CONCRETE LIBRARY OF JSCE N0. 30, DECEMBER 2000
ASR GEL COMPOSITION, SECONDARY ETTRINGITE FORNIATION, AND EXPANSION OF
MORTARS IMMERSED IN NaCl SOLUTION
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(Translation from Proceedings of JSCE, No.641/V-46, February 2000)
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Kenji KATAFUTA
In this study, we pursue the changes in microstructural features and characteristics of ASR gel composition
in mortars containing reactive aggregate after immersion in NaCl solution. These characteristics are
compared with their expansion behavior. Mortars immersed in NaCl solution undergo greater expansion
than those in NaOH solution, and this may be a result of the formation of ASR gel with a relatively low
alkali content. However the coexistence of ettringite with ASR gel is indicated by EDS analyses, so it is
not possible at this stage to infer whether the formation of secondary ettringite increases mortar expansion
or not.
Keywords: ASR, NaCl,- de-icing salt, ASR gels, ettringite, SEM, EDS-BSE analysis
Mitsunori Kawamura is a Professor in the Department of Civil Engineering at Kanazawa University,
Ishikawa, Japan. He has been active in cement and concrete research for 35 years. His reseach interests
include the alkali-silica reaction, concrete durability, and the microstructure of cement-based matetials. He
is a fellow of JSCE.
.
Noriyuki Arano is a researcher in the Division of Cement at Denki Kagaku Kogyo Co. Ltd., Niigata, Japan.
He obtained his D.Eng. from Kanazawa University in 2000. His reserch interests relate to the reaction
products and compatibility of aggregate in concrete. He is a member of JSCE.
Kenjii Katafuta is with the Goyou Kensetsu Co.Ltd.. He obtained his M.S. Degree from Kanazawa
University in 1999.
—273---
1. INTRODUCTION
The effect of NaCl contributed
by deicing salt or seawater on ASR damage to concrete is an important
issue in the field of concrete structure maintenance. A recent survey of the effects of deicing salt on
ASR-induced expansion reported that mortar containing a reactive aggregate may exhibit greater or lesser
expansion when immersed in NaCl solution than in water, depending on the reactivity of the aggregate [1].
Another field investigation,
this time on ASR-affected concrete piles in a partially
submerged marine
environment, has revealed that ASR causes significant damage to the submerged portions of even relatively
newmarine structures, but not to the above-water portions [2]. Thus, if reliable repair of ASR-affected
structures in saline environments is to be achieved, a better understanding of the mechanisms by which
NaCl influences ASR is crucial [3].
The mechanism by which NaCl influences the expansion of hardened
concrete containing reactive aggregate has been studied by a number of groups. At least part of the
expansion in mortar containing reactive aggregate is certainly due to a rise in the OHion concentration in
the pore solution, which is brought about by intrusion of NaCl, and Cl'ions themselves accelerate ASR in
concrete [4,5], However, recent reports [6,7] have suggested that CaA and gypsum as well as alkalis in the
cement play an important role in the expansion of mortar containing a reactive aggregate. In fact, SEM
examinations of opal-containing mortars immersed in 1M NaCl solution for longer than a year showed that
large amounts of chloride-bearing
ettringite had formed in the mortars [8]. However, ambiguity remains as
regards the mechanism by which the formation of secondary ettringite is related to mortar expansion in an
NaCl solution.
The aim of this study is to pursue the changes in microstructural
features indicated by BSE-EDS analysis
and the characteristics
of ASR gel composition in reactive aggregate-containing
mortars when immersed in
NaCl solution, and to relate them to the expansion behavior of the mortars.
2. EXPERIMENTAL OUTLINE
2.1 Materials
Calcined flint with a size fraction of 2.5 mmto 0.6 mmwas used as the reactive aggregate. Its dissolved
silica (Sc) and alkalinity
reduction (Re) in the ASTM chemical test (C 289) were 1063 and 70 mmol/1,
respectively. Japanese Toyoura standard sand was used as a non-reactive aggregate. The CsA content of the
ordinary and sulfate-resisting
Portland cements used was 9.2% and 3. 1%, respectively;
the Na20 equivalent
of the two was 0.67% and 0.59%, respectively. The chemical and mineral compositions of these cements,
as calculated by the Bogue method, are given in Table 1.
Table 1 Chemical
S iO s
Composition
A ha
of Ordinary
O rd in a ry
2 2 .0
5 .2
F e 2O 3
2 .7
C aO
6 3 .6
S u lfa te -re si stin e
2 2 .2
4 .2
4 .6
6 4 .1
M
eO
and Sulfate-Resisting
Portland
Cement (%)
IG O
0 .4 1
4 7
c zs
2 7
C sA
1 .9
N ifc O
0 .4 0
C 3S
1 .5
9 .2
C 4A F
.2
0 .4
2 .2
0 .3 4
0 .3 8
5 1
2 5
3 .4
1 4 .0
S O 3
2.2 Mortar Specimens for Expansion Testing
Mortar was prepared with a total aggregate to cement ratio of 2.0 and a water-cement ratio of 0.5. Reactive
aggregate replaced 30% by mass of the non-reactive aggregate. Mortar bars measuring 25.3 by 25.3 by
285.5 mm,were cured in a sealed state for 28 days, then immersed in 1M NaCl solution at 38 °C. For
expansion tests in 0.6M NaOH solution, a further series of mortars was prepared with an aggregate-cement
ratio of 2.25 and a water-cement ratio of 0.6, with 10% by mass of aggregate replaced by reactive
aggregate. The same cements were used [9].
2.3 BSE-EDS Analysis
Procedure
At 14, 56, 119, and 406 days after immersion in NaCl solution, mortar slices were cut from the center of
the mortar bars for BSE-EDS analysis. These were dried by ethanol replacement, followed by overnight
-274-
vacuum drying at room temperature. The dried slices were impregnated under vacuumwith ultra-low
viscosity epoxy resin. The polished surfaces were sputter coated with a 30 nm layer of gold-palladium
alloy. The samples were then examined using a Hitachi S-2250N SEM equipped with a back scatter
detector and a Horiba EMAX-5770W energy-dispersive
X-ray analyzer.
3. RESULTS
3. 1 Expansion Characteristics
of Mortars in Nad and NaOH Solutions
The expansion curves for mortars made with both
the ordinary and sulfate-resisting
Portland cements
and immersed in 1M NaCl and 0.6M NaOH
solutions are presented in Fig.l. The mortars are
found to expand rapidly
immediately
upon
immersion in both solutions. The rapid progress of
expansion in NaCI solution was maintained up to
about 120 days, reaching a maximumvalue of
about 1.5% in the case of ordinary cement. In the
later stages, the mortars continued to expand at a
lower rate. Even at an early age, however,
expansion rates in NaOH solution were lower than
those in NaCl solution. The expansion rate for
mortars in NaOH solution rapidly decreased after
about 100 days of immersion, reaching an ultimate
expansion of about 1.0%. As Fig.l demonstrates,
the expansion of sulfate-resisting
cement mortars
was less than two thirds that of ordinary Portland
cement mortars.
3.2 ASR Gel Composition
in Immersed Mortars
100
200
300
400
500
600
700
800
TIME (DAYS)
Fig.l
Expansion curves for mortars in NaCl and
NaOHsolution
Careful SEM examinations over the whole of the polished face of mortar samples cured for 28 days in a
sealed state at 38 °C showed that the cement paste developed the normal microstructural features and that
large amounts of ASR gel were present within relatively wide cracks in reactive aggregate particles. Lowand high-magnification BSE micrographs of a reactive aggregate particle found in the polished section of a
mortar sample immersed in NaCl solution for 56 days are presented in Photograph 1. Here, there are many
cracks within the reactive aggregate particles and some cracks are filled with large amounts of ASR gel.
The average mole percentages of SiOz, CaO, and (NaaO+BGO) obtained by BSE-EDS analysis at about 10
spots in ASR gel areas in five different reactive aggregate particles are plotted in the ternary phase diagram
in Fig. 2. These particles were selected from mortars pre-cured for 28 days in a sealed state. Figure 2 also
shows that little change occurred in the mole percentages of (Na2O+IGO) in the gels during immersion in
NaCl solution. However,it is found from the plots of KzO/NasO content ratios at various immersion times
(Fig.3) that potassium in the ASR gels produced in mortars during the pre-curing period was replaced by
sodium during the relatively
early stages of immersion in NaCl solution. Additional
alkalis were not
incorporated in the gels; only Na* and K+ions were exchanged. The replacement of potassium by sodium
mayhave no effect on the expansion ofASR gels [10].
The results of SEM observations of mortars immersed in both NaCl and NaOH solutions show that more
ASR gel is produced in mortars immersed in NaOH solution than in mortars immersed NaCl solution.
However,although the initial content of reactive aggregate in mortars immersed in NaCl solution was three
times that of mortars in NaOH solution (according to the mix proportions), the amount of reactive silica
even in NaOH-immersed mortars may be high enough to maintain the progress of ASR. Unlimited amounts
of OJTions would be supplied to mortars in NaOH solution and the OH ion concentration of the pore
solution of these mortars is higher than that of mortars in NaCl solution. In NaOH-immersed mortars, the
formation of ASR gel with low viscosity and higher fluidity,
as a consequence of the higher alkali content
associated with increased OH ion concentration, may be lead to greater expansive stress relaxation.
-275-
(b)
Photograph
1 BSE micrographs for reactive aggregate particle (a) and ASR gel (b) in ordinary
cement mortar immersed in NaCl solution for 56 days
portland
C
o
Si02
o-A-i oo
Mole Percent
0.4
cc
LU
100
Na20+K20
Fig.2
^i ob
Ternary phase diagram for ASR gels in
mortars
3
IMMERSION
Fig.3
Mole Percent
106
NaaO+KaO
Fig.4
Ternary phase diagram for ASR gels in
mortar s
-276-
00
400
TIME (DAYS)
KzO/NaaO ratios of ASR gel in mortars
immersed in NaCI solution
As shown in Photograph 2, gel remained inside reactive aggregate grains even in mortar immersed in NaCl
solution til the age of 120 days. On the other hand, it is confirmed that gel in NaOH solution was carried
outside the reactive aggregate grains as a result of its low viscosity.
The CaO content of the gel was also investigated, and shown to be the same level in both NaCl and NaOH
immersion. This demonstrates that CaO content has little effect on the expansion behavior of mortars. In a
separate study [9] the authors confirm this limited effect of CaO content in the ASR gel on the expansion
of mortar.
(a) NaCl solution
Photograph
2
3.3 Microstructural
SEM micrographs
(b) NaOH solution
for ordinary cement mortars immersed in NaCl solution
solution for 120 days
and NaOH
features of Cement Paste in Mortars in NaCl Solution
SEM observations turned up no particularly characteristic
features in the microstructure of cement paste in
mortars immersed in NaCl solution for 14 days. However, after 56 days in NaCl solution, many
ASR-induced microcracks were found to be partly filled with ettringite. In the BSE images in Photograph
3, belts of ettringite are seen in cracks approximately 10 um wide by 50-200 um long. No rim cracks filled
with ettringite
were found around aggregate particles,
and particles of ettringite
appear to partly fill
ASR-induced cracks propagating through the cement paste. However, not all cracks were filled with
ettringite,
although cracks caused by ASR cannot be differentiated
from those caused by sample
preparation.
Welooked at 10 different areas of ettringite deposits and carried out EDS analysis for about 5 spots in
each individual
area for a total of about 50 spot analyses of ettringite deposits in the polished face of a
mortar sample. A typical EDS result was that relatively large amounts of (NazO+BGO) were detected when
analyzing string-like
ettringite
areas. The mole percentages of (NazO + l&O) and SiOa obtained by EDS
analysis are plotted in Fig.5. Part of the SiCh and (NazO + IGO) content may result from some
contamination that occurred during analysis. It should be noted that the alkali contents obtained in these
analyses were considerably higher than the 2% found as an impurity in ettringite [1 1]. The CaO, AhOa, and
SOs contents, obtained by excluding other elements being normalized to 100 percent , are plotted in a
ternary phase diagram shown in Fig. 6. The plots lay on and around a line connecting the stoichiometric
composition of ettringite with points obtained for ASR gel in air voids within the mortars. Thus, these
results show that string-like ettringite deposits found in cracks are mixtures of ettringite and ASR gel. The
movementof plotted points towards the CaO apex over time means that the proportion of ASR gel in the
string-like deposits and/or the CaO content of the ASR gel itself increases with time. It should be noted
that we found no such ettringite deposits in ASR-induced cracks in the sulfate-resisting
cement mortars.
-277-
55fc£<?
(a) reactive aggregate particle
(b) cracks filled
Photograph
3
with ettringite
deposits
BSE micrographs for ordinary
cement mortar immersed in
NaCl solution for 56 days
(c) ettringite
12 \- Immersion
O^
6
Time 56 days
V^"
.
%à"
^?
o
5
SiO,
Fig.5
Alkali
and silica
10
(MOLE%)
contents
in cracks
of ettringite
deposits
Fig.6 Ternary phase diagram of ettringite
in cracks
4. DISCUSSION
As observed above, some cracks in the cement paste phase were filled with ettringite. The formation of
ettringite like this requires a supply of Ca2+,SO42",and Al(OH)4" ions. It has been shown that the solubility
of ettringite in the system consisting of ettringite-water-NaCl
is about three times that in the ettringite-water
system, and that monosulfate-water-NaCl systems are less stable than ettringite-water-NaCl systems [12]. It
275-
Table
2
Expansion,
Ettringite
String-like
Formation in Cracks, ASR Gel Composition, Width
Ettringite Areas in Ordinary Portland Cement Mortars
T im e(D ay s)
E x pan sion
String in
-likCerack
E ttring
s ite
0
N o ne
N o ne
14
56
119
V ery A ctiv e V ery A ctiv e V ery A ctiv e
N o ne
P resent
P resen t
A v erag e W id th o f
B a n d A re a s o f
E ttrin eite
A v erag e L en gth of
B a nd A rea s of
E ttrin eite
S tate o f A S R G el
N o ne
N o ne
9.4 u m
N o ne
N o ne
S o lid
S olid
and Length
26 6
S lo w ly
P resent
40 6
V ery S low ly
P re sent
12 .4 um
17 .8 um
14 .O u m
37 0u m
2 34 um
2 72 um
2 13 u m
S olid
S o lid
P a r t ly
E v acu ated
M o s t ly
E vacu ated
of
is known that SO/" ions are released during the conversion of the monosulfate to Friedel's salt under NaCl
attack. So, taking into consideration the fact that secondary ettringite deposits were hardly found in cracks
in the cement mortar samples made with a sulfate-resisting
cement, despite immersion in NaCl solution up
to 406 days, the monosulfate can be concluded to be a major source of SO/" ions.
As shown in Fig.l, the expansion of the sulfate-resisting
cement mortar was much less than that of mortar
containing ordinary cement. The equivalent percentages of NasO in the two types of cement are little
different, but the amount of unreacted CsA in the sulfate-resisting
mortar when immersion began would
have been much less than in the latter. As a result, it is impossible
to infer whether the much lower
expansion of sulfate-resisting
cement mortar in NaCl solution was due to the absence of secondary
ettringite formation or not.
In order to elucidate the role of ettringite formation in the expansion of mortars, we tabulate the rate of
expansion, ettringite formation, average width and length of ettringite-filled
cracks, and the state of ASR
gel within aggregate particles for various immersion times in Table 2. The width and length values given in
this table are averages of 10 measurements for different cracks filled with ettringite. The width of these
ettringite-filled
cracks appears to increase with increasing expansion. However,even after immersion for
406 days, an expansive ASR gel with relatively low alkali content was present within reactive aggregate
particles,
although a great portion of this gel had been evacuated from some of the particles. Thus, it is
unclear if secondary ettringite formation led to increased mortar expansion. Grattan-Bellow [13] and Davis
& Oberholster [14] have reported that the increased expansion in concrete supplied with NaCl is induced
not only by ASR as promoted by the increasing concentration of Na+and OH"ions formed in the reaction
between NaCl and CsA or Ca(OH)j duced, but also by the production of expansive monochloro-aluminate
and chloro-surfo-aluminate. As far as these results of immersion tests in NaCl solution up to 406 days are
concerned, it is not possible to infer whether the formation of secondary ettringite
in many pre-existing
ASR-induced cracks increased mortar expansion or not. The greater expansions of mortars in NaCl solution
than in NaOH solution found in this study could result from the production of low-alkali ASR gel with
high viscosity, which would give rise to lasting expansive pressure within the mortar over a long period.
5. CONCLUSIONS
The conclusions reached in this investigation
can be summarized as follows:
(1) Mortar immersed in 1M NaCl solution expanded more than that in 0.6M NaOH solution. The greater
expansion of mortar in NaCl solution might result from the formation ofASR gel with relatively low alkali
content and high viscosity.
(2) Many ASR-induced micro-cracks were found to be partly filled with ettringite in mortars containing
reactive aggregates and immersed in NaCl solution for periods longer than 14 days.
(3) String-like
ettringite deposits found in cracks were mainly a mixture of ettringite and ASR gel.
(4) From this work, it is impossible
to infer whether the formation of ettringite in ASR-induced cracks
increases mortarexpansion or not.
279
-
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