydration,
after which the samples were centrifuged for 10 min to
separate the pore solution from the solids. The results in
Figure 9show that [Ca] during the first 60 min of hydra-
tion stays on a comparably constant level of ∼20 mmol/L
in sample CN_0 and ∼45 and 145 mmol/L in samples
CN_2 and CN_5, respectively. [S] is steadily declining in
all samples during the first 60 min of hydration and is
reduced in the presence of Ca(NO3)2,from∼86 mmol/L
in sample CN_0 after 15 min of hydration to ∼17 mmol/L
in sample CN_5 at the same time. [Si] is reduced in the
760 DORN et al.
presence of Ca(NO3)2from 80 μmol/L in sample CN_0 to
∼20 μmol/L in the samples CN_2 and CN_5 after 15 min
of hydration. Furthermore, [Si] slightly decreases between
30 and 60 min of hydration in the samples CN_0 and
CN_2, whereas it slightly increases in the sample CN_5
during the same period. Moreover, [Al] is reduced from
11.6 μmol/L in sample CN_0 to 8.5 μmol/Linsample
CN_5 after 15 min of hydration. After 60 min of hydration,
the sample CN_5 shows the highest [Al] in the pore
solution.
3.6 Thermodynamic calculations
Thermodynamic modeling is classically used to estimate
the phase assemblage in thermodynamic equilibrium of
a chemical system. In the context of inorganic binders,
this means the phase assemblage after an infinitely long
hydration time. Nevertheless, the dissolved quantity of
each precursor phase at a given point of time can be
used to estimate the kind and quantity of precipitates and
the ion concentrations in the pore solution as a function
of time.60–62 This approach has successfully been used
to model the evolution of solids and liquid phase com-
position in Portland cements starting at a few minutes
of hydration.60–62 In the current work, thermodynamic
calculations for the pseudo-equilibrium during the ini-
tial dissolution and at complete hydration are performed
depending on the Ca(NO3)2addition.
For the pseudo-equilibria, during initial dissolution, it
was assumed that of the calcium sulfates in the cement,
only bassanite dissolves completely during the first min-
utes of cement hydration.63 Further, the initial dissolution
of 1.8-g C3A per 100 g of cement64 and the complete dis-
solution of arcanite63 were assumed for this calculation.
As the ionic concentrations measurement indicate the
presence of silicon and some sodium (Figure 9), the dis-
solution of 0.1 g of C3S was assumed and 0.05-g Na2O
was added as well to the starting parameters of the calcu-
lation. The precipitation of gibbsite and microcrystalline
Al(OH)3was suppressed, as these phases cannot form in
the timeframe of interest.65 Figures 10 and 11 show the
thermodynamically predicted hydration products and ele-
ment concentrations in the pore solution, calculated based
on the initial dissolution of cement phases in dependence
of the Ca(NO3)2addition. These calculations are intended
as an indicator for the development of the system. It is
unlikely that the system will ultimately reach the cal-
culated (pseudo-)equilibrium states, and mutual kinetic
influences between the phases are not possible to account
for. Nevertheless, the calculations can be used to see in
which direction the need of minimizing the Gibbs free
energy will force the system.
FIGURE 10 Thermodynamically calculated hydration
products based on the pseudo-equilibria of the initially dissolved
phases in dependence of Ca(NO3)2addition
FIGURE 11 Thermodynamically calculated ion
concentrations in solution based on the pseudo-equilibria of the
initially dissolved phases in dependence of Ca(NO3)2addition
Thermodynamic modeling predicts a substantial
increase of the initially forming ettringite quantity for
increasing Ca(NO3)2additions, which can be verified by
in situ XRD (Figure 3). In contrast, the predicted develop-
ment of AFm phases (NO3-AFm and strätlingite) cannot
be detected by in situ XRD, which is likely explained by
the low crystallinity of early forming AFm phases.
The thermodynamic prediction of the ion concen-
trations in the pore solution correlates well with the
trends revealed by pore solution analysis (Figure 9). Cal-
culated and measured ion concentrations indicate that
[Ca] rises with increasing Ca(NO3)2additions, whereas
DORN et al. 761
FIGURE 12 Calculated phase composition of the fully
hydrated cement (assuming calcite as inert) in dependence of the
Ca(NO3)2addition relative to cement mass
the alkali concentrations remain nearly constant. The
slightly decreasing trend predicted for [Si] (except a rise
at ∼1.75 wt.% Ca(NO3)2) was also confirmed by the results
of pore solution analysis as well as the decline of [S] and
[Al], predicted for increasing Ca(NO3)2additions. Further-
more, [N] has been predicted to increase sharply at low
Ca(NO3)2additions. Especially the decline of [Al] might
be particularly interesting for the acceleration of cement
hydration by Ca(NO3)2andwillberecalledinthefollowing
discussion.
For the total hydration of the cement, two cases were
assumed: no participation of calcium carbonate in the
reaction and full participation of calcium carbonate. These
two cases were considered as calcium carbonate may par-
ticipate slower in the reaction than the clinker phases.61,63
Consequently, the experimentally observed phase assem-
blage should be comparable to a state in-between these
two models. In the performed calculations, the formation
of goethite and hematite was suppressed, as these phases
are stable at room temperature but do not form within a
reasonable time due to kinetic hindrances.65,66
Figure 12 shows the thermodynamically calculated
hydrate assemblage of the fully hydrated cement in depen-
dence of the Ca(NO3)2addition, assuming no participation
of calcium carbonate. The results suggest that the vol-
ume of ettringite increases slightly at the expense of
OH-containing SO4-AFm for small Ca(NO3)2additions but
stay unchanged for Ca(NO3)2additions exceeding 0.5 wt.%.
Furthermore, the volume of C–S–H increases slightly but
continuously with increasing Ca(NO3)2additions. The vol-
ume of NO3-AFm increases significantly with increasing
Ca(NO3)2additions, partially at the expense of the Fe–
Si-containing hydrogarnet, the volume of which slightly
FIGURE 13 Calculated phase composition of the fully
hydrated cement (assuming calcite as reactive) in dependence of the
Ca(NO3)2addition relative to cement mass. The formation of
thaumasite was suppressed for the calculation
declines. The volumes of portlandite and OH-hydrotalcite
stay constant over the entire Ca(NO3)2addition range from
0to5wt.%.
The enhanced formation of NO3-AFm and, to a smaller
extent, ettringite and C–S–H in the presence of Ca(NO3)2
leads to an increase of the solid phase volume in the fully
hydrated cement of ∼4cm
3/100 g of cement at an addi-
tion of 5 wt.% Ca(NO3)2. This mechanism is comparable
to the volume increase by CaCO3addition, which leads
to a replacement of SO4-AFm by CO3-AFm phases and
ettringite.61,65,67
When the full participation of calcite is assumed,
the modeling results are comparable in many points,
but the formation of thaumasite is predicted at high
Ca(NO3)2additions (Figure S2). Nevertheless, thaumasite
was observed in none of the current samples. A potential
reason for this discrepancy may be that thaumasite forma-
tion commonly occurs only at temperatures <15◦C (ideally
0–5◦C)68,69 and is very slow,70,71 although thaumasite may
be stable also at an elevated temperature depending on the
system.65,72 As no thaumasite was detected in XRD, the
formation of thaumasite was suppressed for the thermo-
dynamic calculation of the results presented in Figure 13.
In this case, ettringite is CO3-containing at low Ca(NO3)2
additions, and its quantity decreases slightly over the
whole Ca(NO3)2-addition range, as it was also observed
for portlandite and Fe,Si-containing hydrogarnet. C–S–H
and calcite increase slightly, whereas NO3-AFm increases
strongly with increasing Ca(NO3)2additions. CO3-AFm
(mono) is formed only at low Ca(NO3)2additions, whereas
calcite is a part of the equilibrium phase assemblage over
the whole analyzed Ca(NO3)2concentration range. The
762 DORN et al.
overall volume of solid phases increases over the whole
range of Ca(NO3)2additions, as it was also found for the
assumption of complete non-reactivity of calcite.
Several discrepancies between the thermodynamically
calculated results and the measured results of QXRD
must be addressed. No CO3-AFm is predicted by ther-
modynamics for samples containing ≥0.5 wt.% Ca(NO3)2
whereas QXRD results show the formation of up to 2 wt.%
CO3-AFm (mono) in sample CN_5 after 28 days, which
might indicate the carbonation of QXRD samples. Further-
more, no hydrogarnet could be detected by XRD analysis.
However, Dilnesa et al.66 showed that hydrogarnet is
the dominating Fe-containing phase in hydrated OPCs.
According to their study, hydrogarnet is difficult to detect
by standard analytical techniques, such as XRD of nons-
electively dissolved samples, which might explain why it
was not detected in QXRD analysis.
4DISCUSSION
Isothermal heat flow calorimetry, in situ XRD, and the
pore solution analysis indicate significant influences of
Ca(NO3)2on the early cement hydration. The increased
heat flow during the induction period in the presence of
Ca(NO3)2, measured by isothermal calorimetry, indicates
an increased reactivity at this early stage of hydration.
The processes responsible for this increased reactivity
arerevealedbyinsituXRDandporesolutionanaly-
sis and corroboration of the results by thermodynamic
calculations.
In situ XRD shows that the initial gypsum reflex intensi-
ties are significantly higher in the presence of Ca(NO3)2as
compared to the neat reference sample CN_0. On the one
hand, this observation might indicate that the presence of
Ca(NO3)2depresses the dissolution of gypsum and that,
during the 8 min of sample preparation for in situ XRD,
a fast initial dissolution of gypsum occurred only in the
neat reference sample. On the other hand, the observed
increased gypsum reflex intensities might indicate that
Ca(NO3)2enforces a rapid precipitation of additional gyp-
sum by increasing the availability of calcium in solution.
Jansen et al.,73 who analyzed the early hydration of an
additive-free cement paste by quantification of in situ XRD
results, found that in their cement paste, no rapid reaction
of gypsum occurred during the first 4 h of hydration. Based
on this observation by Jansen et al.,73 it was assumed that
fast precipitation of gypsum occurred during the initial
minutes of hydration in the Ca(NO3)2containing samples
CN_2 and CN_5. Moreover, Justnes et al.19 assumed that
the fast precipitation of gypsum might occur in hydrating
Ca(NO3)2-containing cement pastes. The sulfate required
for this initial gypsum precipitation presumably originates
from the fast dissolving alkali-sulfate arcanite,19 of which
the analyzed cement contains 2 wt.% according to QXRD,
as well as the initial dissolution of bassanite. The depressed
sulfate concentration during the first 60 min of hydra-
tion in the presence of Ca(NO3)2,measuredbyICP-OES,
supports the assumption of initial gypsum precipitation.
Although Justnes et al.19 assumed that the precipitation
of gypsum would slow the formation of ettringite due to a
scarce solubility of gypsum, the opposite is found to occur
in the present study.
Although the reflex intensity of gypsum (measured by in
situ XRD) stays on an approx. constant level during the first
4 h of hydration in the neat cement paste CN_0, gypsum
reflex intensities measured for CN_2 and CN_5 immedi-
ately start to decline, suggesting a fast dissolution of the
initially precipitated gypsum. Moreover, the rate of anhy-
drite dissolution increases during the first hours of cement
hydration, whereas the rate of C3A dissolution appears
to be unaffected during the first 8 h of hydration in the
presence of Ca(NO3)2.
As the increased dissolution of cement phases enforces
increased precipitation of hydration products, parallel to
the fast dissolution of gypsum and anhydrite, ettringite is
precipitated at a higher rate in samples CN_2 and CN_5
as compared to sample CN_0. As discussed earlier, due to
their low initial crystallinity, the formation of AFm phases
could not be observed in in situ XRD. However, besides the
increased ettringite formation, QXRD of the dried samples
showed that NO3-AFm had been formed in sample CN_5
during the first 24 h of hydration, and that the determined
amount of NO3-AFm in sample CN_5 further increased to
4 wt.% after 28 days of hydration.
Thermodynamic modeling of the initial phase compo-
sition and the phase composition in the fully hydrated
cement corroborate the presented measurement results
regarding the effect of Ca(NO3)2on the formation
of aluminate-containing hydration products during the
hydration of a commercial OPC. Both the initial formation
of an increasing amount of ettringite and the formation of
NO3-AFm, at the expense of SO4-AFm or CO3-AFm, due
to increasing additions of Ca(NO3)2, were indicated by the
results of the thermodynamic calculations, which is in-line
with the results of Balonis et al.14
Besides a heat flow increase during the induction period,
isothermal heat flow calorimetry (Figure 2)shows,for
increasing Ca(NO3)2additions, a reduction of the time
that passes until the heat flow maximum is reached. This
suggests an acceleration of alite hydration, which is most
significant for the highest analyzed Ca(NO3)2dosage of
5 wt.% in sample CN_5. In situ XRD confirms this accel-
eration of alite hydration in the presence of Ca(NO3)2as
indicated by the early onset of alite reflex intensity decrease
and portlandite reflex intensity increase (Figure 3).
DORN et al. 763
The increased calcium concentration in the pore
solution is regularly discussed as the primary cause
for the acceleration of alite hydration in the presence
of Ca(NO3)2.18,26 However, the present study results
suggest that the effect of Ca(NO3)2on the aluminate
concentration in solution might also contribute to the
observed acceleration of alite hydration.
The presence of Ca(NO3)2increases, as discussed ear-
lier, the formation of the aluminum-containing hydration
products ettringite and NO3-AFm, whereas the dissolution
of C3A appears to be unaffected during the initial period
of hydration. Consequently, a decrease of [Al] in the
initial pore solution was expected and the thermodynamic
calculations, based on the pseudo-equilibria of the initially
dissolving phases, indicated the described reduction of
[Al] in the initial pore solution for increasing Ca(NO3)2
concentrations.
Moreover, the analysis of the pore solution composi-
tion by ICP-OES showed a reduction of the [Al] from
11.6 μmol/L in sample CN_0 to 6.4 μmol/LinsampleCN_5
in early pore solution composition after 15 min of hydra-
tion. CN_5 also has after 30 min still the lowest [Al].
After 60 min of hydration, however, the highest aluminum
concentration in the pore solution was measured in sam-
ple CN_5 with 32.6 μmol/L, which might be attributed
to a further progressed state of hydration of the sample
CN_5 at that time and the onset of aluminum release
from dissolving alite.63 However, it must be considered
that the measured [Al] concentrations are very low, even
though they are well above the limit of quantitation of
0.07 μmol/L, and that minor and unavoidable deviations
in sample preparation might influence the pore solution
composition at such low concentrations.
Nevertheless, based on the results of thermodynamic
calculations and the indications of pore solution analy-
sis, the addition of Ca(NO3)2can lead to a reduction of
aluminum concentration in pore solution, which might
contribute to the observed early onset of alite hydration.
The mechanism causing the retarding effect of aluminum
on the alite hydration has not yet been fully resolved and
could not be further investigated in the present study.
Begarin et al.74 proposed that in the presence of aluminum,
the formations of C–A–S–H nuclei, which are not acting
as seeds for further C–S–H growth, are responsible for the
retarded onset of fast alite hydration. Nicoleau et al.27 and
Pustovgar et al.75 were, however, able to show that it is
rather a poisoning of the alite surface by aluminum that
inhibits alite dissolution in the presence of aluminum.
Regarding the compressive strength development,
Ca(NO3)2slightly reduces the compressive strength
measured after 1 day of hydration and enhances the
compressive strength measured after 7 and 28 days of
hydration, which has also been reported in literature.17,76,77
According to Oey et al.,17 the slight reduction of 1-day
compressive strength in Ca(NO3)2-accelerated samples
might originate from a rapid decrease of clinker particle
surfaces in direct contact with the pore solution, due to
the initially increased rate of hydration product formation.
After the hydration maximum, the rate of hydration
in the accelerated samples decreases faster than in the
unaccelerated samples. After ≥24 h, the hydration in
accelerated and unaccelerated samples continues at a
moderate rate, and QXRD results after 7 and 28 days
show that the amount of remaining clinker phases is
comparable in samples CN_0, CN_2, and CN_5. This
indicates that the progress of hydration is independent of
the Ca(NO3)2addition after the initial hours of hydration.
The thermodynamic calculation of the phase composition
of the fully hydrated cement (Figures 12 and 13)shows
that Ca(NO3)2increases the total phase volume of the
hydrated cement. The reason for that is the formation of
NO3-AFm and to a significantly smaller amount also the
increase of C–S–H as well as, depending on the assumed
CaCO3reactivity, ettringite. This observation suggests
that the increase in the total phase volume and the denser
structure of the hydrated cement61,62 leads to the increase
in compressive strength in Ca(NO3)2-accelerated samples
observed on the tested cement paste cubes after 7 and
28 days of hydration. The influence of Ca(NO3)2on the
compressive strength development was significantly less
pronounced for the mortar prisms made according to EN
196-135 compared to the 2 cm cubes made from cement
paste. No weakening effect of Ca(NO3)2on the interfacial
transition zone between aggregates and hardened cement
paste has been reported in the literature. Consequently,
the enhanced effect of Ca(NO3)2on the tested paste cubes
might be explained by the fact that the volume share of the
sample affected by Ca(NO3)2is higher in the paste cubes,
which do not contain sand, as in the sand containing
mortar prisms.
5 CONCLUSIONS
The presented results were obtained by analyzing one
commercial CEM I 52.5 R in a series of experiments and
thermodynamic calculations. The experimental results
and the results of the thermodynamic calculations indi-
cate that Ca(NO3)2accelerates the silicate reaction of
a hydrating OPC paste and significantly influences the
initial formation of AFm phases and ettringite. In the
presence of Ca(NO3)2, fast initial precipitation of gypsum
occurs. This initially precipitated gypsum then quickly dis-
solves at a higher rate than the gypsum present in the
neat reference sample. Moreover, the dissolution of anhy-
drite has been enhanced in the presence of Ca(NO3)2.
764 DORN et al.
Besides the increased dissolution rate of the two calcium
sulfates, gypsum and anhydrite, the ettringite formation
is enhanced, especially at the highest analyzed dosage
of 5 wt.% Ca(NO3)2. Furthermore, significant amounts of
NO3-AFm are formed in the presence of Ca(NO3)2.No
indications for an increased rate of C3A dissolution in the
presence of Ca(NO3)2were found during the initial 8 h
of hydration. Regarding the silicate reaction, it could be
shown that Ca(NO3)2decreases the time that passes until
the fast alite hydration starts. Based on the thermodynamic
calculation of the initial pore solution composition as well
as the analysis of early pore solution composition by ICP-
OES, it appears that besides a significant increase in [Ca],
a decrease in the [Al] in pore solution might contribute
to the acceleration of alite hydration in the presence of
Ca(NO3)2.
ACKNOWLEDGMENTS
The authors acknowledge Thomas Matschei for fruitful
discussions in the context of this publication. Further-
more, the authors want to thank the assigned reviewers for
their constructive criticism, which helped us to improve
the quality of our manuscript. This work was funded by
the German Federal Ministry of Education and Research
in the scope of the project BauProAddi (FKZ: 03XP0122A).
Open access funding enabled and organized by Projekt
DEAL.
ORCID
Tobias Dorn https://orcid.org/0000-0001-6431-1199
Tamino Hirsch https://orcid.org/0000-0002-0938-4261
DietmarStephan https://orcid.org/0000-0002-1893-6785
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SUPPORTING INFORMATION
Additional supporting information can be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Dorn T, Hirsch T,
Stephan D. Working mechanism of calcium nitrate
as an accelerator for Portland cement hydration. J
Am Ceram Soc. 2023;106:752–766.
https://doi.org/10.1111/jace.18782
