Reaction of siliceous fly ash in blended Portland cement pastes and its effect on the chemistry of hydrate phases and pore solution
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Reaction of siliceous fly ash in blended Portland cement
pastes and its effect on the chemistry of hydrate phases and
pore solution
Einfluss der Reaktion von Steinkohleflugasche auf die Chemie der Hydratphasen und der
Porenlösung eines flugaschereichen Mischzements
Der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität
Erlangen-Nürnberg
zur
Erlangung des Doktorgrades Dr. rer. nat.
vorgelegt von
Florian Deschner
aus Nürnberg
Als Dissertation genehmigt
von der Naturwissenschaftlichen Fakultät
der Friedrich-Alexander-Universität Erlangen-Nürnberg
Tag der mündlichen Prüfung: 03. Juli 2014
Vorsitzender des Promotionsorgans: Prof. Dr. Johannes Barth
Gutachter: Prof. Dr. Jürgen Neubauer
Prof. Dr. Christian Kaps
Dr. Frank Winnefeld
Acknowledgements
The work for the present thesis was carried out at the Swiss Federal Laboratories for Ma-
terial Science and Technology (Empa) in cooperation with Schwenk Zement KG, BASF
Construction Chemicals GmbH, Steag Power Minerals GmbH and the GeoZentrum Nord-
bayern, Chair for mineralogy at the Friedrich-Alexander University of Erlangen-Nuremberg.
I acknowledge Schwenk Zement KG and BASF construction chemicals GmbH for the fi-
nancial and analytical support. Moreover, I want to thank all members of the project for
their ideas and engagement leading to fruitful discussions.
I want to thank many people who have contributed to the successful outcome of this the-
sis.
First, I would like to express my gratitude to my co-supervisor Prof. Jürgen Neubauer for
calling my attention on the research project behind this doctoral thesis and for his manifold
support during the last 4 years. I always appreciated his comments and the common dis-
cussions with him.
Additionally, I sincerely thank my supervisors at Empa, Dr. Frank Winnefeld and Dr. Bar-
bara Lothenbach for guiding me through the doctoral research, teaching me many aspects
of cement related research work and promoting my personal development. I greatly appre-
ciated that they always had an open ear for me and time for the discussion of my results.
Moreover, I want to thank them for their humour and encouraging optimism.
Special thanks are due to Dr. Barbara Lothenbach for supporting my work with GEMS and
teaching me various aspects of the swiss german language.
My sincere thanks are extended to Dr. Beat Münch for his friendly help during the devel-
opment of the image analysis program and for sharing his experience in tending cows.
I also want to thank Dr. Wolfgang Kunther, Dr. Belay Zeleke Dilnesa, Dr. Lucia Ferrari, Dr.
Mohsen Ben Haha, Dr. Klaartje De Weerdt, Axel Schöler and Dr. Andreas Leemann for
many interesting discussions and shared experiences not only about cement or concrete,
but also about fishing, music and other important issues of life.
Furthermore I want to thank Boris Ingold for the frequent preparation of microscopy sam-
ples and musical support. Luigi Brunetti and Angela Steffen are acknowledged for the
technical support and ion chromatography measurements. My gratitude is extended to all
employees of the laboratory of concrete and construction chemistry. I enjoyed the friendly
atmosphere in the lab and it was a great pleasure to work with them.
Special thanks are due to Walter Trindler for his motivating engagement, frequent inspira-
tions for BBQ or other festivities and outstanding support during great fishing challenges at
the lake Greifensee.
My thanks go also to Tobias Danner, Markus Bernhardt, Dr. Daniel Jansen, Sebastian Dit-
trich and and all of my friends from Erlangen and Switzerland for many pub visits, outdoor
adventures and good friendship.
I
Last but not least I want to thank my family and my beloved Franzi for all the care and
support during the last 4 years.
II
Abstract
Global warming as a consequence of the anthropogenic emission of greenhouse gases is
an issue of global concern. Therefore, the United Nations – Framework Convention on
Climate Change has set the goal to keep the increase of the global warming below 2 °C.
To realise this goal, the global greenhouse gas emissions need to be reduced to a level of
60% of the emissions in 2010, until the year 2050. Due to the fact that about 5-8 % of the
global CO2 emissions originate from the production of Portland cement clinker, the request
for lower CO2 emissions is of great concern for the cement industry.
Besides process optimisation and the use of alternative fuels, the replacement of Portland
cement clinker by supplementary cementitious materials (SCM) is an efficient way to re-
duce the CO2 emissions related to cement production. Siliceous fly ash is one of the most
available SCM in Germany. This material is a by-product of the combustion of hard coal in
power plants.
The present thesis studies the effect of siliceous fly ash on the hydration of blended Port-
land cement containing 50 wt% of siliceous fly ash. To achieve this goal, a multi-method
approach using various techniques to characterise the hydration kinetics, solid hydration
products, pore solution and the microstructure has been used. To differentiate between the
exclusively physical effect of the addition of fillers and the chemical reaction of the fly ash,
the investigated samples were compared to a reference containing 50 wt% of quartz pow-
der instead of fly ash. The quartz powder has proven to be a suitable reference material
due to its practically inert behaviour in alkaline solutions. A test of the quartz powder in 0.3
mol/l KOH solution (pH ≈ 13.4) with Ca(OH)2 during 90 days has merely shown any poz-
zolanic reactivity at 23 °C.
Apart from the filler effect, no effect of fly ash on the hydration could be measured before 2
days. Afterwards, evidence of the pozzolanic reaction was given by a decrease of the port-
landite content and the formation of C-S-H with decreased Ca/Si atomic ratios and in-
creased amounts of incorporated Al. The effect of the pozzolanic reaction was reflected in
the pore solution chemistry, which showed decreased concentrations of Ca and increased
Al and Si concentrations.
The analysis of the microstructure showed the formation of hydrate phases on the surface
of fly ash particles within the first days of hydration. The origin of the first hydrate phases
was however assumed to mainly originate from the hydration of the Portland cement and
only to a minor extent from the dissolution of fly ash. After 7 days, the microstructure of the
fly ash particles showed the formation of an inner hydration product, characterised by a
gel-like consistency with a low density or a high content of water, respectively.
Additionally, a strong impact of the temperature on the hydration of fly ash blended Port-
land cement was found. The rate of the fly ash reaction was accelerated with increasing
temperature, and hence the affection of the hydration by the pozzolanic reaction was shift-
ed to earlier hydration times. Above 50 °C, the solubility of ettringite is increased, which led
to the destabilisation of this hydrate phase. The thereby released sulphate was enriched in
the pore solution and the C-S-H phase.Ca and Al were precipitated as siliceous hy-
III
drogarnet, whose formation was kinetically hindered at room temperature and occurred
only at higher temperatures or after very long hydration times. The formation of siliceous
hydrogarnet was further enhanced by the fly ash reaction due to the considerable amount
of released aluminate from the reaction of the fly ash glass. The temperature related
changes in the phase assemblage were confirmed by thermodynamic modelling, per-
formed with the help of GEMS (Gibbs energy minimisation software). The change of the
microstructure was similar as in pure OPC. Elevated temperatures led to the formation of a
more heterogeneous matrix of hydrates with a higher density of the C-S-H.
To directly investigate the reaction kinetics of fly ash within blended Portland cement an
image analysis procedure has been developed using grey level segmentation, grey level
filtering and morphological filtering to quantify the content of anhydrous fly ash in BSE im-
ages. The analysis of 600 images per sample yielded comparable results to conservative
techniques, like selective dissolution using a solution of Ethylenediaminetetraacetic acid
(EDTA) and NaOH.
IVZusammenfassung
Um der globalen Klimaerwärmung und deren Konsequenzen vorzubeugen, haben sich die
Vereinten Nationen (United Nations Framework Convention) das Ziel zu Eigen gemacht,
den Anstieg der globalen Durchschnittstemperatur auf maximal 2 °C einzuschränken. Um
dieses Ziel zu erreichen, soll die Emission von Treibhausgasen bis 2050 auf 60 % des Le-
vels von 2010 reduziert werden. Da die Produktion von Portlandzementklinker etwa 5 bis
8 % der globalen CO2-Emissionen ausmacht, steht auch die Zementindustrie in der Pflicht,
entsprechende Maßnahmen zur Minderung der CO2-Emissionen zu ergreifen.
Neben prozesstechnischen Optimierungen und der Verwendung alternativer Brennstoffe,
können die CO2-Emissionen bei der Zementherstellung insbesondere durch die Reduktion
des Klinkergehalts, bei gleichzeitig vermehrtem Einsatz mineralischer Zumahlstoffe, ge-
senkt werden. Als Zumahlstoffe werden unter anderem, industrielle Nebenprodukte wie
Hochofenschlacke oder Flugasche verwendet. Flugaschen, die bei der Verbrennung von
Steinkohle in Kraftwerken anfallen, erweisen sich in Deutschland auf Grund ihrer hohen
Verfügbarkeit als besonders geeignete Zumahlstoffe.
Die vorliegende Dissertation zeigt den Effekt von Steinkohleflugasche auf die Hydratation
eines Portlandzements mit 50 % Flugascheanteil. Dazu wurden entsprechende Zement-
pasten mit 50 % Portlandzement und 50 % Steinkohleflugasche mit verschiedenen Me-
thoden untersucht. Die Hydratationskinetik, die Entwicklung der Hydratphasen, die Poren-
lösungschemie und die Mikrostruktur wurden charakterisiert. Um zwischen dem rein phy-
sikalisch wirkenden Füller-Effekt der Flugasche und dem Effekt der chemischen Reaktio-
nen der Flugasche zu unterscheiden, wurden Proben mit 50 % Quarzmehl anstatt Flug-
asche als Referenz benutzt. Auf Grund des praktisch inerten Verhaltens des Quarzmehls
in 0,3 molarer KOH Lösung (pH ≈ 13,4) mit Ca(OH)2 über einen Zeitraum von 90 Tagen
bei 20 °C, hat sich das Quarzmehl als geeignetes Referenzmaterial erwiesen.
Neben der physikalischen Wirkung als Füllstoff, konnten bei der Hydratation der unter-
suchten Zemente während der ersten 2 Tage keine weiteren Einflüsse der Flugasche auf
die Hydratation gemessen werden. Zu späteren Zeiten wurde die puzzolanische Reaktion
der Flugasche durch den steten Verbrauch von Portlandit (Ca(OH) 2) und die Bildung von
C-S-H Phasen mit reduziertem Ca/Si Atomverhältnissen und einem erhöhten Einbau von
Al aufgezeigt. Die Effekte der puzzolanischen Reaktion spiegelten sich auch in der Ent-
wicklung der Porenlösungszusammensetzung wieder: die Konzentration an Ca nahm ab,
während die Al und Si Konzentrationen zunahmen.
Die Analyse der Mikrostruktur mittels Rasterelektronenmikroskopie zeigte, dass sich auf
den Oberflächen der Flugaschepartikel während der ersten 2 Tage ein Saum aus Hyd-
ratphasen bildete. Es wurde allerdings angenommen, dass diese Hydratphasen größten-
teils auf die Hydratation des Portlandzementklinkers zurückzuführen waren. Nach 7 Tagen
wies die Mikrostruktur der Flugaschepartikel erstmals ein inneres Hydratationsprodukt auf,
welches sich durch eine niedrige Dichte, bedingt durch einen hohen Wassergehalt, aus-
zeichnete. Während des gesamten Untersuchungszeitraums wurde mit der langsam fort-
Vschreitenden Auflösung der Flugasche ein steter Zuwachs des inneren Hydratationspro-
duktes beobachtet.
Darüber hinaus zeigte sich, dass die Umgebungstemperatur einen bedeutenden Einfluss
auf das Abbinden der untersuchten Zemente hatte. Bei erhöhten Temperaturen wurde die
Flugaschereaktion beschleunigt, wodurch der Effekt der puzzolanischen Reaktion auf die
Zementhydratation früher zum Tragen kam. Die erhöhte Löslichkeit von Ettringit bei an-
steigenden Temperaturen führte zur Freisetzung von Sulfat, welches sich in der Porenlö-
sung und in C-S-H Phasen anreicherte. Freigesetztes Ca und Al wurde in Silikat-reichen
Hydrogranat, welcher sich vermehrt bei höheren Temperaturen und steigendem Flug-
aschereaktionsgrad bildete, gebunden. Die Temperatur-bedingten Veränderungen der
Phasenzusammensetzung konnten mittels thermodynamischer Berechnungen mit GEMS
(Gibbs energy minimisation software) modelliert und nachvollzogen werden. Darüber hin-
aus wurde mit erhöhter Temperatur eine zunehmend ungleichmäßigere Mikrostruktur und
eine erhöhte Dichte der C-S-H Phasen festgestellt.
Um die Reaktionskinetik der Flugasche zu bestimmen, wurde ein Bildanalyse-Verfahren
zur Auswertung von Rasterelektronenmikroskopbildern entwickelt. Bei diesem Verfahren
kann durch Kombination verschiedener Morphologie- und Graustufenfilter, sowie Segmen-
tierungen, der Gehalt an nicht-hydratisierten Flugaschepartikeln bestimmt werden. Die Er-
gebnisse der Analyse von 600 Bildern pro Probe waren vergleichbar zu konventionellen
Verfahren, wie der selektiven Auflösung mit Ethylendiamintetraessigsäure (EDTA) Lösung
und NaOH.
VIList of papers
The thesis includes the following papers:
1. Investigation of a model system to characterize the pozzolanic reactivity
of two low Ca fly ashes and a quartz powder
Deschner F., Lothenbach B., Winnefeld F., Schwesig P., Seufert S., Dittrich S.,
Neubauer J.
Tagungsband der Tagung Bauchemie der GDCh, Hamburg 2011, pp. 127-132.
2. Hydration of Portland cement with high replacement by siliceous fly ash
Deschner, F., Winnefeld F., Lothenbach B., Seufert S., Schwesig P., Dittrich S.,
Goetz-Neunhoeffer F., Neubauer J.
Cement and Concrete Research 42, pp. 1389-1400.
3. Effect of temperature on the hydration Portland cement
blended with 50 wt% of siliceous fly ash
Deschner F., Winnefeld F., Lothenbach B., Neubauer J..
Cement and Concrete Research 52, pp. 169-181.
4. Quantification of fly ash in hydrated, blended Portland cement pastes
by back-scattered electron imaging
Deschner F., Münch B., Winnefeld F., Lothenbach B.
Journal of Microscopy 251, pp. 188-204.
VIIContent
Abstract............................................................................................................................. III
Zusammenfassung............................................................................................................V
List of papers................................................................................................................... VII
Glossary of notations and terms .................................................................................... IX
1 Introduction............................................................................................................. 1
1.1 Portland cement ................................................................................................... 1
1.2 CO2 emissions related to cement production........................................................ 1
1.3 Measures to mitigate CO2 emissions related to cement production ..................... 2
2 Objective ................................................................................................................. 4
3 Summary of used methods.................................................................................... 5
4 Background............................................................................................................. 7
4.1 Ordinary Portland cement (OPC).......................................................................... 7
4.2 Portland cement blended with siliceous fly ash .................................................... 8
5 Main achievements............................................................................................... 11
6 References ............................................................................................................ 16
VIIIGlossary of notations and terms
Oxides
C - CaO S - SiO2 A - Al2O3 F - Fe2O3
$ - SO3
Cement minerals and hydration products
Alite C3S Ca3SiO 5
Belite C2S Ca2SiO4
Aluminate C3A Ca3Al2O6
Brownmillerite C4AF Ca4Al2Fe2O10
Gypsum C$H2 CaSO4 * 2H2O
Ettringite C3A$3H32 Ca6Al2(SO4)3(OH)12 * 26H2O
Monosulphate C4A$H12 Ca4Al2SO4(OH)12 * 6H2O
Methods
BSE Backscattered electron
DTG Differential thermogravimetry
GEMS Gibbs energy minimisation software
ICP-OES Inductively coupled plasma optical emission spectroscopy
SEM Scanning electron microscopy
TGA Thermogravimetric analyses
XRD X-ray diffraction
Materials
OPC / CEM I Ordinary Portland cement
SCM Supplementary cementitious materials
IXOther notations
UNFCCC United Nations – Framework Convention on Climate Change
wt% weight percent
X1 Introduction
1.1 Portland cement
Cementitious materials and concrete are used since thousands of years by mankind. The
oldest archaeological finds of concrete in the broadest sense (mix of sand, rock fragments
and cementitious binder) in human history date back to about 5,600 B.C., used in the
floors of huts in Serbia [1]. The romans used mortar and concrete based on lime and poz-
zolana, which was the basis of the strength and durability of Roman architecture of which
some buildings, such as the Panthenon, still stand today [2]. Portland cement was first pa-
tented by Joseph Aspdin in 1824 [3] and a modern version of this cement is nowadays
used in numerous applications ranging from various kinds of mortar and concrete over
plaster and screed to special products like tile adhesives.
Today, Portland cement based concrete is the most used, solid material on earth. In the
year 2011 estimated 3.4 Gt of cement were produced world-wide [4] and the demand for
cement is predicted to grow in the medium-term future [5]. Obviously, an industry branch
of this size is also involved in the present discussions about global warming caused by an-
thropogenic greenhouse gas emissions. Scientific studies show that the global warming
since 1900 is at least partially due to anthropogenic components like the emission of
greenhouse gases [6]. As a consequence, the reduction of the greenhouse gas emissions
was decided by the United Nations – Framework Convention on Climate Change (UN-
FCCC) conference of the parties in Cancun in 2010 in order to restrict the increase of the
global average temperature below 2 °C above pre-industrial levels [7]. Scenarios that meet
this 2 °C limit have global emissions in 2050 of a level, which is roughly 40% below the
emissions in 1990 and roughly 60% below the emissions in 2010 [8].
1.2 CO2 emissions related to cement production
The direct and indirect CO2 emissions related to the production of cement account for
about 8 % of the global anthropogenic CO2 emissions [9, 10]. For the production of 1 t of
Portland cement clinker about 0.7-1.0 t of CO2 are emitted, depending on technical pa-
rameters of the production process and the types of fuels used [11-13]. Portland cement
clinker is made of limestone, clay or rocks of similar composition, which are ground and
heated up to about 1450 °C. A typical Portland cement clinker has a CaO content of 64-
67 wt%. Since the source of CaO is usually carbonatic, 510 kg CO 2 are emitted per t of
Portland cement clinker by the decalcification of CaCO3 (Eq. 1).
1161 kg CaCO3 650 kg CaO + 510 kg CO2 (Eq. 1)
Other CO2 emissions are mainly related to the combustion of fuels. Indirect CO2 emissions
originate from quarrying, transport processes and the use of electricity in general.
11.3 Measures to mitigate CO2 emissions related to cement production
The most obvious approaches to reduce the CO 2 emissions are to optimise the energy ef-
ficiency of the production process, e.g. by installing a pre-calciner or the usage of alterna-
tive fuels, like refuse-derived fuel or sewage slag instead of coal powder or oil. Although
these measures constitute an improvement of the energy efficiency in many older cement
plants around the world, a modern cement plant with the best available Portland cement
manufacturing technology using alternative fuels, leaves not much potential to improve the
energy efficiency [10].
Another way to reduce the CO2 emissions of the cement production is the replacement of
the Portland cement clinker by supplementary cementitious materials (SCM). Typical SCM
are extracted from natural resources, like limestone powder, calcined clays and natural
pozzolans, or from industrial by-products, like blast furnace slag, silica fume or fly ash.
The use of SCM within cement produced in Europe is regulated by the European stand-
ards EN 197-1 (Fig. 1). The ordinary Portland cement (OPC or CEM I) may contain up to
5 wt% of minor constituents. The other cement types are blended with various amounts of
SCM. With a market share of about 55 % in the year 2010 in Europe [14], the CEM II, also
called Portland composite cement, is the most sold cement in Europe. The CEM II family is
subdivided in CEM II A and CEM II B, which contain 6-20 wt% and 21-35 wt% of SCM, re-
spectively. CEM III are blastfurnace slag cements with a clinker replacement of up to
95 wt%. CEM IV is called pozzolanic cement with up to 65 wt% of clinker replacement.
The cement with the lowest market share in Europe is the CEM V or composite cement
with up to 80 wt% of clinker replacement.
2CEM I
CEM II/A
CEM II/B
CEM III/A
CEM III/B
CEM III/C
CEM IV/A
CEM IV/B
CEM V/A
CEMV/B
0% 20% 40% 60% 80% 100%
Minimum content of Portland cement clinker
Maximal content of fly ash, blast furnace slag, silica fume, natural
pozzolana, limestone, burnt shale
Maximal content of fly ash, natural pozzolana, silica fume
Maximal content of granulated blast furnace slag
Fig. 1. Composition and minimum Portland cement clinker content of cements specified
according to the European standard EN 197-1.
Besides the ecological aspects, blended cements are used due to the beneficial effects of
certain SCM on the durability of concrete. Especially fly ash, blastfurnace slag and silica
fume improve the resistance to sulphate attack and alkali-silica reaction and reduce the
permeability and chloride diffusion of concrete [15-19].
32 Objective
The use of SCM as clinker replacement in Portland cement is always also a question of
their regional availability. In Germany, the most abundant SCM is siliceous fly ash (Type V,
EN 197-1), which is a by-product of the coal combustion process. Especially since the de-
cision of the German Federal Government in 2000 to allow no new constructions of nucle-
ar power plants and to withdraw slowly from nuclear energy production, several new con-
structions of coal-fired power plants were planned. Therefore, the availability of siliceous
fly ash and the potential of its use as clinker replacement was from then on about to be fur-
ther increased. With this background and the goal of the cement industry to reduce the
CO2 emissions of the cement production, the demand for cements blended with increased
amounts of siliceous fly ash increased.
The drawback of the replacement of large amounts of Portland cement clinker by siliceous
fly ash is the reduced early age strength [20, 21], which is related to the late onset of the
fly ash reaction [22]. Many studies carried out in order to enhance the reactivity of fly ash
showed that the effect of the physical or chemical activation of fly ash is usually too low to
compensate for the decreased early age strength and often also too cost- or energy-
intensive, especially when using high replacement levels [23-27].
In order to find new suitable activators, first of all, the details of the reaction of fly ash and
its hydration mechanism in blended Portland cement need to be fully understood.
Therefore, the primary goal of this study is to investigate the effect of the fly ash reaction in
blended Portland cement on the development and composition of hydrate phases as well
as the pore solution. To achieve this, the focus of the investigations is a system consisting
of a CEM I 42,5 R blended with 50 wt% of siliceous fly ash. The high replacement level of
Portland cement was chosen intentionally to maximise the effect of the fly ash reaction. A
multi-method approach to analyse the assemblage and composition of hydrate phases, the
chemistry of the pore solution and the reaction degree of fly ash in the fly ash blended
Portland cement is chosen. The four principal scientific topics of this study are the investi-
gation of the:
Pozzolanic reactivity of fly ash and quartz powder
Effect of fly ash on the hydration of fly ash blended Portland cement at 23 °C
Effect of temperature on the hydration of fly ash blended Portland cement
Quantification of the reaction degree of fly ash during the hydration of blended Port-
land cement by means of image analysis
43 Summary of used methods
Compressive strength
Mortar prisms (40x40x160 mm3) were prepared and tested for certain hydration times ac-
cording to EN 196-1.
Thermogravimetric analyses (TGA)
The hydrated samples were ground, immersed in isopropanol for 15 minutes, washed with
diethylether, and filtered. Afterwards, the samples were dried for 10 minutes at 40 °C to
evaporate the remaining diethylether and analysed in a Mettler Toledo TGA/SDTA 851e
device at a heating rate of 20 K/min in N2-atmosphere. The analysed bound water of all
hydrate phases and the crystal water within Ca(OH)2 were determined by the weight loss
in the temperature intervals 50-500 °C and 400-470 °C. The exact boundaries for the tem-
perature interval of portlandite were read from the derivative curve (DTG). The results are
expressed as percentage of the dry sample weight at 500 °C [28]. Triple preparation and
measurement of the samples after 1, 2 and 7 days of hydration at 23 °C showed an abso-
lute error of up to 2 wt% for the bound water, and up to 0.6 wt% for the water loss related
to the decomposition of Ca(OH)2 . This includes the errors caused by preparation, meas-
urement and sample inhomogeneity.
X-ray diffraction (XRD) measurements
5 mm thick disks of the hardened paste samples with a diameter of 30 mm were sliced
from the cast samples, immersed in isopropanol for 2 days to stop the hydration and
stored under N2-atmosphere until measurement. Measurements were performed in a
PANalytical X’Pert Pro MPD diffractometer with attached X’Celerator detector.
Scanning electron microscopy (SEM)
The hydration was stopped by cutting the sample into slices of 5 mm thickness with a di-
ameter of 30 mm and keeping them for 3 days in isopropanol and drying them for 3 days
at 40 °C. Subsequently, the samples were impregnated with a modified bisphenol-A-
epoxy-resin and polished by polycrystalline diamond suspension at grades from 9 μm
down to 1/4 μm. Finally, the samples were carbon-coated and investigated under high
vacuum conditions in a Philips ESEM FEG XL 30. EDX measurements of the hydrate
phases were carried out at a beam voltage of 10 keV and 10 mm working distance.
Analysis of the pore solution
The pore solutions of the hardened samples were extracted by the steel die method [33]
using pressures up to 250 N/mm2. The solutions were filtered immediately with nylon filters
with a mesh size of 0.45 μm. The free OH- concentrations of the pore solutions were calcu-
5lated with the help of pH measurements with a pH electrode, calibrated against KOH solu-
tions with known concentrations. The K, Na, Ca, Al, Si and sulphur concentrations were
measured by means of inductively coupled plasma optical emission spectroscopy (ICP-
OES).
64 Background
4.1 Ordinary Portland cement (OPC)
Portland cement clinker is a product made of limestone and clay or rocks of similar com-
position. The milled raw materials usually pass through a rotary kiln, where they are heat-
ed up to 1450 °C. During this process, calcium silicate phases (alite and belite) and melts
are formed by the reaction of raw materials. Upon rapid cooling and due to the incorpora-
tion of other ions into the crystal structure, high temperature polymorphs of the calcium sil-
icates are maintained as metastable phases [29, 30]. Upon cooling, calcium aluminate,
calcium aluminate ferrate (brownmillerite) and other phases, such as alkali sulphates crys-
tallize from the melts [31, 32].
The content of the main clinker phases in the OPC used in this project is 57.1 wt% C3S,
17.2 wt% C2S, 4.0 wt% C3A and 13.0 wt% C4AF. The reaction of the calcium silicates is
described by Eq. 2a&b.
C3S + (3+m-n) H Cn-S-Hm + (3-n) CH (Eq. 2a)
C2S + (2+m-n) H Cn-S-Hm + (2-n) CH (Eq. 2b)
with n and m being typically in a range between 1-2 and 1.5-2.5.
Alite reacts quickly with water, leading to the formation of C-S-H and CH. This process is
responsible for the main strength development in OPC during the first 28 days. Belite re-
acts at a much slower rate and contributes to the later strength development [29].
The calcium aluminate phase also reacts very quickly with water. To regulate this reaction,
calcium sulphate is added as set regulator. In the presence of calcium sulphate, C3A re-
acts according to Eq. 3a, resulting in the formation of ettringite. After the depletion of the
calcium sulphate source, ettringite transforms to monosulphate in a reaction with the re-
maining C3A and water (Eq. 3b).
C3A + 3 C$H2 + 26 H C6A$3H32 (Eq. 3a)
C6A$3H32 + 2 C3A + 4 H 3 C4A$H12 (Eq. 3b)
The Al in monosulphate may be substituted by Fe and the SO3 by H2O. In the presence of
carbonate, analogous phases with CO2 instead of SO3 form. The family of these isostruc-
tural phases is called AFm (aluminium iron monophases).
The hydration of calcium aluminate ferrate (C4AF) in the presence of gypsum leads to the
formation of ettringite with Al partially substituted by Fe [33]. However, since Al-ettringite is
7more stable than Fe-ettringite [34], Fe-AFm and mainly Fe-rich siliceous hydrogarnet are
formed [33, 35, 36].
4.2 Portland cement blended with siliceous fly ash
Fly ash is a by-product of the coal combustion process. Mineral components of the raw,
ground coal (e.g. clay minerals, calcite and quartz) melt during the combustion process.
Droplets and fragments of this melt, stream upwards together with the combustion gas into
the chimney, get quenched and solidify. These particles are retained as fly ash in the elec-
trostatic filters. Fly ash is a silica and alumina rich material with variable chemical composi-
tion depending on the type of burnt coal. Depending on the Ca-content, distinction is made
between Type V (10 wt% CaO) fly ash, according to the Eu-
ropean Standard EN 197-1. Fly ash is a very heterogeneous material in terms of morphol-
ogy and chemistry. Although (partially hollow) spherical particles are most abundant, also
many crystalline and sponge-like or odd shaped particles can be found within fly ash (Fig.
2).
4 3
1
2
Fig. 2. BSE images of raw fly ash particles.
1: spherical particle, 2: crystalline particle, 3: spherical particle with crystalline iron oxide
lamellae, 4: sponge-like, odd shaped particle.
The variable chemistry of most of the glassy fly ash particles is characterised by mainly
SiO2 and Al2O3 (network formers) and minor amounts of network modifiers, such as CaO
or alkali oxides (Fig. 3). Additionally, special particles rich in CaO, MgO and P2O5 can be
found. The principal crystalline phases are mullite, quartz and iron oxides such as hematite
and magnetite. Siliceous fly ash also contains readily soluble alkali and calcium salts,
which precipitate from the combustion gas on the surfaces of the fly ash particles.
8SiO2
wt.-%
0.00
0 100
1.00
25
0.25
0.75
75
50
0.50
50
0.50
75
0.75
25
0.25
CaO+MgO 100
1.00
0
0.00 Al2O3+Fe2O3
wt.-%
0.00
0 0.25
25 0.50
50 0.75
75 1.00
100 wt.-%
Fig. 3. Chemical composition of randomly analysed particles of one siliceous fly ash used
in this study.
Due to its heterogeneous particle size distribution and the mainly spherical particle shape,
fly ash is a suitable mineral additive to improve the workability of concrete [37, 38]. Its use
in concrete has also many other beneficial effects like the improvement of the ultimate
strength and increase of the durability [15-19, 37].
The effect of the sole presence of fly ash on the hydration of OPC, just like any other inert
filler material, is referred to as filler effect [28, 39-45]. One aspect of the filler effect is that,
by adding a filler and keeping the water-to-solid ratio constant, the effective water-to-
cement ratio is increased and more space for the growth of hydrate phases is available.
Another aspect is, that the particle surfaces of the filler material act as sites for heteroge-
neous nucleation of hydrate phases. These two aspects of the filler effect lead to an in-
creased reaction rate of the OPC.
Besides its good properties as a filler material, siliceous fly ash (Type V, EN 197-1) is a la-
tent hydraulic material, which hydrates in the alkaline pore solution of Portland cement
[46]. The pH of the pore solution is governed by the dissolved ionic species in solution. Ini-
tially, Ca, alkali and sulphate ions dominate the pore solution and a pH of 12.6-13.0 is
reached. After the depletion of the calcium sulphates, the pH rises to about 13.5. In these
alkaline conditions, the aluminosilicate glass network of the fly ash is attacked by OH- and
the hydroxylated silicate reacts with Ca(OH)2, originating from the hydration of the OPC, to
form C-S-H.
9From the dissolution of the fly ash, also aluminate is released and partially incorporated in
the C-S-H or contributing to the formation of AFm phases or siliceous hydrogarnet [47].
The dissolution rate of the fly ash glass is dependent on the pH [41, 48]. The onset of the
pozzolanic reaction, measured by the consumption of Ca(OH) 2, is reported to start after 7
days [41, 43, 49] and in individual cases after 3 days [50] or after 28 days of hydration [51].
105 Main achievements
The study is focussed on the investigation of the effect of siliceous fly ash on the hydration
of blended cement pastes. The study is subdivided in four parts. In this section the main
achievements of each part are reported and discussed.
1. Investigation of a model system to characterise the pozzolanic reactivity of two low Ca
fly ashes and a quartz powder
Prior to the investigation of the effect of fly ash in blended Portland cement, the pozzolanic
reactivity of the two used fly ashes and the reference quartz powder need to be investigat-
ed. To achieve this, model cement pastes consisting of 100 g of fly ash or quartz powder,
70 g of portlandite, 10 g of calcite and 180 g of an aqueous solution with 0.3 mol per litre
KOH are used. The KOH solution is chosen to mimic the pH of a matured cement pore so-
lution. The portlandite and the calcite serve as reactants for the silicate and aluminate
components of the fly ash glass.
The hydration of these mixes is stopped after 2, 7, 36 and 90 days of hydration. The char-
acterisation of the hydration products by TGA, XRD and SEM show the formation of C-S-H
and AFm phases, mainly monocarbonate. The consumption of Ca(OH)2, measured by
means of TGA, is used as an indicator for the extent of the pozzolanic reaction. The two fly
ashes show a similar pozzolanic reactivity, only the fine, air separated fraction shows a
significantly higher reactivity. The quartz powder shows no significant pozzolanic reactivity
and proves therefore to be a suitable, inert reference material for other studies.
2. Hydration of Portland cement with high replacement by siliceous fly ash
In this part of the study, the reaction of fly ash in blended Portland cement paste and its ef-
fect on the chemistry of the solid hydrate phases, the pore solution and the microstructure
are investigated. Two different fly ashes are used as SCM replacing 50 wt% of Portland
cement. To distinguish between the pozzolanic reaction and the physical filler effect of the
fly ash, a sample with 50 wt% of practically inert quartz powder is used as reference. The
process of fly ash hydration within blended Portland cement is shown schematically in Fig.
4. Although the pH of the pore solution rises within 1 hour to 13, which is sufficiently high
to enable the dissolution of the fly ash glass, no measurable evidence of the fly ash reac-
tion is found before the first 2 days of hydration. After 8 hours, hydration products on the
surface of the fly ash are formed, which are supposed to be mainly originating from the
hydration of the OPC. Between 10 and 24 hours, the calcium sulphate phases deplete.
Due to the ongoing precipitation of ettringite, the sulphate concentration in the pore solu-
tion drops, and hence the pH increases. The rise of the pH is supposed to promote the
dissolution of fly ash. After 7 days, evidence of the fly ash reaction is found by a decrease
of the portlandite content and the change of the pore solution compared to the reference
sample. The analysis of the pore solution shows a decrease of the Ca concentrations due
to the consumption of Ca(OH)2 and an increase of the Al and Si concentrations due to the
dissolution of the fly ash. The C-S-H composition changes analogous to the pore solution
11composition. SEM-EDX analyses of the matrix of cement hydrates reveal lower Ca/Si and
higher Al/Si atomic ratios in the C-S-H as a consequence of the pozzolanic reaction.
After 7 days of hydration, the formation of an inner hydration product of the fly ash is ob-
served. The hydrate phases in this area are characterised by a low density and a high con-
tent of water. The formation of these hydrate phases is related to the low availability of Ca
within the inner hydration product. Depending on the chemistry of the specific fly ash parti-
cles, in some cases hydrotalcite or siliceous hydrogarnet can be found within the inner hy-
dration product. Liesegang rings within the inner hydration product observed after 550
days of hydration demonstrate the gel-like consistency of the hydrate phases in this area.
After long hydration times, the pH decreases slightly due to the binding of alkali within the
C-S-H. Additionally, the amount of pore solution is decreased and the continuous for-
mation of hydrate phases leads to a densification of the matrix. Therefore, the continuous
hydration of fly ash is slowed down.
12a) < 8 h Al(OH)4- b) 8-24 h Al(OH)4-
SO42-
OH- OH-
Ca2+
Al Al
Si(OH)62- Si(OH)62-
Si FA Si FA
Fe Fe
OH-
OH-
Fe(OH)4- Fe(OH)4-
c) 1-7 d d) > 7 d
Al(OH)4 - Al(OH)4-
SO4 2-
SO42-
Al(OH)4-
OH- OH- Ca2+
Ca2+ Ca2+
Al Al
Si(OH)62- Si(OH)62- Si(OH)62-
Si FA Si FA
Fe Fe
Ca2+
OH- OH- Fe(OH)4 -
Fe(OH)4-
C-A-S-H Inner hydration
AFt, AFm Mullite
(Al/Si ~ 0.2) product
(Fe-) Si-
(Fe-) hydrotalcite C-A-S-H
hydrogarnet
Fig. 4. Schematic process of the hydration of a typical aluminosilicate glass fly ash particle
within blended Portland cement.
133. Effect of temperature on the hydration of Portland cement blended with siliceous fly ash.
With a similar setup as in part 2 of the thesis, the effect of temperature on the hydration of
fly ash blended Portland cement compared to the reference sample containing quartz
powder is investigated. Besides the known effect of temperature on the hydration of OPC,
a strong impact of temperature on the hydration kinetics of fly ash is found. All effects on
the chemistry of hydrate phases and pore solution related to the pozzolanic reaction of fly
ash observed in part 2 of the study are shifted to earlier hydration times at elevated tem-
peratures. At 50 °C and higher temperatures the pozzolanic reaction of fly ash is found to
start before 1 day of hydration. At 7 °C, no evidence of the pozzolanic reaction is found be-
fore 90 days of hydration.
At temperatures above 50 °C, ettringite is destabilised due to its increased solubility and
the released Al and sulphate is found to be partially incorporated in C-S-H and further con-
tributing to the formation of considerable amounts of siliceous hydrogarnet. This effect is
pronounced in systems blended with siliceous fly ash due to the reaction of the aluminate
fraction of fly ash.
In accordance with the experimental results, the change of the hydrate phase assemblage
as a function of temperature is modelled by thermodynamic calculations using GEMS [52]
together with the thermodynamic data from the PSI-GEMS database [53] expanded with
additional data for solids that are expected to form under cementitious conditions [33, 35,
36, 54].
The effect of temperature on the microstructure of fly ash blended Portland cement is
found to be similar as in pure OPC. At elevated temperatures, the heterogeneity and
coarse porosity of the microstructure is increased. The C-S-H phase appears brighter in
the BSE image as a consequence of its lower water content and higher density at elevated
temperatures.
4. Quantification of fly ash in hydrated, blended Portland cement pastes by backscattered
electron imaging.
To investigate the effect of parameters like the addition of activators on the reaction kinet-
ics of fly ash in blended Portland cement, the aim of this part of the study is to develop a
method quantifying the content of anhydrous fly ash from the segmentation of BSE imag-
es. A new image processing routine is developed, which shows how characteristic features
of fly ash in the microstructure of hydrated cement pastes can be used to segment anhy-
drous fly ash particles from BSE images. For the image processing, a combination of grey
level segmentation, grey level filtering and morphological filtering is used. The analysis of
600 images per sample at a magnification of 2000 yields the content of anhydrous fly ash,
anhydrous clinker, porosity and portlandite. The analysis of the presented dataset reveals
a standard deviation of the reaction degree of fly ash of up to 4.3 %. However, the accura-
cy of the results is not found to be better than the determination of the fly ash reaction de-
gree by conventional techniques, like selective dissolution [55-57].
The limitations of the accuracy of the method are due to the morphological and composi-
14tional heterogeneities of fly ash and similarities between fly ash and hydrate phases.
Although the method proves to be promising, the accuracy of the method could be proba-
bly improved by the implementation of element mappings and clustering analyses.
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19Paper I Investigation of a model system to characterize the pozzolanic reactivity of two low Ca fly ashes and a quartz powder F. Deschner, B. Lothenbach, F. Winnefeld, P. Schwesig, S. Seufert, S. Dittrich, J. Neubauer Tagunsband der Tagung Bauchemie der GDCh, Dortmund 2010, p.127-132. ISSN/ISBN 978-3-936028-69-0
Investigation of a model system to characterize the pozzolanic reactivity of two low Ca fly ashes and a quartz powder F. Deschner a, B. Lothenbach a, F. Winnefeld a, P. Schwesig b, S. Seufert b, S. Dittrich c, J. Neubauer c a Empa, Swiss Federal Laboratories for Materials Science and Technology, Dübendorf/CH b BASF Polymer Research, Polymers for Inorganics, BASF Construction Chemicals GmbH, Trostberg c GeoZentrum Nordbayern, Mineralogy, University of Erlangen- Nuremberg Introduction Fly ash (FA) is increasingly used as clinker replacement in blended Portland cements in order to reduce the CO2 emissions /1/. The reactivity of the FA changes depending on its chemical and mineralogical composition and the pre-treatment, e. g. grinding or leaching of the raw FA /2/. A test method was developed to evaluate and compare the pozzolanic reactivity and the hydration products of different SCM, at conditions similar to the hydration of an OPC. Materials and Methods To illustrate the effectiveness of the reactivity test a low Ca FA as delivered (F1-u) and a fine air-separated FA (F1-f), both from the same batch of one coal power plant, were used. Another untreated FA (F2-u) from a different coal power plant, characterized by a higher Ca-content and the presence of lime, periclase and anhydrite, was used to be compared with F1. Finally an untreated and a screened Qz powder (Qz-u & Qz-f) of similar fineness as the two respective FA were used as reference materials. The mineralogical and chemical composition of the raw materials, determined by means of X-ray flourescence and quantitative X-ray diffraction (according to /3/), is shown in table 1. For the experiments 100 g of FA or Qz, respectively, were homogenized together with 70 g of portlandite and 10 g of calcite. The materials were subsequently mixed with 180 g of 0.3 m KOH solution in a vacuum mixer for 2 min. The portlandite and the calcite serve as reactants for the FA to form calcium silicate hydrates (C-S-H) and carbonate aluminium iron monophases (AFm). According to thermodynamic calculations /4/ the used amount of portlandite was enough to react 62 wt% of F1-u, before it is totally consumed. The
0.3 m KOH solution imitated the pH of a typical OPC pore solution
after the consumption of the calcium sulphates (pH 13.4). The pastes
were finally sealed in polyethylene bottles and stored at 23 °C.
After 2 d, 7 d, 36 d and 90 d of hydration, the samples were analyzed
by means of thermogravimetric analysis (TGA), X-ray diffraction (XRD)
and scanning electron microscopy (SEM). To stop the hydration, the
samples were ground, immersed in isopropanol for 15 minutes,
washed with diethylether and filtrated. The residue of the filtration was
dried for 7-8 minutes at 40 °C to evaporate the remaining diethylether
to be instantly measured in a Mettler Toledo TGA device at a heating
rate of 20 K/min. The XRD investigations were carried out in a
PANalytical X’Pert Pro MPD diffractometer with attached X’Celerator
detector. For the SEM investigations, pieces of the hardened samples
were kept for 2 days in isopropanol and afterwards dried at 40 °C for
several days. After impregnating the samples by a modified bisphenol-
A-epoxy-resin, they were polished, carbon-coated and investigated
under high vacuum conditions in a Philips ESEM FEG XL 30.
Table 1. Chemical and mineralogical composition of the investigated
materials.
chemical composition [wt%] mineralogical composition [wt%]
F1-u F1-f F2-u Qz-u Qz-f F1-u F1-f F2-u
SiO2 50.9 47.2 45.0 99.7 98.3 Mullite 8.2 6.8 19.5
Al2O3 24.7 29.7 26.5 1.1 1.6 Quartz 7.0 1.8 7.6
Fe2O3 7.3 6.5 8.5 0.5 0.5 Hematite 0.7 0.1 1.2
CaO 3.7 2.6 5.6 0.0 0.1 Magnetite 0.8samples were monocarbonate, hemicarbonate and minor amounts of
ettringite, which may arise from small quantities of SO3 in the FA. Due
to its X-ray amorphous properties no C-S-H could be detected. The
qualitative assemblage of hydrate phases did not change between 2 d
and 90 d. The reactivity tests of the samples containing quartz powder
did not show the development of any crystalline hydration products.
a) 7 d b) 36 d
Mc CH Mc
Hc Hc CH
F1-f
F1-u
E E F2-u
8 9 10 11 12 13 14 15 16 17 18 19 8 9 10 11 12 13 14 15 16 17 18 19
°2 Theta Cu K-alpha °2 Theta Cu K-alpha
Figure 1. Details of the XRD diffractograms of the reactivity tests of the FA
samples after (a) 7 d and (b) 36 d of hydration. E – ettringite, Hc –
hemicarbonate, Mc – monocarbonate, CH – portlandite.
100 0.12
90 0.08
80 0.04
weight loss [wt%]
C-S-H CH Cc
DTG [1/°C]
70 0.00
60 -0.04
50 F1-u -0.08
F1-f
40 AFm Qz-u -0.12
(Mc) Qz-f
F2-u
30 -0.16
100 200 300 400 500 600 700 800 900
temperature [°C]
Figure 2. Relative weight loss (upper curves) and DTG (lower curves) of the
reactivity tests of the investigated samples after 36 d of hydration. Mc –
monocarbonate, CH – portlandite, Cc – calcium carbonate.You can also read
