Influence of selected activating methods on hydration processes of mixtures containing high and very high amount of fly ash
Journal of Thermal Analysis and Calorimetry
Influence of selected activating methods on hydration processes of mixtures containing high and very high amount of fly ash
Iwona Wilin´ ska 0 1
Barbara Pacewska 0 1
Activation 0 1
Hydration 0 1
0 Faculty of Civil Engineering, Mechanics and Petrochemistry, Institute of Chemistry, Warsaw University of Technology , 17 Łukasiewicza St., 09-400 Plock , Poland
1 & Barbara Pacewska
Binders containing fly ash as cement substitute have been used for many years. Typical amount of fly ash in cement composite does not exceed 35% of mass of binder. Replacement of substantial amount of cement mass by fly ash results in retardation of hydration of binder and deterioration of some concrete properties, e.g. delay in setting time and reduction in early compressive strength. These inconveniences may be reduced using proper method of activation. In this paper, possibilities to activate binding mixtures containing high and very high amount of fly ash and low amount of Portland cement were discussed basing on the literature reports as well as on our own research results. Several activating methods were described, such as mechanical activation, exchanging of part of components of binder (cement, fly ash or both of them) by more reactive fine-grained material, chemical activation by the use of alkaline activators as well as combined methods. Possibilities of using these methods were discussed in relation to amount of fly ash introduced as replacement of cement and kind of fly ash used. Usefulness of calorimetry and thermal analysis methods was also presented.
Portland cement is a common binder used in construction
materials. However, its production (especially
clinkerisation process) generates several ecological and economic
problems, such as consumption of great amount of energy
and natural resources as well as emission of CO2. Different
activities are undertaken to minimise these inconveniences,
especially concerning reduction in emission of CO2 [
One of them is utilisation of the supplementary
cementitious materials (e.g. fly ash) in manufacture of cement and
concrete. Cements containing different clinker substitutes,
e.g. fly ash or slag, are currently often used. For example,
cement CEM II may contain from 6 to 20 or from 21 to
35% of fly ash according to standard PN-EN 197-1:2012
]. Fly ash of adequate properties (acc. PN-EN 450-1:2012
]) may be also used as type II additive for concrete. In
this way, using supplementary cementitious materials some
reduction in problems related to cement production is
possible to obtain. Moreover, using for this purpose
byproducts arising in industry (e.g. fly ash from hard coal
combustion produced in energy industry) gives additional
ecological advantages. Such actions are part of a
sustainable development policy.
Theoretically, the higher the level of substitution of
Portland cement by other aluminosilicate material, the
greater the ecological benefits should be expected.
However, replacement of substantial amount of cement in
binder mass by fly ash gives significant deterioration of
concrete properties and limitation of possibilities of
practical use of such material. Thus, fundamental question
appears: what is the amount of fly ash that can be used in
this way? The simplest answer is: it depends on role of fly
ash in binding mixture.
Several groups of binding mixtures depending on the
amount of fly ash can be found in the literature as it is
presented in Fig. 1. Portland cement as typical, classical
binder composed without fly ash is at the one of extreme
point in the diagram. From the opposite side, there is alkali
activated binder composed of 100% of fly ash [
Between them one can specify:
• fly ash cements containing not higher than 35% of fly
ash (it is typical amount of fly ash),
• high volume fly ash binders containing about 50% and
higher of fly ash as cement replacement [
• very high volume fly ash mixtures containing above
70% of fly ash [
Cement hydration and pozzolanic reaction of fly ash
(reaction of active silica and alumina with Ca(OH)2) are
well known [
]. Such processes take place in mixture
of cement and fly ash in the presence of water.
Development of early compressive strength of composites
Fig. 2 Typical ways of
activation of fly ash–cement
Hydration in highly
alkaline activation of fly ash
alkaline activation of fly ash,
Hydration in water
containing pozzolanic replacements of cement depends on
amount of supplementary cementitious material used, its
] and conditions of hardening. Replacement of
substantial amount of cement mass by fly ash gives
important inconveniences, such as retardation of the binder
hydration and, as a result, deterioration of some concrete
properties, e.g. delay in setting time and reduction in early
strength. In case of high volume fly ash concrete, these
inconveniences may be reduced by significant reduction in
water to cement ratio with simultaneous use of
superplasticisers (e.g. in works [
12, 24, 25
]) or by optimal amounts
of components and proper curing [
]. Better results of
early strength in the case of the presence of
superplasticisers may be explained by better dispersion of grains of
binders making their surface more available for water .
However, despite improved dispersion, some water
reducing admixtures may cause significant delay of initial
reactions of hydration [
]. For mixtures containing very
high amount of fly ash, extremally low amount of cement
causes that hydration in water may be insufficient. Such
materials need special activators for hardening.
There are few methods of activation of fly ash–cement
binders resulting in improvement in their properties
especially in early periods of hardening: mechanical activation,
chemical activation, thermal treatment and combined
methods joining different methods of activation (Fig. 2).
Each of these methods influences physicochemical
processes occurring in the binders in the presence of water.
Several mechanisms of improvement in properties of such
binding mixtures may be considered: acceleration of
cement hydration and/or pozzolanic reaction, improvement
in reactivity of supplementary cementitious material by
enhancement of dissolution of its aluminosilicate grains,
arising additional products in the presence of chemical
Aluminosilicate supplementary cementitious materials
containing low amount of calcium components (e.g.
conventional pulverised fly ash from hard coal combustion) do
Possibilities of activation of fly ash-cement mixtures
Acceleration of cement
hydration and pozzolanic
Activation by modification of alumino
silicate composition of fly ash-cement
mixture (introduction of more reactive
enhancing pH of
Faster development of early
properties of composite
Acceleration of dissolution of
not exhibit self-cementing properties and therefore do not
harden in water. They undergo reaction with Ca(OH)2
(pozzolanic reaction), but they can also harden under other
alkaline solutions of high pH (e.g. NaOH solution). That is
why alkaline components are necessary most of all in case
of non-clinker binders (geopolymers) composed from
aluminosilicate material, such as fly ash, without cement.
Elevated temperature of hardening often is needed in case
of such mixtures. Aluminate and silicate species dissolve
from fly ash grains in highly alkaline conditions. Then,
they undergo complex processes of polymerisation and
polycondensation. Consequently, solid product
precipitates. Depending on the type of aluminosilicate source
material as well as conditions of reaction (such as
concentration of alkali and temperature), this product may be
zeolitic or amorphous [
4–6, 8, 9
]. There are also research
reports concerning utilisation of fly ash from fluidised
combustion of coal as activator for slag component in
nonclinker mixture [
High volume fly ash composites can be considered as
sustainable ecological products—‘‘green’’ concretes. There
are possibilities of applications for such materials despite
the deterioration of some properties compared to cement
concrete. The examples may be: controlled low-strength
] or massive foundation slab [
Concrete containing high amount of fly ash as replacement of
cement can develop its properties with time. Proper design
and realisation of high volume fly ash concrete results in
high-performance material. Such composites containing
about 50% of fly ash of good quality in binder, properly
selected type and amount of cement, low amount of water
and superplasticising admixture and subjected to properly
long moist-curing can achieve high ultimate strength and
]. Binder containing very high amount of
cement replacement (about 65% of mixture of fly ash and
slag) and alkaline activator was produced and investigated
as pre-industrially manufactured hybrid cement [
The necessity to discover new, ecologically friendly
binders which could be an alternative to Portland cement
necessitates that research works recognising mechanism of
hardening of activated mixtures are still needed. Such
studies are important from scientific as well as practical
points of view, as they provide a basis for predicting the
long-term properties of hardened composite. It is possible
to obtain material of good properties during alkaline
activation of aluminosilicates. However, alkali activated
nonclinker binders (geopolymers) need high alkaline solutions
and often elevated temperature for hardening. It causes
some ecological problems and raise in price of the final
product. Some problems with safety of workers relating to
using high alkaline solutions also appear [
recently, special emphasis is put to develop activated
binders which can harden in more mild conditions.
The aim of this review is to discuss (basing on the
literature reports as well as on our own research results and
experience) ways of activation of binding mixtures
containing high and very high amount of fly ash. Some
traditional and new trends in the activation of binders basing on
fly ash were described. Effectiveness and mechanisms of
the selected activating processes depending on the amount
and type of fly ash were discussed. Special emphasis was
put on indication usefulness of calorimetry and thermal
analysis for such studies. These methods are frequently
used in case of studies of hydration processes of cement in
the presence of different additives (e.g. [
]). They are
also useful in case of investigations of hydration/activation
processes of high and very high volume fly ash mixtures.
Early hydration of non-activated mixtures containing high and very high amount of fly ash
Fly ash is a fine-grained by-product of power industry. Its
properties depend on several factors such as: type of
combusted solid fuel (e.g. fly ash from combustion of hard
coal, brown coal, rice husk ash), technology of combustion
(e.g. pulverised combustion, fluidised combustion),
technology of off-gasses desulfurisation. Fly ash reactivity in
cement or in fly ash–lime pastes is low at room temperature
and strictly depends on its properties, amount and
conditions of hardening [
21, 38, 43–45
]. This by-product can
react with Ca(OH)2 (pozzolanic reaction), which is formed
during hydration of cement or it may be introduced as an
additional component of mixture. Fly ash containing
sufficient amount of calcium and/or sulphate components
exhibits some self-cementing properties in the presence of
]. Low-calcium fly ash cannot harden in the
presence of water.
Different ways of fly ash–cement interactions may be
considered contributing to the development of properties of
final composite. In general, fine grains of fly ash may act as
nucleation centres for the precipitated products of
hydration, thus fly ash may accelerate hydration of cement.
Pozzolanic reaction between Ca(OH)2 and active silica and
alumina from fly ash contributes to the formation of
additional amount of hydrated calcium silicates (the
socalled C–S–H phase) and calcium aluminates. Non-reacted
grains may act as microfillers.
In general, in mixtures containing high amount of fly ash
and low amount of cement, these two components
influence each other the reactivity. Fine grains of fly ash may
accelerate hydration of cement, while the presence of
cement activates reactivity of fly ash by progressive
increase in the pH, delivery of Ca(OH)2 for pozzolanic
reaction as well as improvement in the dissolution of grains
25% fly ash
50% fly ash
75% fly ash
Fig. 3 Exemplary curves of heat release rate (a) and total heat
released (b) of low-calcium fly ash–Portland cement mixtures during
48 h of hydration with water (fly ash from pulverised combustion of
hard coal, water/binder = 0.5, initial temperature 25 C, BMR
calorimeter) (data for 50% replacement according to [
binder = cement ? fly ash)
of fly ash by increase in temperature of the mixture
(exothermal process of cement hydration) [
in case of high amount of fly ash in fly ash–cement
mixtures dilution effect predominates during the early stages of
hardening. Firstly, fly ash needs adequate pH of solution
and Ca(OH)2 for exhibition of its pozzolanic properties.
Due to this fact, the first step must be hydration of cement
and production of Ca(OH)2. Thus, early hydration periods
depend on cement which is in the mixture in low amount.
Secondly, fly ash needs some time for pozzolanic reaction.
The reaction is slow in ambient temperature, because of
low solubility of aluminosilicate grains at pH provided by
hydration of cement. Thus, hydration processes and
formation of solid products go significantly slowly in fly ash–
cement mixtures compared to hydration of Portland cement
21, 49, 50
]. Exemplary, it is clearly seen in Fig. 3, which
presents calorimetric results for hydration of mixtures
containing different amount of conventional low-calcium
fly ash from combustion of hard coal.1 In general, shapes of
1 Commercially available Portland cement CEM I 32.5R was used in
all measurements results of which were presented in Figs. 3, 4, 5, 6, 8
and 9, average chemical composition (the main components) of
cement [mass%]: SiO2 20.1, Al2O3 4.4, Fe2O3 2.3, CaO 62.6, SO3 3.2,
Na2O ? K2O 1.2, MgO 2.7, average chemical composition (the main
components) of fly ash [mass%]: SiO2 50, Al2O3 20, Fe2O3 7, CaO 5,
25% fly ash
50% fly ash
75% fly ash
25% fly ash
50% fly ash
75% fly ash
calorimetric curves of the rate of heat release are similar to
those for Portland cement presented, e.g. in [
are several characteristic periods on these curves: quick
release of heat after addition of water (wetting of grains,
dissolution of some components), then induction period
(heat release is inhibited) followed by re-growth of heat
release rate (precipitation of products of hydration) and,
after reaching maximum, fall of rate of heat release (the
processes undergo in solid state). It is visible that the
higher the replacement level is, the longer induction period,
the lower intensity of the effect related to precipitation of
products and the lower total heat released. It indicates that
time of start of setting is the longer and early compressive
strength is the lower together with increasing amount of fly
ash and simultaneously decreasing amount of cement. In
case of such mixtures, Portland cement is mainly
responsible for early hydration and heat emission despite some
Footnote 1 continued
SO3 1, Na2O ? K2O 3, MgO 3, crystalline components of fly ash:
mullite, quartz, anorthite, hematite [
acceleration action of fly ash grains regarding to cement
hydration . This conclusion is consistent with the
information given in [
38, 53, 54
] according to which the
pozzolanic reaction between fly ash and Ca(OH)2 starts
from about 3rd day of hydration or even after 7 days.
Similar results of calorimetric studies of early hydration of
mixtures containing high amount of low-calcium fly ash
were presented in several publications, e.g. [
is also confirmation about slow reaction of fly ash with
Ca(OH)2 and the fact that during early days, hydration of
cement dominates reaction of mixture containing high
amount of Class F fly ash and is mainly responsible for
setting, hardening and development of early compressive
strength . The relationship showing that the higher is
the content of fly ash introduced as Portland cement
replacement, the greater is reduction in compressive
strength repeatedly confirmed in studies by various authors,
]. Such inconveniences related to mixtures
containing high and very high amount of low-calcium
Class F fly ash are caused, inter alia, by inadequate amount
of Ca(OH)2 produced during hydration of small amount of
cement. Calculations made by Myadraboina et al. [
showed that 50% and higher of cement replacement by fly
ash results in insufficient amount of Ca(OH)2. Thus, one
can conclude that mixtures containing very high amount of
low-calcium fly ash need additional amount of calcium
component to develop fly ash activity fully. However, it
should also be emphasised in this place that lowering the
amount of released heat with the replacement of cement by
fly ash is favourable in case of massive concrete. Increasing
temperature inside the concrete slab as a result of
exothermal hydration processes may cause cracks and
reduction in durability. Therefore, binders containing high
amount of fly ash may be favourable in case of massive
concretes. Wang et al. [
] proposed to use high volume fly
ash (45%) concrete for a concrete foundation slab. They
found that the temperature rise in concrete decreased and
cracking risk of such massive concrete was reduced.
Moreover, they showed that proper design of concrete
mixture, taking into account water to binder ratio, the
amount of fly ash, amount of binder and type of cement
enabled the gain of the expected level of 60-day
Investigations of microstructure of pastes containing
high and very high amount of low-calcium fly ash after 6
months of hydration showed the presence of C–S–H phase
coexisting with hydrated gehlenite (C2ASH8,2 typical
product of reaction of pozzolanic material rich in aluminates)
and moreover small amount of Ca(OH)2 and small amount
of ettringite. C–S–H(I) formed in such systems exhibited
lower Ca/Si ratio as compared to C–S–H formed in pure
cement paste [
In case of fly ash containing higher amount of calcium
components (Class C fly ash), shape of calorimetric curve
is different as compared with results for low-calcium fly
ash–cement mixtures (Fig. 4). This type of fly ash exhibits
some self-cementing properties. It means that such kind of
fly ash to some extent is able to form hardened structure in
the presence of water without introducing additional
amount of Ca(OH)2; however, its activity and properties
after reaction with water are significantly lower than in
case of Portland cement. Chemical and phase composition
of calcium fly ash is usually more complicated compared to
compositions of siliceous fly ash. Among others, calcium
ingredients may be present in form of different chemical
compounds, such as CaO, CaCO3, CaSO4, gehlenite,
anorthite, etc. They exhibit different ability to react in the
presence of water; moreover, the presence of high amount
of free CaO is unfavourable [
]. However, due to its
2 Abbreviations used in cement chemistry: C—CaO, S—SiO2, H—
H2O, N—Na2O, A—Al2O3.
binding properties, calcium fly ash may be perspective if it
is considered as a substitute of high amount of cement in
binding mixture. But such utilisation of calcium fly ash
needs further intense studies.
Different contribution of fly ash containing calcium
components (from pulverised combustion of brown coal),
compared to low-calcium fly ash (from pulverised
combustion of hard coal), to the early hydration of blended
mixtures is shown in exemplary results of our
investigations presented in Fig. 4. It is evident that the more intense
the first wetting period is, the higher the amount of calcium
fly ash is in the mixture. As the ash content increases, the
induction period is longer, similarly as is the case of
mixture containing low-calcium fly ash. However, after the
induction period relative intense effect connected with
precipitation of hydrated products appears. Another
difference is the presence of additional thermal effect. It is
intense and appears faster when greater the amount of fly
Figure 5 presents examples of our results of TG/DTG
investigations performed for high and very high volume fly
ash pastes composed of calcium fly ash from combustion of
brown coal. It is evident that shapes of TG/DTG curves
resemble those for Portland cement paste. There are three
main mass losses on TG curves:
• up to about 425 C—dehydration of water bound in
products of cement hydration as well as pozzolanic
reaction (ettringite, C–S–H phase, C–A–S–H, C–A–H
• from about 425 to 480 C—loss of water from Ca(OH)2
• from about 550 to 700 C—decomposition of
The main mass loss is caused by endothermic processes
of release of water: dehydration of hydrated forms and
dehydroxylation of Ca(OH)2 (Figs. 5, 6). Release of water
from individual hydrates occurs at a broad temperature
range. That is why, dehydration processes overlap. For
example, according to data given by authors of [
synthesised C–S–H phase can dehydrate up to about 600 C,
while the main mass loss occurs up to 200 C (extremum
on DTG at about 100 C). The observed effects in Figs. 5
and 6 up to about 125 C can be related to the presence of
ettringite and C–S–H phase in sample as well as loosely
bound water in gel. The effects occurring at higher
temperature range are mainly connected with calcium
aluminates and aluminosilicates (C–A–H and C–A–S–H phases),
AFm phase as well as carboaluminates.
It can be seen that the mass loss up to about 425 C
increases with hydration time. It is a proof that hydration
reactions take place and solid products, such as C–S–H
phase and hydrated aluminate, sulfoaluminate and
aluminosilicate phases, are precipitated. The hydrated
aluminate phases are mainly formed after 3 days of
hydration which is indicated by effects on DTG curves at
about 120 and 280 C visible on 28th day. The amount of
Ca(OH)2 is decreased during the time of hydration—it
proves pozzolanic reaction. In the case of 50% replacement
of cement, this reduction in Ca(OH)2 content is not high. It
indicates that hydration of cement takes place, and it
ensures Ca(OH)2 for pozzolanic reaction with fly ash. It
enables to develop fly ash reactivity also after 28th day of
hydration. On the contrary, in the case of 75% replacement,
Ca(OH)2 is almost not visible on 28th day. It shows that at
such high content of fly ash, the amount of Ca(OH)2 is
insufficient, despite the fact that this fly ash contains
calcium components. It is the effect of the presence of
different forms of calcium components in this fly ash [
some of them exhibit low activity. Comparing with TG/
DTG results obtained for fly ash–cement paste composed
with 80% low-calcium fly ash [
], one can state that also
in case of this kind of cement replacement, Ca(OH)2 was
almost completely consumed during 28 days of hydration.
] investigated high volume fly ash concretes
containing 60% of Class C fly ash as cement replacement.
They registered decreasing of compressive strength as
compared with reference Portland cement concrete through
all the time of investigation, i.e. till 90th day of hydration.
It indicated that calcium fly ash used in high amount did
not fully replace cement in these compositions.
Dakhane et al. [
] investigated mixtures containing higher
amount; it is 70% of Class C fly ash. They confirm
acceleration of setting in the presence of Class C fly ash
compared to samples composed of Class F fly ash.
Compressive strength, both early and after 28 days, for binders
containing calcium fly ash was also higher than for those
with Class F fly ash and microstructure was more compact
]. Gu¨neyisi and Gesog˘lu [
] show that in case of high
replacement (60 and 80%) of cement by Class C fly ash
rapid setting occurs and initial time of setting is reduced. In
case of Class F fly ash, such phenomenon is not observed
because this kind of fly ash does not contain sufficient
amount of active calcium compounds. Early compressive
strength of composite containing high volume replacement
of cement by fly ash was reduced, especially in case of
80% fly ash content. However, starting from 28th day of
hydration, compressive strength of mixtures containing
80% of calcium fly ash was higher than for adequate
sample with Class F fly ash [
]. Beneficial effect of using
of 50% fly ash (as cement replacement) containing calcium
as well as sulphate components on compressive strength of
concrete compared to results for concrete performed with
low-calcium fly ash was confirmed also in our
]. However, in case of using high amount of fly
ash containing increased content of sulphates and calcium
components serious risk connected with durability appears.
In such systems delayed ettringite may arise. Its formation
in hardened structure may cause micro-stresses and as a
result deterioration of durability of fly ash–cement
composite. Thus, this direction of utilisation of such fly ash
needs especially intense investigations on hardening
mechanism, phase and crystal composition of matrix and
long-term properties of final composite.
The kind of fly ash used as replacement of cement is key
but not the only factor influencing properties of fly ash–
cement composites. In general, one can affirm that the kind
of fly ash (or other aluminosilicate material) used as
cement substitute, its amount, the amount of water, the
presence of plastificating admixtures, conditions of curing
and hardening as well as time influence hydration
processes (kinetics especially) and properties of final
composite. High and very high volume fly ash mixtures are
more sensitive to the curing conditions compared to
reference sample composed of Portland cement [
Summarising, utilisation of high amount of fly ash as
cement replacement is favourable especially from
ecological point of view. However, it is not a surprise that such
compositions will show deterioration of some properties,
setting time and early compressive strength especially. One
of reasons is insufficient amount of Ca(OH)2. It seems to be
reasonable that solution of this problem may be utilisation
of Class C fly ash containing calcium components and
exhibiting better reactivity compared to Class F fly ash.
However, fly ashes containing calcium components are
more diverse in composition and properties. Thus, their
influence on features of final material may also differ.
Moreover, calcium compounds may be present in fly ash in
various forms differing in activity and possibility to create
desirable products. Utilisation of such kind of fly ash in
high and very high amount in binding materials requires
special care and in-depth researches.
Mechanical activation can increase reactivity of different
pozzolanic additives including fly ash [
activation takes place through high-energy grinding. During
such mechanical treatment of fly ash, its surface area is
increased as well as physicochemical changes on the
surface of grains and in bulk may occur [
of fly ash grains changes, its spherical particles of different
sizes are broken during the grinding process and they
become smaller and irregular . Different kinds of
milling equipments (ball mills) may be used for this
69, 70, 72
]. Effectiveness of mechanical activation
depends on conditions of milling process; however,
chemical composition of activated material is also
essential. The main factors influencing mechanical activation of
fly ash are following: speed of rotation, size and amount of
milling balls used, the presence of liquid medium and its
type, time of grinding, etc. [
]. In case of mild conditions
of grinding, material undergoes mainly homogenisation,
while changes of grains are not significant. During more
aggressive grinding, surface of grains breaks down.
Finally, grains become smaller reaching the desired size
and surface area. Exemplary results of our investigations
showing changes in fly ash subjected to mechanical
activation are presented in Fig. 7. The results confirm that fly
ash from conventional combustion of coal is composed
from spherical grains of different diameters. Grinding
causes mainly fragmentation of bigger grains leaving the
finer ones almost intact. The average grain diameter
decreases as a result of mechanical activation.
Fly ash grains diameter below 45 lm are beneficial, and
using such fly ash as cement replacement can give higher
strength results than originally fly ash composed of
fractions of different size [
]. Thus, grinding is often
proceeded to obtain such particle diameter, e.g. in studies
described in [
]. However, it should be emphasised in this
place that fine fraction of fly ash may be obtained not only
during mechanical activation, but also as a result of
classification during which more reactive, fine fraction rich in
glassy components is separated [
69, 72, 74
]. In general, the
smaller the particle size, the higher the hydration activity of
fly ash is [
]. Thus, grinding process aimed in
obtainment of fly ash of even more fragmentation is also
desirable. Ultrafine fly ash exhibiting an average particle
size of less than 10 lm may be produced during
highenergy mechanical milling with the use of grinding aids to
improve grinding efficiency [
Fig. 7 General view
(magnification 91000) of
particles of raw fly ash and fly
ash which was subjected to
grinding by 1 h in laboratory
planetary ball mill (a) and
granulometric composition (b,
Raw fly ash
Grinded fly ash
Raw fly ash
.06 .90 .13 2 3 .54 .76 .10 .52 .82 .34 15 77 116 175 262
1 1 2 3
Grinded fly ash
Depending on the conditions of milling, not only
reduction in grains size and increase in surface area take
place. Chemical and phase changes also occur, such as
structural defects and rearrangements as well as phase
]. An increase in the level of
amorphisation of the sample is expected; however, some new
crystal phases during the mechanical activation may be
also formed, e.g. as a result of energy provided by grinding
Fig. 8 Exemplary curves of heat release rate (a) and total heat
released (b) for mechanically activated mixture containing 80% of fly
ash and 20% of Portland cement (fly ash from pulverised combustion
of hard coal, intergrinding of dry components for 10 or 40 min. before
water adding, water/binder = 0.5, initial temperature of hydration
25 C, BMR calorimeter) (average chemical composition (the main
components) of fly ash [mass%]: SiO2 54.2, Al2O3 28.9, Fe2O3 5.2,
CaO 2.3, SO3 0.2, Na2O ? K2O 2.6, MgO 1.1, the same fly ash was
used in case of measurements results of which are presented in Fig. 9)
]. Li et al. [
] show that long ball-milling
treatment of fly ash results in reduction in crystallite size,
loss of crystallinity as a result and an increase in
amorphous phases. They also affirmed changes undergoing in
structure of pores and in functional groups, while the time
of milling was an important factor influencing these
All the alterations being results of mechanical activation
of fly ash influence its reactivity in fly ash–cement system.
Mechanical activation can improve the dissolution process
and reactivity of fly ash by allowing better contact between
reagents as a result of increase in specific surface area of
the grains, their structural defects and higher amorphisation
]. In general, fly ash fineness and specific gravity
increase with grinding time, while the greater the fly ash
reactivity is, the finer fly ash grains are [
Modifications of fly ash/binder properties by mechanical
activation cause that mechanism of hydration reaction may
be changed as it is presented in Fig. 8 showing exemplary
results of our investigations. In these studies, mechanical
activation consisting of grinding together (intergrinding)
fly ash (80%) and cement (20%) during 10 or 40 min. in
laboratory planetary ball mill was used. In such conditions,
both the components undergo activation. As a result, period
of precipitation of products of hydration was more intense
and additional exothermic effect appeared (more intense in
case of longer grinding). It is, probably, the effect of
hydration of aluminates which were more available after
grinding. Mechanical activation did not influence
significantly duration of induction period in this case. Thus, one
can expect that mechanical activation did not influence
significantly time of start of setting in discussed case [
However, next period on calorimetric curve has visible
higher intensity. All of these changes are related to
increasing rate of hydration; thus, it indicates shorter time
of the end of setting. Similar results for mixtures
containing 70% of grinded fly ash were presented in [
Enhancement of reactivity of fly ash, easier dissolution
of its components and better contact of reagents cause that
some properties of fly ash–cement mixtures can be
improved. A lot of research reports proved that it is
possible to enhance compressive strength of mixtures
containing high amount of fly ash by mechanical activation.
There are publications discussing effectiveness of this type
of activation depending on different factors, such as the
kind of fly ash, the way of grinding of dry constituents
(separately or intergrinding), using grinding aids, time of
grinding, as well as time and temperature of hardening of
composites performed with mechanically activated
materials, etc. [
11, 65, 69, 73, 74, 78
]. For example,
Kumar et al.  concluded that 50–60% of mechanically
activated fly ash can be used as clinker replacement and
such blended cement exhibits compressive strength and
other properties comparable to commercial cement. They
found reduction in induction period in case of mechanically
activated mixture containing clinker and 50% of fly ash.
Authors of [
] reported that intergrinding of blended
cement components may give significantly better results of
compressive strength than separately mechanical treatment
of clinker and fly ash. It may be the effect of better
homogeneity. The advantages of intergrinding on separate
grinding of the components were also reported by other
]. Jaturapitakkul et al. [
] received very
fine fly ash of average particle size below 4.0 lm by
grinding coarse fraction of fly ash. They demonstrated that
concrete containing this ground fly ash in amount of 50%
as cement replacement exhibited compressive strength
similar to results for ordinary Portland cement concrete.
Supit et al. [
] investigated high and very high volume fly
ash mixtures (40–70% of Class F fly ash) modified by the
presence of 8% of ultrafine fly ash. They found that
samples with addition of ultrafine fly ash exhibit enhancement
compressive strength compared to those composed without
this very fine component. Beneficial results for samples
containing ultrafine fly ash are probably an effect of better
effectiveness in consuming of Ca(OH)2 in pozzolanic
reaction and densification of the structure. However,
despite an enhancement effect of ultrafine fly ash, 28-day
compressive strength of mortars containing very high
volume fly ash binder was more than three times lower as
compared for reference Portland cement mortar [
Similarly, Roychand et al. [
] showed that 80%
replacement of Portland cement by ultrafine fly ash results in
significant reduction in compressive strength as compared
to Portland cement reference mortar. For this reason, the
authors of [
] proposed partial replacement of this fly ash
by other components.
Summarising, it is possible to enhance reactivity of fly
ash–cement mixture using mechanical activation. In case
of mechanical treatment of both main constituents of the
binder, it is beneficial to use intergrinding instead of
separate grinding. Physical and chemical changes of solid
components of the binder strictly depend on conditions of
mechanical activation and time. It is possible to improve
some properties of composites containing high and very
high amount of fly ash and in this way extend possibilities
of their use. However, in case of very high volume fly ash
mixtures, mechanical activation may be insufficient to
improve setting time and compressive strength satisfactory.
It is because grinding process does not change the amount
of hydraulic components which are necessary to form solid
structure. In this case, activation may be insufficient. Thus,
in such case, another methods of activation of the system
also connected with mechanical treatment of dry mix
constituents should be considered.
Activation by exchanging part of aluminosilicate component by more reactive material
Exchanging of part of aluminosilicate component of
binding mixture by more reactive material aims quicker
development and improvement in early properties of final
composites. One can distinguish two main possibilities of
this method of activation: exchanging of cement or
exchanging of fly ash by other components. In the first
case, one can consider total or partial replacement of
cement by more rapid set one. In this way, acceleration of
setting and hardening and better early compressive strength
may be expected. In the second case, replacement of a part
of fly ash by more reactive pozzolana causes quicker
reaction with Ca(OH)2 and formation of binding phases.
More rapid set cement or additional amount
Bentz and Ferraris [
] investigated hydration of mixtures
containing 50% of fly ash and 50% of cement which was
partially replaced by rapid set cement. They concluded that
the presence of rapid set cement reduces significantly
initial setting time of the mixture. Promising strength results
were obtained by McCarthy and Dhir [
], who used
rapidhardening Portland cement or low-energy clinker in place
of typical Portland cement. However, in case of these
] the amount of cement replacement by
Class F fly ash was only 45%. It was proved in our previous
] that in case of high and very high amount of fly
ash in the mixture (40 or 80% by mass) some improvement
can be achieved by replacing the used Portland cement by a
higher class Portland cement. It was observed that in the
presence of cement of higher class induction period on
calorimetric curve was shorter and precipitation of
products during the next period was more intense; thus, shorter
time of initial setting may be expected.
In mixtures containing fly ash and cement, Ca(OH)2
arising during hydration of cement activates fly ash. For
this reason, introduction of additional amount of Ca(OH)2
or CaO into fly ash–cement mixtures may be considered as
one of methods of activation improving compressive
strength and durability of final composite. Some problems
with the use of Ca(OH)2 for this purpose may result from
low solubility of Ca(OH)2 as well as some retardation of
hydration of cement. Thus, ensuring good homogeneity of
mixture is especially important. Moreover, Ca(OH)2 may
be contaminated by CaCO3 which decreases its activity.
These may be the reasons of different literature reports
concerning efficiency of this activator in high and very
high volume fly ash mixtures. Thus, activation of fly ash by
the presence of additional amount of Ca(OH)2 may not be
revealed significantly at early periods of hydration
especially in case of very high volume fly ash binders and low
amount of Ca(OH)2. It was proved in our calorimetric
]. It was shown that introduction of 2%
of Ca(OH)2 into very high volume fly ash mixture did not
influence kinetics of early hydration significantly.
Velandia et al. [
] investigated reactivity of different
high losses on ignition fly ashes (used as 50% of cement
replacement) in the presence of lime and quicklime. They
showed that the properties of fly ash, especially content of
amorphous phases and grain size, as well as the amount and
the type of used activator influence reactivity of fly ash. In
general, the authors found that influence of quicklime and
lime on the compressive strength is rather little; however,
in some cases early compressive strength was improved.
Barbhuiya et al. [
] showed that introduction of 5% of
hydrated lime results in improvement in early (3-day) and
later (28-day) compressive strength of sample containing
50% of low-calcium fly ash. Results presented in [
showed that compressive strength of very high volume fly
ash mixture (about 70%) is not influenced by additional
amount of Ca(OH)2 till 7th day of hydration. It indicates
that the presence of Ca(OH)2 slightly improves fly ash
reactivity at early stages but provides component for the
pozzolanic reaction at later periods. Studies of content of
Ca(OH)2, based on TG/DTG results, have allowed the
authors to state that the reactivity of fly ash is enhancement
with addition of the hydroxide and as result of it 28-day
compressive strength is improved. Similarly, Lorca et al.
] presented that introduction of Ca(OH)2 into concrete
containing 50 or 70% of low-calcium fly ash as cement
replacement results in improvement in compressive
strength starting from 7th day of hardening. Thermal
analysis after 1 year of hardening revealed the absence of
Ca(OH)2 in high and very high volume fly ash concretes.
Several peaks on DTG curves allowed the authors to
identify some products of hydration and pozzolanic
reaction: C–S–H phase, ettringite and hydrated calcium
aluminosilicate type of gehlenite.
Silica and aluminosilicate pozzolanic materials
Another possibility to improve early as well as later
properties of binding mixtures containing high and very
high amount of fly ash is introduction into the system small
amount of pozzolanic materials exhibiting better reactivity
than fly ash. These active pozzolans may be introduced as
replacement of part of fly ash. Another possibility of their
introduction is proportional reduction in both cement and
ash contents. In some cases, these additives are also
introduced as a replacement of part of cement. Active
finegrained pozzolan can react with Ca(OH)2 earlier than fly
ash. Thus, the inconveniences concerning with the fact that
fly ash needs time to develop its activity are eliminated.
Moreover, the presence of additional component may also
influence (accelerate) hydration of cement (e.g. by
providing of nucleation centres). Filler effect of fine particles
results in more tightly microstructure.
Silica fume or nanosilica is often used for this purpose.
This material is fine-grained and may cause deterioration of
fluidity; however, this effect may be compensated by
spherical grains of fly ash [
]. Accelerating action of
small amount of silica fume on early hydration of high
volume fly ash mixture may be manifested in changes of
shape of heat release curve while improvement in
pozzolanic activity by reduction in Ca(OH)2 basing on TG/
DTG results. Expected effect of accelerating action of silica
fume is better compressive strength of final composite.
For example, it was shown by Zhang and Islam [
in case of 50% cement replacement by fly ash, introduction
of 1% of silica fume results in some accelerating effect. It
was manifested by shorter induction period on calorimetric
curve and slightly higher total heat released after 30 h of
hydration. Compressive strength was also increased in the
presence of silica fume. Increased compressive strength of
high volume fly ash composite modified by the presence of
silica fume was also confirmed by authors of [
54, 55, 90
Elahi et al. [
] revealed that the presence of silica fume
can improve compressive strength of mixes containing
40% of low-calcium fly ash starting from 7th day of
hydration. According to authors of [
] explanation, it is an
effect of better bonding between cement matrix and
aggregate. Results received by Yazıcı [
] confirmed that
addition of silica fume into high volume fly ash composite
can effectively improve compressive strength also in case
when Class C fly ash was used.
Better results can be achieved using nanosilica fume
]. In general, small amount of nanoparticles, such as
nano-Fe2O3, nano-CaCO3, nano-SiO2, can improve
compressive strength of cement as well as microstructure of
hardened matrix on condition that nanoparticles are well
]. Exemplary literature reports concerning
influence of nanoparticles on high volume fly ash mixtures
are presented in Table 1.
Nanosilica, being highly amorphous powder, reacts
easily with Ca(OH)2 and as a consequence C–S–H phase is
quickly formed. This reaction occurs faster than in case of
fly ash. Moreover, nanosilica makes microstructure denser
and contributes greater durability. Very fine grains of
nanosilica act as nucleation centres for precipitating
products; thus, hydration of cement is accelerated [
Interfacial transition zone is improved and compressive
strength of final material is enhanced as a result of action of
this additive despite its small amount used [
87, 92, 95
However, it should be indicated that very fine particles of
nanosilica tend to agglomerate. Thus, ensuring of good
homogeneity of binding mixtures by selection of
appropriate method of mixing is important [
For example, authors of [
] discussed results
concerning effectiveness of silica fume and nanosilica. They
showed that the presence of nanosilica accelerated
hydration of cement (C3S especially) probably by providing
additional sites for nucleation of products as well as
reducing amount of calcium ions in the solution during
pozzolanic reaction. As a result, C–S–H and Ca(OH)2 were
formed faster. Ca(OH)2 is available for pozzolanic reaction
during which additional amount of C–S–H phase may be
formed. The additional peak on calorimetric curve
indicates that the presence of nanosilica probably accelerated
also hydration of tri-calcium aluminate and depletion of
Kawashima et al. [
] revealed, basing on
thermogravimetric results, that addition of nanosilica into blended
cement containing 40% of fly ash causes rapid
consumption of Ca(OH)2 during early days of hydration resulting in
increasing early strength gain. This effect of strength
enhancing is gradually decreased over time. Similar
Shaikh et al.  Roychandat al. 
Hou et al.
relationship between intensity of strength gain and time
was also observed in [
Supit and Shaikh [
] investigated influence of small
addition of nanosilica on concrete composed of 40 or 60%
of fly ash (as cement replacement). They reported that in
case of composite containing the highest amount of fly ash,
small addition of nanosilica results in improvement in early
(3-day) compressive strength while on later ages this
parameter was almost the same as for concrete without
nanosilica. It indicates that acceleration of cement
hydration play significant role in the improvement in early
mechanical properties of high volume fly ash concretes.
In case of very high volume fly ash concretes small
amount of cement and resulting from this small amount of
Ca(OH)2 may be insufficient to ensure high strength of
final material and fully developed pozzolanic activity of fly
ash. Moreover, it is very important to provide good
homogeneity of the system. Exemplary, Roychand et al.
] revealed that addition of hydrated lime to mixture
containing cement, fly ash and nanosilica did not increase
strength. Inversely, in case of greater amount of nanosilica
small decrease in compressive strength was registered. The
authors concluded that created additional part of C–S–H
gel may act as a barrier for further reactions, e.g. it may
block access of Ca(OH)2 to fly ash grains and in this way
hinder pozzolanic reaction of fly ash [
Another way of activation, basing on replacement of
part of material by another component of higher activity, is
using highly pozzolanic aluminosilicate fine-grained
materials. Wei et al. [
] registered improved compressive
strength in case of concretes composed of binder
containing 50% of fly ash and 50% of cement and additionally 5%
of metakaolin. They stated, basing on DTA results, that in
the presence of metakaolin, Ca(OH)2 is significantly
consumed in pozzolanic reaction during 28 days of hydration.
This reduction in amount of Ca(OH)2 in the systems was
higher than in case of sample containing silica fume instead
of metakaolin. Moreover, in case of metakaolin the
presence of hydrated aluminosilicate phases type of C2ASH8
was more clearly visible.
Gu¨ neyisi and Geso g˘lu [
] highlighted that retarding
effect of high amount of fly ash (60% of cement
replacement), manifested by prolongation of initial and final time
of set, was reduced in case of using mixture of fly ash and
metakaolin. Beneficial effect of metakaolin on compressive
strength was observed starting from 7th day of hydration.
In our previous paper [
], it was proposed to use spent
aluminosilicate catalyst from petrochemical industry to
activate mixtures containing very high amount of fly ash.
This spent catalyst is a well-known material exhibiting
high pozzolanic activity and accelerating hydration of
]. It was found that introduction of this
aluminosilicate into the mixture in place of a part of fly ash
results in intensification of heat release after addition of
water, reduction in induction period and faster
consumption of Ca(OH)2 in pozzolanic reaction. Addition of spent
catalyst in the place of part of fly ash can accelerate to
some extent hydration of fly ash–cement pastes and
pozzolanic reaction. Small amount of available Ca(OH)2 is
quickly depleted in the presence of the spent
aluminosilicate. Thus, introducing additional amount of Ca(OH)2 or
other alkaline component is needed . Vela´zquez et al.
] proposed to use mix of the spent catalyst and fly ash in
case of up to 45% of replacement of cement.
Summarising this part of the paper, one can conclude
that in case of very high volume fly ash binders (more than
70% of fly ash) exchange of a part of fly ash by more active
pozzolana is only effective to some extent. It is possible to
accelerate cement hydration and pozzolanic reaction and to
improve in this way early properties of composite, but
small amount of hydraulic components is not sufficient to
allow fly ash to react to a greater extent, and development
of high compressive strength is not possible. It is possible
to improve early properties of the binders to some degree
by adding additional amount of components providing
substrates for pozzolanic reaction (such as CaO, Ca(OH)2)
or more hydraulic reactive components (e.g. rapid set
cement). Chemical, alkaline especially, activation should
be considered in this case for better reactivity of the
mixtures as well as quicker setting, hardening and development
of early compressive strength.
Chemical activation of high and very high volume fly ash–cement mixtures
Binders composed of small amounts of ordinary Portland
cement or cement clinker (20–30 mass%) and fly ash in
amount of 70–80 mass%, additionally containing separate
source of alkali component, can be defined as ‘‘hybrid
18, 19, 43, 74, 102, 103
]. Those activated
systems are mainly investigated by the team of researchers
from Spain [
16, 18, 19, 43, 48, 50, 104
literature reports concerning influence of chemical activators
on hybrid cement are presented in Table 2. This type of
binding mixtures may be produced not only using fly ash as
the main component but also another aluminosilicate
materials such as different kinds of slag [
103, 105, 106
red clay brick waste , mixture of fly ash and slag
] or other multiple components , but such
compositions were not discussed in this paper. Moreover,
chemical activation has been used not only in the case of
binders containing very high volume cement substitutions
but also in case of their lower content (e.g. 40% [
High alkaline activation
In general, it is beneficial to use admixtures or solutions
providing high alkaline environment for reaction. In such
conditions, aluminosilicate grains of fly ash can undergo
dissolution, which is the first step of formation of solid,
hardened structure. In case of very high volume fly ash
mixtures, alkaline activators similar to those used in
geopolymers, i.e. alkali silicates and/or alkali hydroxides,
may be considered. Introduction of alkali hydroxide results
in higher pH of solution than in case of saturated solution
of Ca(OH)2. However, there is some problem with their
use, which was mentioned earlier in the introduction of this
paper. Moreover, usage of highly alkaline solutions
influences hydration of cement in such a way that high
concentration (high pH of solution needed for dissolution of fly
ash) inhibits hydration of clinker silicates and formation of
C–S–H phase [
43, 104, 108, 112–114
]. Moreover, curing
conditions (e.g. temperature) influence hydration processes
and as a result also development of binder properties.
It should be emphasised that the presence of cement,
being a source of calcium ions, in the discussed high alkali
activated mixtures results in hydration processes to
undergo differently as compared to that in typical
alkaliactivated fly ash (geopolymers), and the formed products
are also different. Various types of gels may be
co-precipitated, depending on composition of the binder, type of
fly ash, the kind of alkali activators and time. In case of
alkali activation of hybrid cement containing high amount
of fly ash, the presence of two main types of gel products,
i.e. C–A–S–H and (N,C)-A–S–H, can be expected. More
detailed explanation of nanostructural mechanism of
formation of products and their transformation with time can
be found in [
43, 48, 104
Hindered hydration of clinker silicates in high pH may
result in low compressive strength at early periods
depending on type of alkaline activator, amount of cement,
temperature of hardening, etc. [
Considerably delay of normal cement hydration in the
presence of solution of NaOH ? Na2SiO3 (pH 13.5) was
visible in calorimetric results of early cement hydration
presented in [
]. In such case, thermal effect of
precipitation of products was strongly reduced similarly as total
heat evolved. Despite this retardation, the presence of
soluble silicates beside NaOH is beneficial. Authors of
] showed that in case when waterglass ? NaOH
solution was the hydrating liquid, the compressive strength
was improved both after 2 and 28 days of hydration.
Garcia-Lodeiro et al. [
] observed, basing on calorimetric
measurements, hindering of cement hydration and some
synergy effect between the ash and cement in the presence
of high pH (additional heat effect at early period).
Rivera et al. [
] investigated hybrid material hydrated
with waterglass solution. They confirm that in such systems
Ca(OH)2 is not precipitated and non-reacted alite and belite
(from clinker) are present. It indicates that alkali solutions
change process of hydration of Portland clinker and
activate fly ash. However, not only pH but also the presence of
soluble silica influences hardening of fly ash-rich binder
]. Mej´ıa et al.  proved that using NaOH and
sodium silicate as activators of hybrid cement containing
70% of fly ash may be beneficial in case of fly ash showing
high loss of ignition and exhibiting no commercial value.
Improvement in compressive strength of high alkali
activated binders may be visible not only till 28th day of
hydration but also in later periods. It was evidenced by
authors of [
] who showed that mixture containing 70%
of Class F fly ash and activated by liquid containing
12.5M NaOH and sodium silicate exhibited 365-day
compressive strength higher of about 20% compared to results for
sample hydrated in water.
Moderate alkali activation
One of the advantages of hybrid cements over geopolymers
is that mixtures containing cement do not require elevated
temperature for hardening and forming solid structure
102, 111, 114
]. Moreover, activation methods during
which pH of solution is raised gradually in situ can be used.
It may be realised by introduction of some inorganic salts
into the system.
Inorganic salts, such as Na2SO4 or Na2CO3, in the
presence of calcium ions can undergo reaction during
which some solid products precipitate (CaSO4 2H2O or
CaCO3 in case of use of Na2SO4 or Na2CO3 respectively).
Sodium hydroxide (Na?, OH-) is also produced during the
Fig. 9 Exemplary curves of heat release rate (a) and total heat
released (b) for mixture activated by intergrinding dry components
[fly ash, cement, Ca(OH)2, Na2SO4] for different time [planetary ball
mill PM 100 (Retsch), fly ash from pulverised combustion of hard
coal, water/binder = 0.5, initial temperature of hydration 25 C,
BMR calorimeter] (Ca(OH)2 and Na2SO4 were used in amount 10 and
4% respectively calculated in relation to total mass of cement and fly
reactions and results in growing alkalinity of the solution
16, 19, 36, 43, 49, 60, 74, 103, 110, 115, 116
needed for the reaction may come from different sources:
cement hydration, introduction of additional amount of
Ca(OH)2 into the system or as component of used
supplementary cementitious material (e.g. in case of high
calcium fly ash). This method of activation, i.e. moderate
alkali activation, is more secure compared to the activation
performed with strong alkalis. Solutions of lower alkalinity
may be used in this case, and high pH is generated in situ;
thus, there is greater safety for workers and the
environment. Moreover, the activators have relatively low impact
on cement hydration and elevated temperature of hardening
usually is not necessary [
In general, different products of reaction may appear
with the use of solutions of alkali salts. Gel of hydrated
calcium silicates and aluminosilicates (similarly as in case
of hydration of Portland cement) and hydrated alkali
aluminosilicates (similarly as in case of alkali activated fly
ash) were formed. These products do not precipitate as neat
phases, but they interact and form mix which undergoes
chemical and structural transformations with time.
Generally, existing of different aluminosilicate forms of
variable structures depends on composition of binding
mixture including Ca availability, pH and temperature.
Alkali media (high pH) and availability of aluminate ions
change composition and structure of C–S–H phase, while
calcium ions interact with N–A–S–H gel forming (N,C)-A–
S–H gel. In case of high lime content in the reacting
solution, formation of C–A–S–H gel is favoured.
Secondary products such as: ettringite, carbonates, etc., appear
depending on the type of activator used as well as
conditions of reaction [
19, 43, 62, 105
Na2SO4 is known not only as component contributing to
the increase in pH and improving ash solubility. Its
activating effects in relation to cement hydration are also
known. Na2SO4 belongs to cement-hardening accelerators.
One of hypothesis explaining this effect is faster binding of
sulphate ions in form of ettringite and decreasing of
concentration of calcium ions [
]. Thus, in case of mixtures
of fly ash–cement activated by Na2SO4 both influence on
hydration of cement and improvement in dissolution of fly
ash grains should be considered. Additional amount of
ettringite may appear. However, the presence of ettringite
and its stability is strongly dependent on the composition of
the liquid phase and pH. Thus, depending on properties of
fly ash (e.g. Class C or Class F fly ash), the type of cement
as well as other factors (such as amount of activator, water
to binder ratio, the presence of plasticising admixtures,
temperature, etc.) intensity of action of Na2SO4 and
detailed mechanism of activation may be different.
For example, we confirmed in our previous work [
accelerating action of Na2SO4 in relation to the hydration
of cement–fly ash paste in case of typical amount of fly ash
(30 mass%) in the binder. However, using the same
amount of Na2SO4 in case of very high volume fly ash
(80 mass% of low-calcium fly ash) binder resulted in small
degree of accelerating during early hydration process, as it
was evidenced in calorimetric investigations [
In practice, the effect of Na2SO4 should result in
improving strength of the binder. Acceleration of setting
and satisfactory early compressive strength is very
important. Wang et al. [
] reported that binder mortars
composed of 40% of fly ash and 60% of cement and activated
with Na2SO4 (standard curing at 20 C) exhibited almost
two times higher early compressive strength compared to
non-activated sample. However, in case of steam curing at
60 C the compressive strength was similar as for the
nonactivated reference. Velandia et al. [
] showed that in case
of 50% cement replacement by fly ash, activation of such
systems by Na2SO4 gives better compressive strength on
early days as compared to activating by lime and
quicklime. This relationship was observed mainly in case of fly
ashes containing low amount of Fe2O3; thus, the authors of
] concluded that the content of Fe2O3 in fly ash affected
Donatello et al. [
] showed that 2-days compressive
strength of mixture containing about 80% of Class F fly ash
was about two times higher than the result registered for
about 80% of sand (both mixtures activated by Na2SO4).
Total heat evolved during 2-days of hydration was also
higher for the first mentioned composition. It confirms that
strength development in the presence of Na2SO4 is not only
result of acceleration of cement hydration but also
activation of fly ash grains and their reaction.
Development of compressive strength in the presence of
Na2SO4 activator is undoubtedly connected with
transformations occurring in precipitated aluminosilicate phases.
However, the presence of additional amount of sulphate
ions may favour formation of ettringite which can be
responsible for setting and early compressive strength and
formation of which may densify the hardened structure.
Results obtained by different researchers give
contradictory informations about the possibility of ettringite
formation in the presence of Na2SO4. For example formation
of ettringite was observed in [
] (50% of cement
replacement by fly ash). Donatello et al. [
] noted the
presence of small amount of ettringite during early stages
of hydration of mixture containing higher amount, i.e.
about 80%, of grinded bottom ash. Because in the case of
this investigation, gypsum was not present in initial
mixtures (Portland cement clinker free of gypsum was used),
appearing of ettringite confirms that Na2SO4 is responsible
for their formation. Formation of some amount of ettringite
during early hydration of very high volume fly ash binders
activated with Na2SO4 was also described in [
Moreover, it was also affirmed in [
] that more ettringite is
formed when CaSO4 was used instead of Na2SO4, while in
case of mixture containing fine sand and activated with
Na2SO4 ettringite did not appear.
Thus, formation of this phase is dependent on the
amount of calcium ions in the systems and availability of
aluminates strictly connected with pH of reaction
environment. Alkalinity of reaction solution is a key factor.
Ettringite formed in alkali activated hybrid cements has
poor stability. For example, structure of ettringite may be
affected by partial substitution of Ca by Na [
]. At high
alkaline media and high concentration of Na?, formation of
Phase U and monosulphate are rather favoured. However, it
also depends on content of sulphates in mixture [
Understanding the role of calcium in these systems is
very important as it influences binding phases. Research
concerning influence of Na2SO4 on fly ash–lime paste
showed that early pozzolanic reaction is accelerated and
ettringite may be formed. The presence of Na2SO4
accelerates consumption of Ca(OH)2, and hydration products
such as C–S–H and hydrated aluminosilicate phases are
formed faster [
44, 60, 117
]. Li et al. [
] reported that in
case of fly ash activated by Ca(OH)2, CaSO4 and Na2SO4
(blended together) and temperature 60 C, ettringite is
formed. It confirms that sulfoaluminate phases type of
ettringite may be formed not only in reaction of C3A
(constituent of cement) but also with the aluminate ions
coming from breaking of glass in grains of fly ash. Shi and
] confirmed that addition of Na2SO4 results in
increase in early and later strength of pastes containing
80% of ground volcanic fly ash and 20% of Ca(OH)2.
Kulasuriya et al. [
] also showed that introduction of
activator (Na2SO4, Na2SiO3 or both of them) into fly ash–
lime mixture results in improving compressive strength.
However, better results of compressive strength were
obtained for the same chemical activation of mixture
containing not only fly ash and lime but also Portland
cement (about 50% of fly ash, 20% of lime and 30% of
Portland cement). In this case, the best results of
compressive strength were registered with the use of Na2SO4 as
an activator. Beneficial influence of addition of Na2SO4
and CaO into fly ash–cement mixture on up to 28-day
compressive strength was presented by authors of [
Using lime and Portland cement as constituents of the
activated binder containing fly ash seems to be especially
important in the case of very high volume fly ash binders.
In such compositions, Ca(OH)2 formed during hydration of
cement appears in very low amount. Moreover, it is
relatively quickly depleted what can be evident on TG/DTG
]. It causes that pozzolanic activity of fly ash cannot
fully develop. In case of additionally introduced Ca(OH)2
component, Ca2? ions are quickly available for reaction
with activator (alkali salt) and alkali activation of fly ash
can start quickly. At later periods of hydration, reaction
between fly ash and Ca(OH)2 can contribute to the
development of mechanical properties of the final composite.
Received by us calorimetric results of very high volume
fly ash (about 80%) mixture activated by Na2SO4 and
Ca(OH)2 indicate that simple hand mixing of components
of binder do not influence induction period significantly.
For this reason, we proposed to use combined activation
method: chemical (Na2SO4 ? Ca(OH)2) and mechanical
activation, during which all dry components were grinded
together in planetary ball mill before adding water. In such
a way, benefits from these two methods of activation can
give advantageous synergy effect, what was preliminary
shown by us in [
]. Further calorimetric results for
hydrating pastes are very promising. It was shown that
significant reduction in induction period, visible
intensification of the next period of heat release related to
precipitation of hydration products as well as intensification of
total heat release during early 48 h after addition of water
take place (Fig. 9). These changes are the more intense, the
longer time of grinding was used. Few mechanisms of early
activation take place in this case. Mechanical treatment of
dry mixture causes better homogenisation; however,
activation of grains by crushing and breaking down their
surface also appears. It depends on the time and intensity of
grinding. On the other hand, mechanical breach of grains
causes that active components are better available to react
in the presence of water. In case of fly ash ?
cement ? Ca(OH)2 ? Na2SO4 composition, ettringite or
other sulfoaluminate phases are probably formed. Early
formation of ettringite-type phases seems to be very
possible, when calorimetric results of mechanical activation
(grinding of fly ash and cement mixture) and combined
activation (grinding of fly ash, cement, Ca(OH)2 and
Na2SO4 mixture) were compared (Figs. 8, 9). In the first case,
additional exothermic effect in form of intense peak during
precipitation period appears. In the case of introduction of
Ca(OH)2 and Na2SO4 into the activated mixture, this peak
is not visible and induction period is effectively shortened.
In the case of mechanical activation only, aluminate ions
become more available and precipitate in the form of
hydrated calcium aluminates, while in the presence of
Na2SO4 these ions react fast forming ettringite or similar
aluminosilicate phases. The exact mechanism of action of
combined activation as well as optimisation of conditions
of mechanical activation to receive the best setting time
and strength results requires additional research and
analysis and will be the objective of forthcoming publications.
Beneficial effect of mechanical activation (grinding of
fly ash) used together with chemical activation by Na2SO4
was confirmed in [
Additional amount of calcium ions in chemically
activated fly ash–cement mixtures may come not only from
additional introduction of lime or Ca(OH)2. They may also
appear as component of some types of fly ashes.
Dakhane et al. [
] found that in case of Class C fly ash
containing mixture (the level of cement replacement was
70%), activation with Na2SO4 resulted in the formation of
ettringite as well as higher degree of polymerisation of
silica was affirmed earlier than in case of low-calcium fly
ash binder. Main product of hydration/activation was gel of
(N,C)-A–S–H in case of binders containing low-calcium
fly ash and C–A–S–H in case of sample containing higher
amount of calcium components.
Summarising, chemical activation is an effective
method for enhancement reactivity of fly ash–cement
system and improving setting and compressive strength.
Mechanisms of action of activator are different depending
on the type of chemical activator used. In general, these
methods of improvement in fly ash reactivity depend on
raising pH and improving solubility of aluminosilicate
grains of fly ash. In case of some activators, secondary
phases are also formed contributing to properties of final
composite. Activating techniques allowing the increase in
pH in situ as effect of chemical reaction with activator are
especially desirable. This way of raising the reactivity of
the system is possible with Na2SO4 in the presence of
calcium ions. Using combined methods of chemical and
mechanical activation is especially promising.
In this paper, selected methods of activation of binding
mixtures containing high and very high amount of fly ash
have been presented. Their effectiveness, influence on
kinetics of hydration and on formed solid products,
modifications of setting times and compressive strength have
been discussed. Possibilities of using calorimetry and
thermal analysis methods for investigation of activation
processes have been shown. Several conclusions have been
formulated basing on the review of the literature reports as
well as the results of our investigations and experience.
1. High amount of fly ash in binding mixture results in
delay of hydration, lengthening time of start of setting
and reduction in compressive strength especially in
early ages. It is particularly visible in the case of
lowcalcium fly ash. Using fly ash containing higher
amount of calcium components may diminish these
inconveniences because of some hydraulic properties
of this type of fly ash. However, in some cases this
type of fly ash may endanger durability of the final
material. Thus, utilisation of calcium fly ash in high
and very high amount in binding mixture is promising
but needs further research.
2. Choice of activation method depends on the amount of
fly ash, its kind and role in the system.
3. Mechanical activation enhances reactivity of mixture,
but it may improve properties of composite only to
some degree. It is very important that mechanical
activation of both components of binder, it is fly ash
and cement, gives better results, especially in case
when intergrinding is used. Depending on conditions
of grinding, physical and chemical changes in grains
occur. This method of activation can give beneficial
results in case of high volume fly ash mixture, however
may be insufficient in case of very high content of fly
ash and low content of cement.
4. Exchange of part of fly ash by more reactive
pozzolanic materials results in faster developing
properties of fly ash–cement mixture. It is because more
active pozzolana reacts quickly forming binding
phases and fly ash can contribute to the development
of properties of mixture later. Moreover, very fine
grains of active pozzolana may accelerate hydration of
cement. However, Ca(OH)2 can be quickly depleted
causing neutralisation of pH. Fly ash cannot fully react
in such case. Thus, in case of low amount of calcium
components in the system, especially in case of very
high amount of low-calcium fly ash, this method can
be effective only to some degree.
Chemical activation consisting in raising pH causes
faster dissolution of fly ash grains. High alkaline
activators may inhibit hydration of cement causing
reduction in early compressive strength. Solid products
that are formed in such conditions are modified as
compared to products of cement hydration. Different
types of gel may coexist depending on type of alkali
activator and conditions of reaction.
Recently hybrid cements composed of very high
volume of fly ash and activated with some inorganic
salts have met with great interest as alternative binders
for Portland cement. Such materials do not require
very strong alkaline solutions and elevated temperature
to harden, so they are relatively safe for workers and
for the natural environment.
Promising results may be obtained using combined
method of activation, for example chemical activation
with Na2SO4 and Ca(OH)2 and mechanical activation
consisting in intergrinding of dry components. Further
advanced investigations of such systems as well as
other new compositions of hybrid cements are needed.
These are the main topics of our ongoing research.
Calorimetry and thermal analysis are very useful for
investigation of activation processes in fly ash–cement
binders. Calorimetry enables to observe early
hydration/activation processes. Thermal analysis gives
answer about some reaction products and their estimate
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