V. BÍLEK et al. EFFECT OF A COMBINATION OF FLY ASH AND SHRINKAGE-REDUCING ADDITIVES ...
813–817
EFFECT OF A COMBINATION OF FLY ASH AND
SHRINKAGE-REDUCING ADDITIVES ON THE PROPERTIES OF
ALKALI-ACTIVATED SLAG-BASED MORTARS
VPLIV KOMBINACIJE LETE^EGA PEPELA IN DODATKA ZA
ZMANJ[ANJE KR^ENJA NA LASTNOSTI MALTE IZ Z
ALKALIJAMI AKTIVIRANE @LINDRE
Vlastimil Bílek, Luká{ Kalina, Jan Koplík, Miroslava Mon~eková,
Radoslav Novotný
Brno University of Technology, Faculty of Chemistry, Materials Research Centre, Purkyòova 118, 612 00 Brno, Czech Republic
xcbilekv@fch.vutbr.cz, bilek@fch.vut.cz
Prejem rokopisa – received: 2015-06-29; sprejem za objavo – accepted for publication: 2015-10-26
doi:10.17222/mit.2015.133
This study is aimed at reducing the drying shrinkage of alkali-activated slag (AAS) through the use of low-calcium fly ash (FA)
and commercially available shrinkage-reducing additives (SRAs) originally developed for OPC-based binders, since there are no
such admixtures tailored for AAS systems. Generally, all the SRAs tested are based on modified alcohols. All the mortars were
based on slag activated by waterglass, with the water-to-slag ratio equal to 0.40 and the sand-to-binder ratio 2:1. In the first step,
the effect of the partial replacement of slag by FA (25, 50 and 75) % of mass fractions on the drying shrinkage and compressive
strength was investigated. On the basis of the obtained results a mortar with 50 % FA in the binder was chosen for subsequent
experiments, where the influence of three types of SRAs on the drying-shrinkage behaviour was examined. It was observed that
while 25 % of the FA did not affect the drying shrinkage significantly, 50 % and 75 % of FA in the binder decreased the drying
shrinkage by 57 % and 78 %, respectively. However, with an increasing content of FA, the compressive strength markedly
decreased. All the tested SRAs had a similar effect on the drying shrinkage of the slag/fly ash (50/50) mortar: at a dose of
0.50 % (by mass of slag) the shrinkage was reduced only slightly, whereas 1.0–3.0 % of SRA resulted in a decrease by 49–66 %.
Also, the drying shrinkage rate during the first days of drying was modified. However, all the SRAs reduced the compressive
strength as compared to the neat slag-FA mortar, especially when the doses were higher than 0.50 %.
Keywords: alkali-activated slag, fly ash, shrinkage reducing additives, shrinkage, strength
Namen {tudije je zmanj{anje kr~enja pri su{enju z alkalijami aktivirane `lindre (AAS) z uporabo lete~ega pepela (FA), z majhno
vsebnostjo kalcija in komercialno dostopnega dodatka za zmanj{anje kr~enja pri su{enju (SRA), originalno razvitega za veziva
na osnovi OPC, ker do sedaj ni bilo takega dodatka primernega za AAS sisteme. Na splo{no so vsi preizku{eni SRA temeljili na
modificiranih alkoholih. Vse malte so bile iz `lindre, aktivirane z vodnim steklom, z razmerjem voda:`lindra (0,40) in
pesek:vezivo v razmerju 2:1. Na prvi stopnji je bil preiskovan vpliv delne nadomestitive `lindre s FA (25, 50 in 75) % masnih
odstotkov, na kr~enje pri su{enju in na tla~no trdnost. Na osnovi dobljenih rezultatov je bila za nadaljevanje preizkusov izbrana
malta s 50 % FA v vezivu, kjer se je preiskoval vpliv treh vrst SRA na skr~ek pri su{enju. Opa`eno je bilo, da medtem ko 25 %
FA ni vplivalo pomembno na skr~ek pri su{enju, je 50 % in 75 % FA v vezivu, zmanj{alo skr~ek pri su{enju za 57 % oziroma
78 %. Vendar pa se je z nara{~ajo~im dele`em FA tla~na trdnost opazno zmanj{ala. Vse preizku{ene SRA so imele podoben
vpliv na kr~enje pri su{enju malte, z razmerjem `lindra:lete~i pepel (50:50), pri koli~ini 0,50 % (masa:`lindra) se je skr~ek
zmanj{al samo delno, medtem ko se je pri 1,0–3,0 % SRA skr~ek zmanj{al za 49–66 %. V prvih dneh se je spremenila tudi
hitrost kr~enja pri su{enju. Vendar pa je SRA zmanj{ala tla~no trdnost, v primerjavi z maltami iz `lindre in FA, {e posebno, ~e
sta bila njuna dele`a ve~ja od 0,5 %.
Klju~ne besede: z alkalijami aktivirana `lindra, lete~i pepel, dodatki za zmanj{anje kr~enja, skr~ek, trdnost
1 INTRODUCTION
Concretes based on Portland cement (PC) provide a
very good mechanical performance for arelatively low
cost. Besides some durability problems that may occur,
PC production is connected with the significant emis-
sions of greenhouse gases such as CO2 and NOx and a
high energy consumption. It is estimated that the PC
industry contributes 7 % to the global CO2 emissions.
The utilization of some industrial wastes or by-products
such as blast-furnace slag (BFS) and fly ash (FA) may be
a possible way to partially solving the above-mentioned
problems. BFS and/or FA can be activated by an alkaline
activator to formulate so-called alkaline cements with simi-
lar or even better properties than PC-based materials.1
Alkali-activated slag (AAS), especially when the
activator is waterglass, shows similar mechanical proper-
ties to PC-based materials.2 However, a high shrinkage,
leading to cracking, usually occurs,3 which is considered
to be the most serious complication for the use of AAS
in practice.4 The fact that the drying shrinkage of AAS
concrete is several times higher thana that of PC-based
concrete is often associated with a higher mesopore
volume in the AAS matrix, which results in substantial
capillary tensile forces at the water-air meniscii and the
material shrinks.5
Materiali in tehnologije / Materials and technology 50 (2016) 5, 813–817 813
UDK 67.017:621.763:621.745.4 ISSN 1580-2949
Professional article/Strokovni ~lanek MTAEC9, 50(5)813(2016)
One possible way to mitigate shrinkage is the use of
shrinkage-reducing admixtures (SRAs). Generally, SRAs
belong to the group of chemical substances called sur-
factants. The molecules of surfactants are composed of a
polar head (ionic or non-ionic) and a non-polar tail.
Considering the water-vapour interface, the polar head is
attracted to the water phase, while the non-polar tail is
oriented towards the gaseous phase, which means that
surface tension is reduced and thus the capillary stresses
decreas.6 On the other hand, there are several doubts
about the importance of surface tension and capillary
stresses on shrinkage. For example, F. Wittmann7 stated
that even a significant reduction in the surface tension of
the pore solution did not noticeably affect the shrinkage
of a cementitious system and found the disjoining pres-
sure to be more important. Also, M. J. Setzer8 empha-
sized the role of the disjoining pressure and changes in
the surface energy. Nevertheless, SRAs are successfully
used in concrete production and a reduction in shrinkage
of up to 50 % was reported.6
A limited number of studies concerning the use of
SRAs in AAS are available. C. Bilim et al.9 and also M.
Palacios and F. Puertas10 reported a reduction in shrink-
age through the use of an SRA based on polypropylene
glycol, especially in the case of moist curing. T. Bakha-
rev et al.11 achieved a lower shrinkage thanks to some
non-standard SRA and also when using some air-entrain-
ing agent.
2 EXPERIMENTAL WORK
2.1 Materials and sample preparation
Ground granulated BFS with specific surface area of
400 m2/kg was used as a reference binder and siliceous
sand as a fine aggregate. The sand-to-binder ratio was
2:1. The BFS was activated by liquid sodium silicate
with a SiO2/Na2O ratio of 1.85 at the doses of 4.2 % by
binder mass. The water-to-binder ratio was 0.40.
In the first series the BFS was partially replaced by
low-calcium FA. The weight ratios of the S/FA were
100/0 (marked as S), 75/25 (FA25), 50/50 (FA50) and
25/75 (FA75). The mortar FA50 was additionally modi-
fied using three commercially available SRAs in doses of
0.50, 1.0, 2.0 and 3.0 % by mass of BFS + FA. Based on
the safety data sheets, these SRAs were generally a
mixture of alcohols and glycols. The first one (A) was a
mixture of 5-ethyl-1,3-dioxane-5-methanol, 2-ethylpro-
pane-1,3-diol and 2-ethyl-2-(hydroxymethyl)propane-
1,3-diol, the second one (B) was based on
2,2-dimethylpropane-1,3-diol and 2-butylaminoethanol
and the third one (C) consisted mainly of 2-methoxy-
methylenoxy propanol and propane-1,2-diol.
At the beginning of the mixing procedure, all the
liquid components were combined and the BFS + FA
were added. After 30 s of mixing the sand was added.
The total mixing procedure took 4 min. Then the fresh
mortar was cast into the moulds. After 24 h of moist
curing, the specimens were de-moulded and the strength
or shrinkage testing followed.
2.2 Compressive strength testing
In the case of the first series (BFS partially replaced
by FA), the compressive strength was tested on broken
parts of the specimens (from the three-point bending
test) with the dimensions 20 mm × 20 mm × 100 mm,
while the influence of the different dosages of SRAs was
studied on broken parts of the specimens with the dimen-
sions 40 mm × 40 mm × 160 mm. All these tests were
performed for 24 h, 7 d and 28 d after the mixing. After
their de-moulding, all the specimens were submerged in
water until the start of the test.
2.3 Shrinkage tests
The drying shrinkage was measured as length
changes on the basis of the procedure described in
ASTM C596. After the de-moulding the specimens were
immersed in water, in which they were stored for 3 d.
Then they were taken out to the laboratory conditions
(approximately 50 % relative humidity and 23 °C), their
surfaces were dried with a wet towel and measurements
of their length changes began. Also, their mass changes
during drying were measured and evaluated.
3 RESULTS
The results obtained for the first series, i.e., the
influence of the partial replacement of BFS by FA on the
compressive strength development, the drying shrinkage
and the mass changes during drying, are presented in
Figures 1 to 3, respectively. On the basis of these data,
the mortar FA50 with a relatively low drying shrinkage
and a concurrently satisfactory compressive strength was
V. BÍLEK et al. EFFECT OF A COMBINATION OF FLY ASH AND SHRINKAGE-REDUCING ADDITIVES ...
814 Materiali in tehnologije / Materials and technology 50 (2016) 5, 813–817
Figure 1: Effect of fly ash on the compressive strength of alkali-acti-
vated slag mortar
Slika 1: Vpliv lete~ega pepela na tla~no trdnost malte iz z alkalijami
aktivirane `lindre
chosen for further experiments, where the effect of
different dosages of different SRAs on the compressive
strength (Table 1), the drying shrinkage (Figure 3) and
the mass loss during drying (Figure 4) was investigated.
In order to compare the compressive strengths of the
mortars containing SRAs with the reference mortar FA50
by using the specimens of the same dimensions, the
FA50 mortar was prepared once more. Thus, the desig-
nation R for the FA50 mortar was used in the second part
of this study.
4 DISCUSSION
As can be seen from Figures 1 and 2, the partial
replacement of the BFS in the binder by FA led to a
gradual decrease in the compressive strength, but also to
a significant shrinkage reduction in the case of mortars
with 50 % and 75 % of FA in the binder. These results
correspond with those reported by M. Marjanovi} et al.12,
where the compressive strength of mortars with a pre-
dominant content of BFS in the binder were significantly
higher than those with a predominant content of FA.
Also, the slightly higher shrinkage of the mortar with a
BFS/FA ratio of 75/25 than that of the 100/0 mortar was
recorded in the same study. The increasing content of FA
led to a substantial increase in the mass loss during the
drying process under laboratory conditions, as can be
seen from Figure 3. This could imply that a smaller
quantity of hydration products was formed in the speci-
mens with higher doses of FA, because the alkali activa-
tion of FA is generally favoured by an elevated tempe-
rature and a larger amount of Na2O in the mixture.12
Moreover, the main hydration product of FA activation,
the N-A-S-(H) phase, does not chemically bind water,13
which may also affect the mass changes during drying.
Table 1 shows the impact of SRAs on the compres-
sive strength on the reference mortar R, whose compo-
sition was the same as the mortar FA50. The difference
was only in the size of the specimens for compressive
strength testing, as stated in Section 2.2. However, simi-
lar results for the compressive strength were obtained,
when compared Table 1 and Figure 2. Regardless of the
type of SRA used, the compressive strength of the refe-
rence mortar was markedly reduced when SRAs were
applied. Generally, the compressive strength decreased
with the increasing dose of SRAs. The exception was the
mortar with 3 % of SRA C, where a noticeably higher
strength than that of the mortars with 1 % and 2 % of
this additive were observed during the whole testing
period.
The effect of SRAs on drying shrinkage of the
reference mortar is shown in Figure 4. It is clear that all
the SRAs tested modified the drying shrinkage develop-
ment in a similar way. If only 0.5 % of SRA was used,
the drying shrinkage slightly decreased, while a signifi-
cant decrease in shrinkage was observed when the dose
of SRA was raised from 0.5 % to 1.0 %. A further
increase in the SRA dosage only led to a minor decrease
in the drying shrinkage. A noticeably different situation
was observed for the mass loss during the drying of these
mortars (Figure 5), which exhibited a rather gradual
V. BÍLEK et al. EFFECT OF A COMBINATION OF FLY ASH AND SHRINKAGE-REDUCING ADDITIVES ...
Materiali in tehnologije / Materials and technology 50 (2016) 5, 813–817 815
Figure 3: Effect of fly ash on the mass changes during drying of
alkali-activated slag mortar
Slika 3: Vpliv lete~ega pepela na zmanj{anje mase med su{enjem
malte iz z alkalijami aktivirane `lindre
Figure 2: Effect of fly ash on the drying shrinkage of alkali-activated
slag mortar
Slika 2: Vpliv lete~ega pepela na skr~ek pri su{enju malte iz z alkali-
jami aktivirane `lindre
Table 1: Effect of SRAs (A, B, C) on the compressive strength development (in MPa) of alkali-activated BFS/FA 50/50 (R) mortar
Tabela 1: Vpliv SRA (A, B, C) na razvoj tla~ne trdnosti malte BFS/FA 50/50 (R), aktivirane z alkalijami
Mixture R A B C
SRA dose (%) 0 0.5 1.0 2.0 3.0 0.5 1.0 2.0 3.0 0.5 1.0 2.0 3.0
1 d 8.1 4.4 5.0 – – 4.7 3.0 – – 4.1 3.8 1.2 5.7
7 d 16 9.9 9.1 2.7 1.5 11 5.7 2.4 1.9 7.9 5.7 4.0 11
28 d 54 40 16 3.6 2.8 44 19 2.2 2.2 40 7.6 4.2 14
increase in the mass loss with the increasing content of
SRAs. The reason for such an increase in the mass loss is
likely to be associated with some retardation effect of all
the SRAs used on the hydration of alkali-activated
BFS/FA mortars, which is indicated by its compressive
strength development. The SRAs also modified the
shrinkage rate during the first days of drying, where a
significant reduction as compared with the reference
mortar was observed. After this the period of most
intensive shrinkage followed, while since approximately
the 10th day of exposure to a dry climate, all the mortars
had almost the same shrinkage profile.
Although its porosity was not measured, we can
expect a significant decrease in the amount of hydration
products of the mortars containing 1.0 % and more SRA
and therefore a higher volume of larger pores filled with
easily evaporable water before the start of the drying.
This raises considerable doubts about the compatibility
V. BÍLEK et al. EFFECT OF A COMBINATION OF FLY ASH AND SHRINKAGE-REDUCING ADDITIVES ...
816 Materiali in tehnologije / Materials and technology 50 (2016) 5, 813–817
Figure 5: Effect of SRAs (A, B, C) on the mass loss of alkali-acti-
vated BFS/FA 50/50 (R) mortar during drying
Slika 5: Vpliv SRA (A, B, C) na zmanj{anje mase pri su{enju malte
BFS/FA 50/50 (R), aktivirane z alkalijami
Figure 4: Effect of SRAs (A, B, C) on the drying shrinkage of alkali-
activated BFS/FA 50/50 (R) mortar
Slika 4: Vpliv SRA (A, B, C) na kr~enje pri su{enju malte BFS/FA
50/50 (R), aktivirane z alkalijami
of the tested SRAs with the studied alkali-activated sys-
tem. Some of the authors mentioned in the introduction
achieved a significant reduction of the drying shrinkage
without any significant changes in the AAS hydration,
but they used SRAs based on polypropylene glycol,
while the SRAs used in this study mainly comprised
low-molecular-weight diols.
5 CONCLUSIONS
This paper looks at the possibilities of drying-shrink-
age reduction for mortars based on AAS by FA and
SRAs. The compressive strength and mass changes
during the drying were also evaluated. From the results
obtained it can be concluded that despite a certain
strength decrease the FA can be successfully used for the
shrinkage reduction of AAS. Considering both the
compressive strength and the drying shrinkage, the
mortar with the ratio of BFS/FA 50/50 seems to be the
most favourable FA. A further decrease in the shrinkage
of this mortar was achieved through the application of
commercial SRAs based mainly on diols and designed
for Portland-cement-based systems. However, all these
SRAs had a very negative impact on the compressive-
strength development, which makes them unsuitable for
the studied alkali-activated BFS/FA system.
Acknowledgement
This outcome was achieved with the financial support
by the project: Materials Research Centre at FCH BUT-
Sustainability and Development, REG LO1211, with
financial support from National Programme for Sustain-
ability I (Ministry of Education, Youth and Sports).
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