Cement & Concrete Composites 18 (1996) 67-76
0 1996 Elsevier Science Limited
Printed in Great Britain. All rkhts reserved
SO958-9465(96)00002-9
ELSEVIER
Interfacial Interactions in Lightweight Aggregate
Concretes and their Influence on the Concrete
Strength
R. Wasserman & A. Bentur *
National Building Research Institute, Faculty of Civil Engineering, Technion, Israel Institute of Technology,
Israel
(Received 14 April 1995; accepted 28 January 1996)
INTRODUCTION
Abstract zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
There is considerable increase in the application of high strength lightweight concretes using
lightweight
aggregates.
Therefore,
there
is
renewed interest in the role of aggregates in
such concretes, particularly since they are the
weak link in these systems. Usually, the strength
of such concretes is believed to be controlled by
the strength of the aggregates and the strength
of the paste. Recent studies into the microstructure of such concretes
suggest
that the
interaction of the lightweight aggregate and the
matrix can be quite different
than that of
normal aggregate. i-’ The interfacial microstructure can be quite similar to that of normal
concrete if the aggregates are wetted, but it can
be considerably different and much denser if
the aggregate is used dry. In some studies it has
that
pozzolanic
reaction
been
suggested
between the aggregate and the paste matrix is a
factor contributing to strength43 while in others
no such interaction was observed.337
In view of the influence that the lightweight
aggregate-paste
matrix interaction may have on
the concrete strength, a systematic study was
undertaken
with sintered fly ash lightweight
aggregates to resolve some of the mechanisms
by which they interact with the matrix, and
assess their potential influence on the strength
of the concrete. To achieve this goal, commercial Lytag aggregate was modified by heat and
polymer treatments
to obtain aggregates with
variable properties, i.e. differences in strength,
absorption and pozzolanic activity. The inter-
The interactions betw een sintered fly ash lightweight aggregates and the matrix in portland
cement concretes was studied to resolve factors
other than aggregate strength which influence the
concrete strength. Aggregates of variableproperties
w ere produced and concretes of equal effective
waterlcement ratio w ere prepared and tested for
strength and microstructure. It was found that differences in concrete strength could not alway s be
accounted for by diflerences in the aggregate
strength. These trends could be related to phy sical
and chemical interfacial processes, which have an
influence on the overall strength beyond that of
the aggregate strength. The phy sical process identified was densification of the interfacial transition
zone due to absorption of the aggregates; this process has considerable influence already at early
age. The chemical processes w ere associated with
pozzolanic activity of the aggregate and deposition
of CH in the pores in the shell of the aggregate;
these processes became effective only at later age,
beyond 28 days. The enhancement in strength due
to these influences ranged betw een 20 and 40% .
Such influences should be taken into account
when predicting the concrete strength or in the
design of lightweightaggregate of optimal properties.
Key words: Aggregates,
concrete
strength, fly ash.
*Corresponding author.
67
68
R. W asserman, A. Bentur
action of these aggregates with the paste matrix
was studied. In a previous paper9 the properties
and microstructure
of the aggregates as well as
the strength of concretes of equal effective
water/cement ratio were reported. It was shown
that the differences in strength could not be
accounted only on the basis of the aggregate
strength. In the present paper the microstructural study of these concretes is reported, in
particular
the interfacial
characteristics.
The
interfacial processes resolved can explain differences in the strength of lightweight aggregate
concretes which could not be accounted
for
merely by the strength of the aggregate.
EXPERIMENTAL
They were all demolded after one day and than
cured continuously in water at 20°C up to 90
days. In the present study additional concretes
were prepared also using normal aggregate, and
curing of the lightweight and normal aggregate
concretes at elevated temperature
to resolve
pozzolanic effects. This curing procedure consisted of 28 days in water at 20°C followed by
water curing at 60°C up to a total curing time of
90 days.
The concretes were tested periodically for
compressive strength (70 mm cubes), and their
internal structure was characterized
by SEM
observations and micro-chemical analysis of polished
surfaces.
The
specimens
were
impregnated with low viscosity epoxy and polished to obtain a flat surface for micro-analysis.
Energy dispersive X-ray analysis was used to
determine the Ca and Si contents, and profiles
of C/S ratio across the aggregate-paste
interface were drawn.
The lightweight aggregates used in the present
study were sintered fly ash. Commercial Lytag
(UK) aggregate was treated by heat (heating to
1200, 1250 and 1300°C and rapid or slow cooling
afterwards)
or by polymer
(partial
impregnation
and hydrophobization)
to obtain
RESULTS AND DISCUSSION
aggregates with controlled difference in absorption and pozzolanic reactivity. Details of the
SEM observations
aggregate preparation
and their internal structure are given in Ref. 9. Table 1 summarizes
SEM observations were carried out at one day
their main characteristics.
It can be seen that
and at a later age of 90 days. At one day there
they can be classified according to their strength
seemed to be intimate contact between the
into two groups with each having aggregates of
aggregate and the matrix only in the concrete
similar strength but different in other properfrom the original Lytag aggregate (Fig. l(a)). In
ties. Accordingly,
the concretes
were also
all the treated
aggregates
separation
was
classified into two groups.
observed, as shown for two examples in Fig.
Concretes
were prepared
with aggregate
l(b) and (c). The separation does not necesblend of 51% lightweight aggregate and 49%
sarily mean that there is a void in the interfacial
graded normal weight fine aggregate, at an
zone; it could reflect the presence of a very
effective w/c ratio of 0.40 (after allowing for the
porous
material
which shrinks considerably
early absorption of the aggregate). Details of
during the drying in the preparation
of the
concrete compositions and preparation methods
specimens for the SEM observation, with evenand their strength values are given in Ref. 9. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCB
tual separation.
Indeed, observations
at the
Table 1. Properties of lightweight aggregates
Group
I
II
Name
Treatment
Ly tag
untreated
125O”C,rapid cooling
13OO”C,slow cooling
12OO”C,rapid cooling
polymer
L1250
L13oOSC
L1200
polymer
*Determined on the basis of reaction with CH.
Crushing
strength,
M Pa
W ater
absorption,
% Vol.
90 Day s
pozzolanic
activity of
the outer shell *
14.9
14.7
15.6
19.3
19.9
14.5
9.7
9.2
10.6
3.8
0.15
0.40
0.52
0.28
-
Interfacial interactions in lightweightaggregate concretes
69
after the aggregate
paste surface exposed
removal show a denser microstructure
in the
concrete with the untreated
Lytag aggregate
(Fig. 2(a)) and much more porous material in
the treated aggregates
as seen in the two
examples shown in Fig. 2(b) and (c). It should
be noted that the differences are not only in the
overall porous nature but also in the type of
microstructure
detected, showing more needlelike material, apparently ettringite, in Fig. 2(b).
At 90 days there seems to be intimate contact
at the interface in all of the concretes, except
the one with the polymer treated aggregate,
which even at 90 days showed separation at the
Fig. 1. The aggregate-matrix
interface at one day: (a)
untreated Lytag, (b) L1250 and (c) polymer treated.
Fig. 2. View of the matrix side of the aggregate-matrix
interface: (a) untreated Lytag, (b) L1300 and (c) polymer
treated.
70
R. W asserman,A. Ben&r
interface (Fig. 3). The surface of the original
Lytag and heat treated aggregates is quite fuzzy
and the structure of the aggregate is masked
without being able to resolve its porous nature
(Fig. 4(a) and (b)). It seems that the pores in
the aggregates have been penetrated by hydration products, and in the case of the untreated
Lytag aggregate there is evidence for presence
Fig. 3. The aggregate-matrix
interface at 90 days: (a)
untreated Lytag, (b) 1300SC and (c) polymer treated.
of large crystals in the pores (Fig. 5) whereas in
the treated
aggregates
no clear crystalline
morphology could be observed. In contrast to
these aggregates, the polymer treated aggregate
preserved its original nature, and remnants of
fused fly ash particles and some pores could be
clearly observed (Fig. 4(c)).
Fig. 4. View of the aggregate side of the aggregatematrix interface at 90 days: (a) untreated
Lytag, (b)
Ll3OOSC and (c) polymer treated.
Interfacial interactions in lightweight aggregate concretes
71
Composition of the interfacial transition zone
The 1 day and 90 day compositions across the
interface from the paste into the aggregates are
provided in Figs 6-8, in terms of C/S ratio profiles. The C/S ratio of the aggregate shell itself
is about 0.15 and that of the bulk paste is about
3. It can be seen in Figs 6-8 that in the vicinity
of the actual interface there are deviations from
these bulk values which extend into the paste
matrix as well as into the aggregate.
At one day the C/S ratio curves drop sharply
just at the interface to the values typical of the
aggregate shell (-0-E).
This is characteristic of
all the three aggregates in Figs 6-8. However,
in front of the interface, towards the paste
matrix, there is a difference between the untreated Lytag (Fig. 6) and the heat treated one
(Figs 7 and 8). In the former the C/S curve is
Fig. 5. High magnification
of the aggregate surface at
the aggregate-matrix
interface in 90 days cured concrete
with untreated Lytag aggregate.
lgregate
___-,
,
matrix
- - I
\
I
\
\
*, 90 days
I day
-100
0
-50
50
100
150
200
250
300
distance, microns
Fig. 6. C/S ratio profiles
aggregate.
across the aggregate-matrix
matrix
aggregate
interface
in 1 and 90-day-old
concretes
J
-50
0
50
100
150
200
250
300
distance, microns
Fig. 7.
C/S ratio profiles across the aggregate-matrix
Lytag
zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
0.1
-100
with untreated
interface
in 1 and 90-day-old concretes with L1250 aggregate.
R. Wasserman, A. Bentur
72
1 .-
I
\ . 90 days
.
\
.
\
*
0.1 7
-100
-50
0
100
50
150
200
250
300
distance, microns
Fig. 8.
C/S ratio profiles across the aggregate-matrix
interface
almost horizontal at a value of -3.5, from the
bulk paste up to the actual interface. In the
latter there is a zone in front of the actual
interface towards the bulk matrix where the C/S
ratio is high, about 10-40, and it extends over a
distance of several tens of microns. This trend is
also characteristic
of the two other treated
aggregates, at 1200°C and the polymer treated
one (not shown here). The width of the interfacial transition zone in front of the aggregate
can be estimated as the zone in which the C/S
ratio is high, and it is about 4, 25 and 40 ,um for
the original Lytag aggregate and the 1200 and
1300°C treated ones, respectively. The lower
value for the untreated
aggregate
may be
accounted for by the higher absorption which
prevents accumulation of water at the interface;
accumulation
of such water in the fresh concrete is believed to be the cause for the
formation of the porous zone in the vicinity of
the aggregate
surface, resulting in a more
porous microstructure which is rich in CH. Both
of these characteristics were observed here, in
the SEM micrographs and in the composition
profiles. A plot of the width of the interfacial
transition zone as a function of the 6 h water
absorption of the aggregates (Fig. 9) shows a
clear relation, with the width of the interfacial
transition zone decreasing with the increase in
the absorption, as might be expected on the
basis of the explanation provided here.
This trend changes at 90 days, where a high
C/S ratio curve seems to be ‘penetrating’ deep
into the aggregate in the untreated Lytag aggregate (Fig. 6), whereas in the heat treated
aggregates, at 1250 and 13OO”C, the C/S ratio
curve is declining gradually within the aggregate
in 1 and 90-day-old concretes with L13OOSC aggregate.
boundaries, from values of about 3 at the interface to the characteristic aggregate C/S .ratio of
about O-15. This occurs at a distance of about
several tens of microns into the aggregate (Figs
7 and 8). These results suggest that in the original Lytag aggregate
the pore solution can
penetrate
effectively into the pores of the
aggregate, due probably to its large absorption
capacity, and deposits of CH are formed in the
aggregate pores, as evidenced by the high C/S
ratio and the SEM observations. In the higher
temperature
treated aggregates there is probably absorption
of this kind, although to a
smaller extent, and since the aggregates are
more pozzolanic,
CSH rather than CH is
formed preferentially.
In these aggregates, the
45
40
35
30
25
20
15
10
5
0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIH
1
2
0 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSR
3
6 hours absorption,
Fig. 9. Relation
between
interfacial
width at one day and the 6 h absorption
aggregate.
%vol.
transition
zone
of the lightweight
InterJacial interactions in lightweightaggregate concretes
73
Within each group of equal aggregate crushing
strength (groups I and II) the concrete strength
values are not equal. In group II the polymer
treated aggregate
is consistently
weaker by
about 20% than the heat treated aggregate. In
group I the trends change with time: at early
age the strength of the concrete is smaller for
the higher temperature
treated aggregate, but
with time the differences diminish, and by 90
Strength
days they are all practically equal. When comThe effect of the aggregate type on the strength
paring between the groups it can be seen that
levels at 1, 28 and 90 days is shown in Fig. 10.
the higher strength heat treated aggregate in
group II provides a concrete which at early age
is stronger than all the concretes in group I
which is in agreement with the higher strength
(a) zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
of the L1200 aggregate in group II. However,
14
this difference diminishes with time and by 90
0” 12
days the concretes of group I are practically of
f
10
the same strength as the L1200 aggregate con5
8
crete in group II, inspite of the higher crushing
f
e
6
strength of the latter aggregate. The polymer
IO
treated
aggregate in group II always gave a
or 4
weaker concrete than those in group I, inspite
:
2
of its higher strength compared to all of the
0
aggregates in group I.
The fact that the trends in strength observed
at one day change over time, suggests that there
@)
is a ‘dynamic process’ whose nature can be bet60
ter resolved when plotting for each of the
g 50
systems the strength values relative to the 28
days strength (Fig. 11). This figure contains
$40
the relative curve for
also, for comparison,
t” 30
normal aggregate concrete of the same w/c
=z 20
ratio. Up to 28 days the relative strength values
reduction in C/S ratio in the interfacial transition zone in front of the aggregates from high
values of up to 20 at 1 day, to about 3 at 90
days suggest that the larger deposits of CH at
early age were the source for the Ca ions that
penetrated
into the aggregate,
to become
involved in the pozzolanic reaction.
z
N
10
0
z
(cl
P
70
g
60
r‘ 50
‘b
$40
Z
n 30
;
20
;
10
0
0
20
40
60
Am
60
100
Davy
Fig. 11. Development
of relative strength over time in
Fig. 10. Compressive strength values at 1, 28 and 90
the different concretes (strength values for each concrete
days of concretes of groups zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
I and II.
relative to its 28 days strength).
74
R. Wasserman, A. Benrur
similar in all of the concretes. Beyond that
they can be classified into three types: (a) small
increase in strength, less than 5% at 90 days
(characteristic of the normal aggregate concrete
and the aggregates of group I polymer
treated and 1200°C treated), (b) mild increase
in strength, about 20% at 90 days (characteristic
of the untreated Lytag aggregate concrete), and
(c) marked increase in strength, of about 40%
at 90 days (characteristic of the concretes with
aggregates treated at 1250°C and 1300°C).
The differences in the trends at early age and
later ages suggest that different mechanisms
control the strength values at these two ages
and they are superimposed on the effects of the
aggregate strength. When comparing the differences at early age in concretes prepared from
aggregates of similar strength it seems that the
lower strength concretes
are the ones with
aggregates of smaller absorption capacity, as
can be seen from the relations in Fig. 12. This
trend can be explained by the formation of a
more porous interfacial transition zone with the
lower absorption aggregates (Fig. 9). This also
shows up in the SEM observations indicating
aggregate-matrix
separation
in the lower
absorption aggregates (Fig. 1).
At later ages, the SEM observations indicate
a very dense transition zone in all of the concretes except the one of the polymer treated
aggregate (Figs 3 and 4). This may account for
are
71
0
1
2
6 hours absorption,
3
4
%vol.
Relation between the one day compressive
strength in group I concretes and their 6 h water absorption.
Fig. 12.
the much lower strength values of the concrete
with the polymer treated aggregate which is
maintained
even at this age. In all other concretes
the interfacial
transition
zone seems
much denser than in normal strength aggregates
and it is difficult to resolve the border between
the aggregates and the matrix (Figs 3 and 5), in
contrast to normal aggregate concrete where
deposits of CH and some porosity can be
observed at the interfacial transition zone.“’
Although the interfacial region was dense in all
the heat treated and untreated
Lytag aggregates, differences in the composition
of this
zone were seen. In the L1250 and L13OOSC
aggregates, the C/S ratio curves suggest the
presence of CSH within a zone of about 50 pm
within the outer shell of the high temperature
treated aggregate, which could be correlated
with their higher pozzolanic activity (Table 1).
This can account for the relatively high increase
in the strength of the concretes prepared from
these aggregates after 28 days (Fig. 11). It more
than compensates for their lower initial strength
due probably to some reinforcement
of the
outer shell of the aggregate and bond enhancement due to the continuity in the CSH outside
and inside the aggregate. In the concretes prepared with the lower temperature
heat treated
aggregate (1200°C) and the normal aggregate
no pozzolanic activity was observed and no significant strength increase occurred beyond 28
days. The concrete from the untreated
Lytag
aggregate did not seem to behave according to
these trends, and although it was not pozzolanic
it showed some strength enhancement
beyond
28 days. This might be explained by the formation of CH deposits
in the shell of this
aggregate which probably strengthens it, but are
probably less effective in enhancing the bond as
suggested to occur in the pozzolanic active
aggregates.
In order to check the hypothesis of the influence of pozzolanic activity of the aggregates,
three concretes were cured beyond 28 days in
60°C water: concretes with normal aggregate
and the original Lytag, both of which are non
pozzolanic, and the concrete with the 1300°C
aggregate (1300SC) which had the highest pozzolanic activity. The results presented in Fig. 13
show, as expected, that the high temperature
curing did not affect the first two concretes.
However it enhanced the strength of the third
concrete, which could be attributed to mobilizing to a greater extent its pozzolanic activity.
75 zyxwvutsrq
interfacial interactions in lightweight aggregate concretes
118
116
114
--
I -0
--
112
--
110
--
108
.-
I
D’
L13oaSC
-.-
I
.--
I
- I -
--•
l
l
l
l
106
--
104
102
--.
. ,
.
,oo,
l
l
.
l
.
_:
25
l
30
-1
l
lytata
normal
/
l
;
35
“_;“_--*c-40
45
,----_-*--,50
55
60
65
/__-9
70
75
60
85
90
age, days
Fig. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA
13. Relative strength curves of concretes cured at 60°C beyond 28 days (strength
curing).
CONCLUSIONS
(1) Lightweight
aggregates of similar strength
do not necessarily yield concretes
of
equal strength even if the matrix is of the
same effective w/c ratio. Thus, factors in
addition to the aggregate strength should
be considered.
(2) The additional influences that should be
considered were found to be associated
with the physical and chemical characteristics of the aggregates, both of them
affecting the overall strength by processes
which take place at the interfacial transition zone. In the aggregates this zone
extended also into the aggregate itself.
(3) The physical process occurs at early age
and is governed by the absorption
of
water into the aggregate. Higher absorption eliminates accumulation of water in
the fresh matrix in the vicinity of the
aggregate. As a result the interfacial transition zone in lightweight aggregates of
higher absorption
is denser. Thus, for
lightweight aggregates of equal strength,
the aggregate of higher absorption will
provide higher strength concrete due to
its denser interfacial transition zone.
The
chemical process occurs at later age.
(4)
Two types of processes were resolved
here: pozzolanic reaction between the
aggregate and the alkaline pore solution
which penetrates into it, and an ‘impregnation’ process in which CH deposits in
the pores of the aggregates. The latter is
more likely to occur in aggregate having
higher absorption and bigger pores.
values at 60°C relative
to 20°C
(5) The contribution of the processes identified in enhancing the concrete strength
can be evaluated by comparison of the
relative strength values of the different
concretes:
Physical processes: At early age the
strength difference in concretes having aggregates of similar strength is
about 25% due to differences in 6 h
absorption capacity of about 200%;
the higher strength is in the concretes
with
higher
absorption
aggregates.
0 ‘Impregnation’
mechanism
can
result in strength increase of about
20% at 90 days (relative to 28 days).
0 Pozzolanic
activity can lead to
strength increase of about 20% at 90
days (relative to 28 days).
0
(6) This is a basis for considering
these
mechanisms for controlled production of
lightweight aggregates intended for use in
high strength lightweight concrete.
REFERENCES
Zhang, M.H. & Gjorv, O.E., Penetration
of cement
paste into lightweight aggregate. Cement and Concrete
Research, 22 (1991) 47-55.
Zhang, M.H. & Gjorv, O.E., Microstructure
of the
interfacial zone between lightweight aggregate and
cement paste. Cement and Concrete Research, 20
(1990) 610- 18.
Zhang, M.H. & Gjorv, O.E., Pozzolanic reactivity of
lightweight aggregates. Cement and Concrete Research,
20 (1990) 884- 90.
76
R. Wasserman,A. Bentur
4. Khokorin, N. K., The Durability of Lightweight Aggregate Concrete Structural Members, Kuibyhev, USSR,
1973.
5. Knigina, G.J. & Pinayev, A.A., Mikrokalorimetria
i
Stabilonost Phasovogo Sostava Agloporita. Stroitelny
Materialy, 241 (1975) 29-30.
6. Fagerhmd, G., Frost Resistance of Concrete with Porous
Aggregate, Research report 2178. Cement and Concrete
Research Institute, Sweden, 1978.
7. Swamy, R. N. & Lambert, G. N., Mix design and
properties of concrete made with PFA coarse aggregate and sand. Int. J. Cement Composites and
Lightweight Concrete, 5 (1983) 263-74.
8. Sarkar, S.L., Chandra, S. & Berntsson, L., Interdependence of microstructure
and strength of structural
lightweight aggregate concrete. Cement and Concrete
Composites, 14 (1992) 239-48.
9. Wasserman, R. & Bentur, A., Effect of Lightweight
Sintered Fly Ash Aggregate Microstructure
on the
Strength of Concretes, submitted for publication.
10. Bentur, A., Microstructure,
interfacial
effects and
micromechanics
of cementitious
composites,
In
Advances in Cementitious Materials, Ceramics Transactions, Vol. 16, (ed. S. Mindess).
The American
Ceramic Society, 1991, pp. 523-50.