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Belitic calcium sulfoaluminate cement: Hydration chemistry, performance, and use in the United States

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Belitic Calcium Sulfoaluminate (BCSA) cement, has a history of over 30 years of use in North America. To date, more than 2 million tons of BCSA cement have been manufactured in the United States. It has been primarily used in the fast-track rehabilitation of highway and airfield pavement It has also been used widely in high-performance, low carbon footprint mortars. Despite this extensive use in the field, many aspects of the hydration mechanisms have yet to be investigated. The gap between the reactivity of calcium sulfoaluminate and that of belite, for one, has been presented as a challenge to the continuous strength gain of the material. In theory, this gap in reactivity should be reflected in a reduction in the rate of strength gain after hydration of the sulfoaluminate is complete. However, this is not always observed in the field, as was observed in the rehabilitation of the concrete runway of the Seattle-Tacoma Airport. This paper discusses the hydration of B-CSA cement with a particular focus on the intermediate hydration times using quantitative XRD, thermodynamic modeling, TGA and TD-1H NMR. Hydrated BCSA cement contains amorphous phases yet to be defined and characterized, especially in terms of their contribution to strength. The paper discusses these results in the context of the history of the binder in North America, the regulatory aspects controlling its use, and the important opportunities linked to its unique characteristics of speed of construction, durability and low carbon footprint.
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15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
Belitic calcium sulfoaluminate cement: Hydration chemistry,
performance, and use in the United States
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ABSTRACT
Belitic Calcium Sulfoaluminate (BCSA) cement, has a history of over 30 years of use in North America.
To date, more than 2 million tons of BCSA cement have been manufactured in the United States. It
has been primarily used in the fast-track rehabilitation of highway and airfield pavement It has also
been used widely in high-performance, low carbon footprint mortars. Despite this extensive use in the
field, many aspects of the hydration mechanisms have yet to be investigated. The gap between the
reactivity of calcium sulfoaluminate and that of belite, for one, has been presented as a challenge to
the continuous strength gain of the material. In theory, this gap in reactivity should be reflected in a
reduction in the rate of strength gain after hydration of the sulfoaluminate is complete. However, this is
not always observed in the field, as was observed in the rehabilitation of the concrete runway of the
Seattle-Tacoma Airport.
This paper discusses the hydration of B-CSA cement with a particular focus on the intermediate
hydration times using quantitative XRD, thermodynamic modeling, TGA and TD-1H NMR. Hydrated
BCSA cement contains amorphous phases yet to be defined and characterized, especially in terms of
their contribution to strength. The paper discusses these results in the context of the history of the
binder in North America, the regulatory aspects controlling its use, and the important opportunities
linked to its unique characteristics of speed of construction, durability and low carbon footprint.
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
1. INTRODUCTION- BELITIC CALCIUM SULFOALUMINATE IN THE UNITED STATES
Belitic calcium sulfoaluminate, a binder in which the predominant phase is belite and the second major
phase is calcium sulfoaluminate is a promising binder (Juenger 2011). It was invented, has been
manufactured and has been sold in North America since the early 1980s. It is sometimes presented as
a recent invention, but its discovery is owed to Ost in 1975, following Klein’s pioneering work on
sulfoaluminate (Klein 1963). Removing tricalcium silicate C3S from siliceous binders is desirable from
a sustainability point of view because the high temperatures required for its formation contribute to the
high carbon footprint of Portland cements. However, since C3S also provides early strength, its
presence plays a crucial role in such cements. By replacing C3S with ye’elemite, which forms at lower
temperature yet provides early strength, a new single cement can be manufactured which offers the
advantage of not requiring blending with Portland cement. Therein lies the elegance of the Ost patent:
the binder sets quickly, exhibits the good durability associated with belite hydration, and has lower
carbon footprint (Bescher 2012, 2016, 2017). These properties have attracted interest and recently led
to the introduction of a clause in an act of the United States Congress, which outlined the importance
of concrete combining durability, low carbon footprint and fast strength (Title 23, Section 120(c)(3)
United States Code). BYF cements are sometimes described as being in the R&D phase but, in fact,
they have been commercially available in the Americas since the 1980s. It is true that much research
remains to be done. While the early strength gain mechanism is fairly well understood, the reactivity of
phases other than CSA still needs elucidation. This is important because it affects the durability of the
binder and impacts its acceptance in the industry. If BCSA hydration can be shown to create strength-
contributing phases after ettringite formation, the potential use of this cement as a low-carbon
alternative to Portland cement is enhanced- provided that the economics are favorable.
This article explores some of the hydration mechanisms taking place beyond ettringite formation. It
frames the discussion in the context of the history of development of this cement in North America, the
path to industry specifications and it discusses some of the main applications of the binder. Noting that
BCSA is a single binder and not a blend, we conclude by stressing the need to distinguish it from other
types of CSA cements, and we outline a need for a nomenclature of CSA cements, perhaps akin to the
well-established ASTM classification of Portland cements. This will lift the confusion between various
types of CSA-containing cements.
1.1 Materials
A commercial BCSA cement (Rapid Set, CTS Cement Manufacturing Co.) was used for the hydration
study. The chemical composition as well as the mineralogical composition of the cement used are
shown in table 1. A water/cement ratio of 0.487 was used for the study.
Table 1. Mineralogical (XRD) and chemical (XRF) composition of the cement
Phases
wt.-%
Oxides
wt.-%
β-C2S
43.6
SiO2
14.3
α`-C2S
4.4
TiO2
0.58
Anhydrite
10.6
Al2O3
15.4
Bassanite
3.7
Fe2O3
0.9
Quartz
0.4
Mn2O3
-
Ye’elimite
27.4
MgO
1.4
Brownmillerite
1.8
CaO
49.5
Periclase
1.6
Na2O
0.2
Gehlenite
1.7
K2O
0.6
Perovskite (CT)
1.3
P2O5
0.3
Calcite
2.6
LOI
2.2
Dolomite
0.9
SO3
14.9
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
1.2 Methods
1.2.1 Heat flow calorimetry
A TAM Air heat flow calorimeter was used to measure the heat flow curve of the cement used. The
cement used was mixed with water (according to the chosen w/c ratio) externally with a spatula in order
to guarantee a proper mixing of the cement powder with the water. Before mixing the water and the
cement were equilibrated at the same temperature of the calorimeter in order to avoid artifacts due to
different temperatures of sample and calorimeter.
1.2.2 QXRD
For highly time resolved in-situ XRD the cement paste was placed into a temperature-controlled
sample holder and covered by Kapton film in order to avoid carbonation and water evaporation during
the measurement. In sum, 88 powder patterns were recorded within the first 24 hours of hydration.
In order to study the phase development of the cement over a long time period (up to 6 month) at a
constant water/cement ratio the cement paste was filled in dense plastic capsules and stored at 23
°C. At defined hydration times the samples were cut and the slices were measured with a Bruker
diffractometer.
The evaluation of the powder patterns was done applying Rietveld refinement combined with an
external standard method (Jansen et al. 2011).
1.2.3 Thermodynamic modelling
Thermodynamic modelling (according to Lothenbach et al. 2006) was performed using the
geochemical GEMS-PSI software [available at http://gems.web.psi.ch] involving a thermodynamic
database [available at http://www.empa.ch/cemdata, Version cemdata18, Lothenbach et al. 2018].
Thermodynamic equilibrium of the cement used at the w/c ratio chosen for the study was calculated
using the chemical composition determined by means of XRF shown in table 1.
1.2.4 TD-1H-NMR
TD-1H-NMR was applied in order to evaluate the ongoing hydration of the cement as well as the
consumption of the free water (see also Valori et al. 2013, Maus et al. 2006). Due to the relatively high
iron content of the cement and the resulting very short relaxation time the application of the CPMG
pulse sequence did not bring additional information about different mobile reservoirs and was therefore
not applied in this study. A SE pulse sequence was applied and the quantities of the mobile and solid
hydrogen reservoirs in the sample were evaluated.
Due to the missing information from the CPMG pulse sequence no information about the true sample
T2 (impact of the magnetic field inhomogeneities cannot be overcome with SE pulse sequence) and
the pore structures (pore size) of the sample can be provided. However, a quantitative evaluation of the
SE reservoirs certainly can give an idea about the hydration progress of the cement paste as well as
the consumption of the water during hydration.
Evaluation of the curves was done by ILT applying the MinerSys framework (Ectors et al. 2016). In
comparison to a combined Gaussian-exponential fit no significant overestimation of the solid part can
found when applying a multi-exponential decay modelling.
1.2.5 TGA
TGA was used to confirm the results from XRD. Hydration was stopped by stirring smoothly ground
hydrated cement at defined hydration times in isopropanol for 5 minutes. The liquid was then removed
from the sample by vacuum aspiration. Before measurement the samples were additionally dried at 40
°C for 5 minutes. TGA was performed under nitrogen atmosphere with 5°C/min between room
temperature and 1000°C.
2. RESULTS AND DISCUSSION
Heat flow analysis as well as the results from insitu-XRD show that after 1 hour the main reaction of the
ye’elimite takes place synchronously with the reaction of anhydrite. The main hydration phase formed
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
is ettringite. From the XRD patterns recorded within the first days of hydration, no evidence for other
hydrate phases such as Afm can be seen. Figure 1 shows that the consumption of yeelemite and
anhydrite is nearly complete at 10 hours. There is a small amount of ye’elimite which seems not to react
within the first 6 months. After around 3 months reaction of the C2S in the sample is detected. This in
turn appears simultaneously with the formation of straetlingite as observed by XRD.
Figure 1. Ye’elimite and C2S quantities during hydration of the cement used (w/c ratio = 0.487;
T = 23 °C)
Figure 2 shows the detected signals from TD-1H-NMR as recorded at different times during hydration
of the cement. It can be seen that at the beginning of hydration only one exponential decay curve can
be detected representing the amount of water given to the cement. After 50 hours a significant amount
of a solid fraction was formed following rather a Gaussian decay than an exponential decay. It can be
seen that between 8 days and 17 days the signal for the mobile fraction (free water and maybe AH-gel)
disappears and after 17 days and 28 days an ongoing confinement could be detected by a shift of the
curve to shorter relaxation times.
From previous work it is quite clear that the solid part of the curve represents the hydrogen bound in
crystalline phases such as ettringite, Afm and straetlingite (although the crystallinity is not always very
good; see Jansen et al. 2018) and the mobile part represents the amount of hydrogen which is bound
in amorphous gels like C-S-H (Valori et al. 2013, Muller et al. 2013) or AH-gel (Jansen et al. 2017) and
free water.
After 48 days (see figure 3) the mobile fraction of the hydrogen seems to be completely consumed and
the solid fraction dominates the curve recorded for the cement. During the following time until 6 months
after begin of hydration the curve recorded does not change. Due to the fact that no mobile fraction is
formed again at later points in time and no shift of the signal recorded to different times can be detected,
it can be assumed that the water released by the decrease of ettringite is immediately consumed in
order to form other hydrate phases than ettringite (such as straetlingite and Afm) and the microstructure
formed seems not to be affected by the recrystallization of the hydrate phases. This, indeed could be
better proven by determining the real T2 time of the samples applying the CPMG pulse sequence, which
was not possible here due to the high iron content of around 1 wt.-%. However, at later points in time
there is no evidence for a mobile fraction which could be measured with the CPMG pulse sequence.
0,1 1 10 100 1000
0
5
10
15
20
25
3 month
C2S
Ye'elimite
wt.-%
time (h)
Slight reaction of C2S ???
Rapidset XRD/Rietveld results:
6 month
3 days
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
Figure 2. TD-1H-NMR curves (SE pulse sequence) during hydration of the cement used (w/c
ratio = 0.487; T = 23 °C)
Figure 3. TD-1H-NMR curves (SE pulse sequence) during hydration of the cement used (w/c
ratio = 0.487; T = 23 °C)
Figure 4 shows the amounts of the hydrogen reservoirs in the sample. It can be seen that the solid part
increases while the mobile part decreases. This in turn gives a good idea about the ongoing reaction of
the cement and the consumption of the water given to the sample. After 3 months, no significant amount
of free water (mobile fraction) can be quantified after ILT of the recorded curves.
0,001 0,01 0,1 1
0
10
20
30
40
50
60
70
80
90
100
Intensity
time (ms)
48 d
3 month
6 month
1H-NMR Rapidset
No free water
No indication for ongoing structure development or destruction
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
Figure 4. Quantities of the hydrogen reservoirs determined during hydration of the cement
used applying a SE pulse sequence (w/c ratio = 0.487; T = 23 °C)
In order to understand the hydration behaviour of the cement at the chosen w/c-ratio thermodynamic
calculations are considered (Figure 5). The left column shows the composition of the cement used (not
100 wt.-% because only reactive phases are plotted). The right column shows the equilibrium phase
composition for the hydrate phases formed at the chosen w/c ratio. Although the sulfate in the cement
is sufficient to guarantee complete ettringite formation, it can be seen that at equilibrium Afm should be
formed together with ettringite. This can be explained by the fact that Afm incorporates less water than
ettringite. However, the first days of hydration are certainly dominated by the reaction of ye’elimite with
anhydrite forming large amounts of ettringite. It can be seen that after 28 days that the dominating
hydrate phase formed is ettringite with an amount which is higher than the expected amount from
thermodynamic modelling. Between 28 days and 48 days the free water is almost consumed (see figure
7) and the drift of the system to equilibrium occurs very slowly.
Between 3 months and 6 months almost no change in the TD-1H-NMR curve can be seen. However,
XRD indicates that the reaction to equilibrium takes place and the initially formed ettringite reacts
synchronously to the reaction of the C2S forming straetlingite and Afm, which is in accordance with the
calculated thermodynamic equilibrium of the system. TGA in Figure 6 also gives evidence that between
28 days and 6 months ettringite is consumed and Afm, straetlingite and also C-S-H is formed. However,
TGA is only qualitative but the evaluation and interpretation of the curves allows a confirmation of the
above discussed hydration path for the cement used.
Figure 5. Thermodynamic modelling of the equilibrium hydrate phase assemblage of the
cement used compared to the measured phase assemblage (w/c ratio = 0.487; T = 23 °C)
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
Figure 6. TGA curve for the cement used after 28 days and 6 months (w/c ratio = 0.487; T = 23
°C)
These results further our understanding of the hydration of BCSA cements at intermediate ages (from
hours to months). The hydration of the cement and its relationship to strength development was
originally described by Ost as being akin to the strength of a bucket of nails (Ost 1997). This analogy is
mostly apt but it is also incomplete. Microstructure and strength development is continuous even in the
presence of the poorly hydraulic belite, even without resorting to various strategies used to bridge the
reactivity gap between ye’elemite and belite such as the use of ternesite or activating the belite phase
with boron.
3. HISTORY AND DEVELOPMENT OF BCSA IN NORTH AMERICA
That ettringite is not the sole source of strength in BSCA concrete is part of the success of the binder
in North America since 1980. Approximately 2 million metric tons of BCSA cement have been produced
in North America to date. This is due to a number of incremental regulatory approvals obtained in the
United States. The American Chemical Society issued a Chemical Abstracts Service number for BCSA
in 1998 (960375-09-1), making it a parent of Portland cement. The cement also meets ASTM C1600
specifications for rapid setting cements, a performance specification similar to ASTM C 150 and ASTM
C1157 for portland cements. Many State Departments of Transportation in the US have specified the
material. However, a geographical review of where BCSA concrete is used reveals that it is primarily
commercially successful in the most densely populated areas of the United States. This suggests that
it is not a straightforward, economical substitute for Portland cement except in applications in which this
extended closure times of highways and airports are not possible.
Figure 7 Use of BCSA for Pavement in the United States, between 2009 and 2018. (Source:
CTS Cement Manufacturing Co.)
200 400 600 800
-0,5
-0,4
-0,3
-0,2
-0,1
0,0
diff.rel.weight (%/°C)
T (°C)
28 days
6 months
TRU without sand
Less ettringite
This could be CSH
Afm
Here is also stratlingite
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
More than 1,000 lane-kilometers of BCSA concrete pavement has been placed in California to date. In
1994, the Northridge earthquake caused the collapse of two major overpasses in Los Angeles,
California. The emergency repair required the use of a rapid strength concrete which led to the first use
of BCSA concrete by the California Department of Transportation. This project became a catalyst for
the continued and regular use of BCSA in California. Typically, one or two lanes of the freeway are shut
down at 10 pm, old slabs are removed and replaced with BCSA concrete. The economic incentive for
the use of this type of concrete is illustrated by the steep fined imposed in the event of late opening:
$1,000 per minute of delay. The specifications are 2.8 MPa flexural strength for opening and 4.1 MPa
flexural at 28 days. A similar approach to pavement rehabilitation is used by several states in the United
States.
Figure 8 Concrete Pavement Removal (Left) and Replacement with BCSA (Right) in California
Other significant infrastructure projects include 20,000 m3 of BCSA concrete pavement placed in 2018
on the rehabilitation of the Mexico-Querétaro highway, and 30,000 m3 in the rehabilitation of the
Seattle airport concrete runway between 1994 and 2012. Due to restrictions on closure time, the
runway was only available for repair between 11 pm and 6 am. Pavement specifications for the
rehabilitated pavement was 3.8 MPa for opening and a design life of 20 years (Lary and Rothnie,
1994). Flexural strengths achieved during construction were 5.2 MPa at 4 hours, 5.8 MPa at 5 hours;
6.2 MPa at 6 hours and 6.3 MPa at 28 days. In 2012, the BCSA concrete slab was tested at 79.0 MPa
and the flexural strength was 8.0 MPa, indicating a continuous strength gain after 17 years in the field.
It is likely that the continuous strength increase is explained by the slow but continuous hydration of
belite as described above or by the other mechanisms discussed in the present article (Ramseyer &
Bescher, 2016).
Figure 9 Removal and testing of BCSA Airport Concrete Pavement After 17 Years in the Field
at the Seattle International Airport.
BCSA can also address a number of concrete pavement design issues associated with portland
concrete infrastructure, such as joint spacing, lifecycle and maintenance costs. The authors constructed
a concrete slab 11.4 m x 11.4 m x 35.6 cm. Approximately 4 times the size of a traditional Portland
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
slab, this would allow building a typical runway just four-slabs wide, thereby minimizing construction
and maintenance costs. The slab was instrumented to measure shrinkage and warping. At 120 days,
both shrinkage and warping were below the detection limit of the sensors.
Figure 10 Quadruple Sized Experimental Airfield BCSA Slab (11.4 m by 11.4 m)
Slabs of such size cannot be placed using portland cement concrete because the high drying shrinkage
would lead to warping and curling. Other opportunities in improving pavement types such as CRCP,
JPCP or overlays or in other infrastructure applications have been demonstrated (Maggenti 2015,
McNerney 2015, Itani 2017)
BCSA cement is frequently confused with other high-alumina cements or other types of CSA cements.
This calls for the need for a nomenclature for CSA cements. This could be done easily on the basis of
binder characteristics and chemistry. The main mineralogical phases in CSA clinkers include CSA,
belite and calcium sulfates. Depending on the balance of these components, the CSA binders can be
shrinkage-compensating, rapid setting, self-stressing, or combinations of the above. In other words,
CSA cements a class of cements onto themselves. Just as in the case for portland cement types in
ASTM, CSA cements can be classified based on chemistry and applications. We propose a
classification as follows:
Table 2 Types of CSA binders
Ye’elemite
Belite
Calcium Sulfate
Other
Type A - Accelerating
Additive
35 - 45%
0 - 20%
10 - 30%
5 - 55%
Type B - Belitic Calcium
Sulfoaluminate Cement
20 - 30%
40 - 60%
5 - 25%
0 - 35%
Type C - Expansive
Additive
10 - 20%
10 - 30%
40 - 60%
0 - 40%
Type K - Shrinkage
Compensating Cement
1 - 10%
30 - 50%
1 - 20%
20 - 70%
Figure 11 Proposed nomenclature for CSA binders
11.4 m
11.4 m
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
Many of the CSA cements sold in China and in Europe are of Type A and are used blended with Portland
cements. Historically, CSA binders in the United States were first produced for shrinkage compensation
as Type K cement (Woodstrum 1964, Herrin 2003). In the 1980s, the rapid strength gain advantages
of the BCSA binder became the primary impetus for its commercial success as a single, non-blended
cement. The low carbon footprint (i.e. the sustainability) is not attractive economically yet (Bescher et
al. 2012). However, progress in the acceptance of this binder will be helped by the adoption of a proper
nomenclature distinguishing BCSA cements from other CSA binders.
4. CONCLUSIONS
Some features of the hydration of BCSA cement at intermediate times have been explored. Under
closed conditions (no evaporation or uptake of water), the reaction of the very reactive phases ye’elimite
and anhydrite forming ettringite significantly dominates the early hydration of the cement and, as a
consequence, the reaction of the comparably less reactive phases. Thermodynamic modelling predicts
stability of ettringite and formation of a C-S-H phase in the long term. Experimental results indicate the
evolution of an amorphous or poorly crystalline structure which has not been fully characterized,
following the precipitation of ettringite. These results are consistent with the continuous strength gain
observed in BCSA concrete in the field. However, the hydration picture if not complete and there is a
need for further characterization. These results are consistent with the observed durability of BCSA in
the field in the last 30 years. Furthering the acceptance of this binder in industry will be helped by the
adoption of a nomenclature distinguishing BCSA cements from other CSA binders.
5. ACKNOWLEDGMENTS
The very kind assistance of D. Jansen, F. Goetz and J. Neubauer is gratefully acknowledged.
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Prague, Czech Republic, September 16–20, 2019
15th International Congress on the Chemistry of Cement
Prague, Czech Republic, September 16–20, 2019
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Shrinkage Cement. California Department of Transportation. September 1964.
... The hydration of calcium sulfoaluminate forms a crystalline and dimensionally stable mineral called ettringite that contributes to a very high early age strength, while the hydration of belite leads to later age strength (Odler and Colan-Subsuste, 1999;Quillin 2001). With these unique properties, BCSA cements have much lower shrinkage and allows construction of larger slabs (Bescher 2019). ...
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Seattle Tacoma International Airport (SEA) has a 25-year successful history of using BCSA for rapid airfield construction. A test slab of 11.3 m x 12.2 m x 0.5 m (37.5 ft x 40 ft x 18 in.) thick was placed at SEA on November 1, 2019. This report describes the placement, instrumentation, and an analysis of slab behavior in terms of shrinkage and curling, as measured using embedded strain gauges. Preliminary data show exceptionally low strains even for such a large slab. This unusual behavior is probably due to the unique chemistry of BCSA, which involves early strength gain caused by water-retaining ettringite, followed by the minimal shrinkage associated with belite content. Analysis of the data collected over a year shows that this unique chemistry would support improving construction protocols and slab design allowing overnight replacement of four contiguous, regular-sized portland cement concrete slabs with a single BCSA slab on an active taxiway.
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The production of ferrite-rich calcium sulfoaluminate belite (CSABF) cement clinker, also containing MgO, from ladle slag, Fe-slag, and phosphogypsum was translated from a lab-scale to a pilot demonstration in a 7-metre kiln at 1260C. An account of the pilot trials/manufacturing is presented, and the process was robust. Laboratory tests prior to scale-up showed that gehlenite formation can be inhibited in the CSABF clinker by adding excess CaO in the raw meal; however, this reduces the amount of iron (Fe) that can be incorporated into ye'elimite and leads to higher ferrite (C6AF2) content. Detailed microstructural analyses were performed on the clinker to reveal the clinker composition as well as the partition of the minor elements. Different ferrite phases with varying amounts of titanium and iron are distinguished. Eighty-five percent of the clinker raw meal was comprised of side-stream materials and the clinker produced in the kiln had chemical raw-material CO2 emissions 90% lower than that of Portland cement made from virgin raw materials. These results can have a significant impact in regions with a prospering metallurgical industry, enabling industrial decarbonisation and resource efficiency.
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The resistance to chemical sulfate attack was tested for cements containing various amounts of tricalcium aluminate (C3A) and compared to that of calcium sulphoaluminate-belite cement containing no detectable amount of C3A. The six materials tested included a portland Type I (11% C3A), Type II (6% C3A), a Type V (0.5% C3A), and a rapid setting calcium sulphoaluminate (CSA) cement (0% C3A). In addition, two intermediate blends (0.4 and 5.0% C3A) were tested by combining these portland cements with shrinkage compensating CSA cement. Mortar cubes were immersed for two years in a solution containing high concentrations of sodium and sodium/potassium sulfate. Degradation of the mortar cubes was assessed qualitatively via visual observation and quantitatively through measurement of weight change. The magnitude of the degradation by sulfate attack was found to correlate well with C3A content as measured by X-ray diffraction, with Type I portland sustaining the heaviest damage and the CSA rapid setting cement remaining essentially unaffected by the prolonged exposure to the highly sulphated environment.
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Mercury intrusion and nitrogen sorption porosimetry were employed to investigate the pore structure of calcium sulfoaluminate (\( C\overline{S} A \)) and portland cement pastes with cement-to-water ratio (w/c) of 0.40, 0.50, and 0.60. A unimodal distribution of pore size was drawn for \( C\overline{S} A \) cement pastes, whereas a bimodal distribution was established for the portland cement pastes through analysis of mercury intrusion porosimetry. For the experimental results generated by nitrogen sorption porosimetry, the \( C\overline{S} A \) cement pastes have a smaller and coarser pore volume than cement paste samples under the same w/c condition. The relative dynamic modulus and percentage weight loss were used for investigation of the concrete durability in freeze–thaw condition. When coarse aggregate with good freeze–thaw durability was mixed, air entrained portland cement concrete has the same durability in terms of relative dynamic modulus as \( C\overline{S} A \) cement concrete in a freeze–thaw environment. The \( C\overline{S} A \) cement concrete with poor performance of durability in a freeze–thaw environment demonstrates the improved durability by 300 % over portland cement concrete. The \( C\overline{S} A \) concrete with good performance aggregate also exhibits less surface scaling in a freeze–thaw environment, losing 11 % less mass after 297 cycles.
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Two modifications of ye'elimite (namely orthorhombic ye'elimite (stoichiometric C4A3$) and iron-containing cubic ye'elimite (C4A2,7F0,3)) were synthesized and the reactions without additional sulfate and with gypsum were tracked by means of heat flow calorimetry. Without gypsum the reaction differs significantly for both modifications while the addition of gypsum leads to almost comparable reactions. With addition of gypsum two distinct heat flow maxima could be detected while the reaction without gypsum showed only one distinct maximum for both modifications. For the systems with gypsum additional experiments were performed namely in-situ XRD, NMR, thermodynamic modeling, TGA and pore solution composition. It could be shown that the reaction with gypsum shows two steps of ettringite precipitation for both modifications. The two steps show significant differences. During the first step of ettringite precipitation gypsum and ye'elimite and free water are consumed forming ettringite and a highly hydrated AHx. The second ettringite precipitation is powered by ye'elimite consumption but no further gypsum dissolution. In addition, water is removed from the AHx used up for the second ettringite precipitation. The two modifications of ye'elimite react comparable with gypsum but show slightly different heats of hydration when reacting with the same calcium sulfate (gypsum) during early hydration; this is due to slightly lower reaction turnover of the cubic, iron-containing modification leading to slightly reduced ettringite formation.
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After 10 years of service, the condition of two innovative PCC pavements, designed with a reduced number of joints, is reported. A third pavement, a conventional PCC pavement, was constructed at the same time. The performance of the two innovative pavements is compared to the performance of the conventional PCC pavement.
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1H nuclear magnetic resonance has been applied to cement pastes, and in particular calcium silicate hydrate (C–S–H), for the characterisation of porosity and pore water interactions for over three decades. However, there is now renewed interest in the method, given that it has been shown to be non-invasive, non-destructive and fully quantitative. It is possible to make measurements of pore size distribution, specific surface area, C–S–H density and water fraction and water dynamics over 6 orders of magnitude from nano- to milli-seconds. This information comes in easily applied experiments that are increasingly well understood, on widely available equipment. This contribution describes the basic experiments for a cement audience new to the field and reviews three decades of work. It concludes with a summary of the current state of understanding of cement pore morphology from the perspective of 1H NMR.