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Research Paper
Mechanical properties and durability assessment of rice huskash concrete
Rahmat Madandoust a,*, Malek Mohammad Ranjbar a, Hamed Ahmadi Moghadamb,Seyed Yasin Mousavi a
aDepartment of Civil Engineering, University of Guilan, P.O. Box 3756, Rasht, IranbDepartment of Civil Engineering, Lashtenesha-Zibakenar Branch, Islamic Azad University, Lashtenesha, Iran
a r t i c l e i n f o
Article history:
Received 23 May 2010
Received in revised form
4 July 2011
Accepted 25 July 2011
Published online 24 August 2011
* Corresponding author. Tel.: þ98 9113314970E-mail addresses: [email protected]
(H.A. Moghadam).1537-5110/$ e see front matter ª 2011 IAgrEdoi:10.1016/j.biosystemseng.2011.07.009
The effect of rice husk ash (RHA) on concrete properties and durability was studied. To
establish the suitable proportion of RHA for the partial replacement of cement, concrete
mixtures with 0e30% RHA were produced and their mechanical properties were deter-
mined. The effect of RHA on the uniformity of concrete was also examined. The durability
of the specimens exposed to aggressive environments (5% NaCl with wet-dry cycling) was
evaluated for a total of eleven months. The degree of damage was studied by determining
the percentage of reduction in compressive strength and chloride ions penetration as
compared with control specimens that had cured normally. The results indicate that the
partial replacement of cement by RHA improved durability and homogeneity but did not
increase the early age compressive strength of concrete. However, concrete containing
RHA showed higher compressive strength at the later ages. The scanning electron
microscopy (SEM) studies of the microstructure of mortar specimens showed that the RHA
filled up the pores and this explained the superior mechanical performance of the mortar
with RHA.
ª 2011 IAgrE. Published by Elsevier Ltd. All rights reserved.
1. Introduction Rice husks, sometimes called rice hulls, are one of the
A large number of researches have been directed towards the
utilisation of waste materials (Bui, Hu, Stroeven, 2005; Mehta,
1977). For the construction industry, the development and use
of blended cements is growing rapidly. Pozzolans from
industrial and agricultural by-products such as fly ash and rice
husk ash (RHA) are receiving more attention now since their
use generally improves the properties of the blended cement
concrete, the cost and the reduction of negative environ-
mental effects.
; fax: þ98 1316690271.c.ir (R. Madandoust), ran
. Published by Elsevier Lt
major agricultural by-products and are the shells produced
during the de husking operation of paddy rice. It constitutes
20% of the 500 million tons of paddy produced in the world
(Bhanumathidas & Mehta, 2004). Due to the low nutritional
properties of rice-husk, it is not appropriate for use as a feed
for animals. Moreover, its siliceous composition is resistant to
natural degradation, which can produce a large environ-
mental load (Zerbino, Giaccio, Isaia, 2011). Hence, adequate
alternative disposal arrangements must be considered to
avoid environmental effects. In some countries, rice husk has
[email protected] (M.M. Ranjbar), [email protected]
d. All rights reserved.
Fig. 1 e XRD pattern of the RHA.
Fig. 2 e SEM micrograph of the RHA.
b i o s y s t em s e ng i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2 145
been widely used as a fuel for rice mills and electricity
generating power plants as a very effective method for
reducing rice husk volume. When rice husk is burnt, about
20% by weight of the husk is recovered as ash in which more
than 75% by weight is silica. Silica content in RHA may
improve the properties of fresh concrete, reduce heat evolu-
tion, reduce permeability, and increase strength at longer ages
(Ganesan, Rajagopal, Thangavel, 2008; Gemma Rodrıguez de
Sensale, 2006). In addition, the use of RHA decreases the
demand for cement in the construction industry, reduces the
cost of concrete production, and reduces the negative envi-
ronmental impact that CO2 emissions represent in the
production of cement (Bui et al., 2005; Hwang&Wu, 1989). The
use of RHA in concrete was patented in the year 1924 (Pitt,
1972). Initially rice husk was converted into ash by open
heap village-based burning method at temperatures ranging
from 300 �C to 450 �C (Columna, 1974). When the husk was
converted to ash by uncontrolled burning below 500 �C, theignition was not completed and a considerable amount of
unburnt carbon was found in the resulting ash (Al-Khalaf &
Yousiff, 1984). Carbon content in excess of 30% has been
expected to have an adverse effect upon the pozzolanic
activity of RHA (Cook, 1986). The ash produced by controlled
burning of the rice husk between 550 �C and 700 �C inciner-
ating temperature for 1-h transforms the silica content of the
ash into non-crystalline or amorphous silica. The silica
content in rice husk ash is high at approximately 90% and
silica in amorphous form is suitable for use as a pozzolan.
Research and development in various parts of the world
has led to the conclusion that RHA is suitable for partial
replacement because of its very high silica content (Bui et al.,
2005; Mehta, 1977). The reactivity of RHA is attributed to its
high content of amorphous silica, and to its very large surface
area governed by the porous structure of the particles (James
& Rao, 1986; Mehta, 1994). Generally, reactivity is also fav-
oured by increasing fineness of the pozzolanic material
(Kraiwood, Chai, Smith, Seksun, 2001; Paya, Monzo,
Borrachero, Peris, Gonzalez-Lopez, 1997). However, Mehta
(1979) has reported that grinding of RHA to a high degree of
fineness should be avoided, because it derives its pozzolanic
activity mainly from the internal surface area of the particles.
By blending rice husk ash with courser cement, higher
packing can be expected, leading to improved behaviour of
blended systems (Stroeven, Stroeven, Bui, 2002). The amount
of replacement for cement is influenced by the nature of the
silica, fineness of the ash and the presence of other materials
such as carbon.
The enhancement of the resistance to chloride penetration
is one of the benefits of incorporation of pozzolans. It is
generally accepted that incorporation of a pozzolan improves
the resistance to chloride penetration and reduces chloride-
induced corrosion initiation period of steel reinforcement.
The improvement is mainly caused by the reduction of
permeability/diffusivity, particularly to chloride ion trans-
portation of the blended cement concrete (Tumidajski, Chan,
Boltzmann, 1996).
The engineering properties of concrete containing RHA
were studied by determining mechanical properties and
durability characteristic. In the case of mechanical properties,
compressive and tensile strengths were measured on
standard specimens. Also, the uniformity of concrete con-
taining RHA was evaluated by testing a reinforced concrete
beam by means of non-destructive ultrasonic test. The long
term durability was evaluated by chloride analysis, and the
percentage reduction in compressive strength of the speci-
mens exposed to aggressive environment were compared to
the control specimens.
2. Experimental programme
In this study for determining the suitable amount of RHA,
different mixtures with 0e30% RHA replacement by weight of
cement were used. The cement used was type I ordinary
Portland cement complying with the requirement of ASTM
C 150.
The RHA, provided from the north of Iran, had a high
content of silica (90.9%) by weight. Fig. 1 shows X-ray
diffraction (XRD) pattern of the RHA sample. A strong broad
peak of the RHA was centred at about 22� (2q) which was in
keeping with the strong broad peak of a characteristic of
amorphous SiO2. It contained certain amounts of crystallite as
crystalline silica. This indicated that RHA was obtained by
burning at relatively high temperatures in the range of 650 �C
Table 1 e Chemical composition and physical propertiesof binder materials.
Constituents (wt.%) PortlandCement
RHA
Chemical
composition
CaO 64.5 0.8
SiO2 20.0 90.9
Al2O3 6.0 0.83
Fe2O3 4.2 0.6
MgO 1.2 0.56
Na2O þ K2O 0.8 1.55
Physical
properties
Blaine fineness (m2 kg�1) 336.5 376.8
Pozzolanic Activity Index (%) e 81.25
Bulk density (kg m�3) e 429.1
Table 2 e Physical properties of coarse and fineaggregates.
Physical tests Coarse aggregate Sand
Specific gravity 2.56 2.63
Fineness modulus 6.86 2.64
Bulk density (kg m�3) 1580 1720
Water absorption (%) 0.78 1.3
b i o s y s t em s e n g i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2146
which led to the crystallisation of the amorphous silica. To be
used as a pozzolanic material, the ash was ground by means
of a laboratory batch 2ball mill until certain fineness and
surface area (376.8 m2 kg�1) was reached. The RHA passing
through sieve No. 325 (sieve opening 45 mm) was considered
for the partial replacement of cement. As concluded from
Fig. 2, ground RHA grains are mostly angular edged grains
with different particle sizes. The chemical composition and
physical properties of RHA and cement as a binder are given in
Table 1. Also, the particle size distribution curves of this
binder are shown in Fig. 3.
Locally available natural sand was used as the fine aggre-
gate while the crushed limestone with 16 mm maximum
size (sieved) was used as the coarse aggregate. The specific
gravity, the fineness modulus, bulk density and water
absorption of fine and coarse aggregates are shown in Table 2.
Both fine and coarse aggregate grading curves are also shown
in Fig. 4.
The details of the concrete mixtures used for the entire
study are shown in Table 3. The RHA used, was produced by
burning heaped up rice husk obtained from controlled pre-
processing. For possible comparison, mixtures with constant
slump were prepared and therefore the concrete containing
RHA was made incorporating a superplasticiser. The super-
plasticiser was a polyethylene sulphonate complying with
ASTM C-494 type F. The specific gravity of the superplasticiser
as given by the supplier was 1080 kg m�3 and the PH was 6
with chloride content of less than 0.1%.
The concrete mixtures were proportioned to give 28-day
strength of about 30MPa based on 100mmcubeswith a slump
of about 50 mm. Properties of the concrete mixtures including
air content of the freshly mixed concrete by the pressure
0.0
20.0
40.0
60.0
80.0
100.0
0001001011Sieve mesh size; µm
Cum
ulat
ive
pass
ing
(%)
Cement RHA
Fig. 3 e Particle size distribution curves of cement and RHA.
method (ASTM C231), slump of concrete (ASTM C143), unit
weight, and 28-day compressive strength (ASTM C39) were
determined and are given in Table 4 including the cost
saving analysis which was determined in a former study
(Babaiefar, 2007).
For each type of concrete, a number of standard specimens
were cast in order to measure the mechanical properties and
the durability of concrete. The specimens were cast in steel
moulds and full compaction was achieved with the use of
a vibrating table. The specimenswere remove frommoulds on
the following day and cured under water in a curing tank at
21 �C temperature. After 7 days, the specimens for durability
assessment were exposed in a tank containing 5% NaCl
solutionwithwet and dry cycles (at 15 days intervals) until 360
days. Similar control specimens were also cast and cured
under water in a conventional curing tank for the same
period.
Two reinforced concrete beams were also cast for studying
the uniformity of concrete. The concrete was supplied by the
four batches used in each case from a 0.1m3mixer. The beams
were compacted using a poker vibrator in three layers before
levelling and steel trowel. They were cured under damp
conditions and polythene sheeting for 7 days and then in
a laboratory environment of 21 �C and 75% relative humidity
until tested.
Three 100 mm cubes were also cast from each of the four
batches per beam and subjected to a similar curing regime
until both beams and cubes were tested at 28 days. The beams
and cubes were tested using ultrasonic pulse velocitymethod.
This test method has been described in detail by Bungey and
Millard (2004) and the recommendations of ASTM E494 were
followed using a portable ultrasonic unit. Details of the rein-
forced concrete beam with the grid locations of ultrasonic
pulse velocity tests are shown in Fig. 5. The grid locationswere
designed to consider spacing requirements and avoiding
reinforcing steel.
0
20
40
60
80
100
0.01 0.1 1 10 100
Sieve mesh size; mm
Cum
ulat
ive
pass
ing
(%)
Sand Gravel
Fig. 4 e Fine and coarse aggregate grading curves.
Table 3 e Concrete mixture proportions.
Mix Gravel(kg m�3)
Sand(kg m�3)
Cement(kg m�3)
RHA Water(kg m�3)
W/B ratio Superplasticiser(kg m�3)
(%) (kg m�3)
NC 951 844 396 e e 210 0.53 e
CR5 951 844 376 5 20 210 0.53 3.76
CR10 951 844 356 10 40 210 0.53 3.90
CR15 951 844 337 15 60 210 0.53 4.10
CR20 951 844 317 20 79 210 0.53 4.23
CR25 951 844 297 25 99 210 0.53 4.40
CR30 951 844 277 30 119 210 0.53 4.56
110
b i o s y s t em s e ng i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2 147
3. Results and discussion
3.1. Mechanical properties
3.1.1. Compressive strengthCompressive strength of concrete specimens with different
percentage of replacement of RHA (between zero to 30%) was
tested at the age of 28 days which are shown in Fig. 6. This
figure confirms that the reduction in the 28-day strength
increases with the level of cement replacement.
Previous study (Babaiefar, 2007) shows that the inclusion of
RHA in concrete can reduce the total amount of cost up to 7.6%
(Table 4). In addition concerning to the strength gain, it seems
that 20% RHA may be considered as a suitable replacement.
To assess the influence of RHA on engineering properties of
concrete, two types of concrete were developed, normal
concrete and concrete containing 20% RHA. To verify that this
replacement (20%) is beneficial, long term strength develop-
ment was studied.
Table 4 e Properties of concrete mixtures.
Mix Air content(%)
Slump(mm)
Density(kg m�3)
28-day,fcu (MPa)
Cost savinga
(% per m3)
NC 3.6 65 2346 35.4 e
CR5 3.6 60 2345 33.6 �1.6
CR10 3.7 55 2345 32.4 0.3
CR15 3.7 55 2346 32.1 2
CR20 3.8 56 2342 30.8 3.9
CR25 3.9 50 2342 29.7 5.7
CR30 4.1 45 2345 26.6 7.6
a Determined in a former study (Babaiefar, 2007).
Fig. 5 e Details of reinforced concrete beam
Fig. 7 shows the compressive strength development at
different ages up to 360 days for RHA and normal concretes.
For all tests, three specimens were tested at each of the ages
and the mean values are considered in this figure. From the
results, it can be seen that in all cases, compressive strength
increases with age. Furthermore, in the short term, the
strength enhancement for RHA concrete is lower than normal
concrete. This shows that RHA concrete may be unacceptable
for fast track construction due to its reduced strength at early
age. However in the long term, the results in Fig. 7 indicate
higher pozzolanic activity in RHA concrete. Although at the
age of 3 days the amount of compressive strength of RHA
concrete was 65% of normal concrete, this amount increased
to 96% at 90 days and 98% at 180 days. This may suggest that
the concrete quality assessments which normally are taken at
the age of 28 days need to be reconsidered at ages above 90
days. Further study of longer-term strength development, up
with grid positions (Dimensions in cm).
60
70
80
90
100
0 5 10 15 20 25 30
RHA (%)
% o
f St
reng
th g
ain
Fig. 6 e Compressive strength gain for different percentage
of RHA at 28 days.
0
10
20
30
40
50
3 7 28 90 180 270 360
Age; days
Com
pres
sive
Str
engt
h; M
Pa
Normal Concrete
RHA Concrete
Fig. 7 e Development of compressive strength with age.
0
1
2
3
4
5
0 10 20 30 40 50 60
Compressive Strength; MPa
Ten
sile
Str
engt
h; M
Pa
RHA Concrete
Normal concrete
55.0454.0 ct ff =
Fig. 9 e Compressive strength versus tensile strength.
b i o s y s t em s e n g i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2148
to 360 days demonstrates that the strength development is
comparable to normal concrete in 270 days andwill be slightly
greater (by 2%) than normal concrete at the age of 360 days.
3.1.2. Splitting tensile strengthThe tensile strength of concrete containing 20% RHA obtained
at different ages is shown in Fig. 8. By observing the results, it
can be seen that strength development, based on ASTM C496
method, is about the same order as those in compression test.
In other words, the tensile strength for both RHA and normal
concretes will increase with age, and at all ages, the tensile
strength of RHA concrete was lower than normal concrete at
ages up to below 90 days, but it became greater than normal
concrete at the later ages. The ratio of the tensile strength of
RHA concrete to the normal concrete was 79% at the age of 3
days whilst this ratio was 104% at the age of 360 days. This
increase can be attributed to the pozzolanic activity that
occurred in RHA concrete as the concrete aged.
Fig. 9 shows a relationship between compressive and
tensile strengths. The correlation between the compressive
and the tensile strengths for these two types of concrete are
shown in Fig. 9. Regression analysis was conducted to eval-
uate the correlation between the compression and the tensile
test results, and their regression curvewas drawn in the figure
with correlation coefficient of 0.96. As is shown in this figure,
correlation between compressive and tensile strengths is
independent of the type of concrete. Also from this figure, it
0
1
2
3
4
3 7 28 90 180 270 360
Age; days
Ten
sile
Str
engt
h; M
Pa
Control Concrete
RHA Concrete
Fig. 8 e Development of tensile strength with age.
can be observed that the rate of increase in compressive
strength is higher than for tensile strength.
3.2. Concrete uniformity assessment
The effect of 20% RHA on concrete homogeneity was assessed
by using large scale reinforced beams to simulate site
construction. This study was designed to provide information
on uniformity of in situ properties of RHA concrete beams and
to compare the results with ones cast with normal concrete.
Tests on the beams were made using through-transmission
54 kHz ultrasonic pulse velocity measurements. The distri-
bution of concrete strength was determined in the beams
through the ultrasonic test results and by using the correla-
tion curves. Correlation curves for both normal and RHA
concretes were obtained based on the ultrasonic pulse
velocity test results and the cube compressive strength and
they are shown in Fig. 10.
Twenty-eight day pulse velocity results were obtained on
the beams at grid locations indicated in Fig. 5. The pulse
velocity test results have been converted to estimated
compressive strengths using correlation curves given in
Fig. 10. Then concrete strength distributions of control beam
and beam containing RHA have been assessed. For this
purpose, the mean value of ultrasonic pulse velocity and the
estimated compressive strength were evaluated at three
different levels at the height of 100, 250 and 400 mm from the
bottom of the beams.
0
10
20
30
40
50
55.445.3
Ultra pulse velocity; km sec^-1
Com
pres
sive
str
engt
h; M
Pa
Normal concrete
RHA concrete
Fig. 10 e Ultrasonic pulse velocity correlations with
strength.
Table 5eAverage ultrasonic pulse velocity and estimatedcompressive strength across depth of beams.
Concrete type Level in beam V (km s�1) fce (MPa)
Normal Concrete Top 4.060 29.1
Mid 4.121 33.4
Bottom 4.174 37.8
RHA Concrete Top 4.422 28.6
Mid 4.448 30.5
Bottom 4.479 32.8
V: Ultrasonic pulse velocity. fce: Estimated compressive strength.
0
100
200
300
400
500
70 80 90 100 110
Relative strength (%)
Hei
ght a
bove
bas
e; c
m
Normal concrete
RHA concrete
Fig. 11 e Variation in strength through the height of
beams.
b i o s y s t em s e ng i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2 149
Table 5 summarises results of the pulse velocity
measurements and equivalent cube strength at each level.
Average variations in strength across the depths of the beams
shown in Fig. 11, indicate that trend is as expected with
a reduction of strength towards the top of the beams and the
7 6 . 3
9 8 . 2 9 8 . 6
8 8 . 4
8 4
8 6 . 3
7 5 . 2
8
Fig. 12 e Strength relative contour p
9 2 . 3
9 8 .2 9 5 . 8
9 3
8 3 . 9
9 1 . 1
9 5 . 8
9 3
8 8
Fig. 13 e Strength relative contour
results for normal concrete agree closely with previous find-
ings (Bungey & Madandoust, 1994). This phenomenon is likely
to be the result of bleeding and segregation and also the effect
of concrete hydrostatic pressure throughout the height of
concrete beams.
In addition, the influence of concrete type on the homo-
geneity is clearly apparent; the RHA concrete shows the least
variation in strength development with average strength
variation between top and bottom layers of only approxi-
mately 13%, whereas this difference was up to 23%, in normal
concrete. It can be said that by adding the RHA into the
concrete, the effect of bleeding and segregation can be pre-
vented to some extent. The contour plots of Figs. 12 and 13
provide further comparisons of the strength variations
within the twomentioned beams, both across their depth and
along their length. Random variation along the member
length should be noted as well. Once again, from the relative
strength contours, the lowest value for strength was detected
at the top of beam in compare to the lower levels. Results for
normal concrete are similar manner to those found elsewhere
for ordinary reinforced concrete wall (Madandoust, 2001).
An examination of contour lines for normal and RHA
concretes indicate the effects of RHA in concrete. It can
significantly improve the concrete bonding and this can be
related to the microstructure of hardened concrete.
3.3. Durability studies
Increasing the resistance of concrete to the penetration of
chloride ions is the primary way to increase the service life of
structures in aggressive environments. The durability of the
specimens exposed to aggressive environments (5%NaCl with
wet and dry cycling) was evaluated for entire eleven months’
test results.
9 7 . 21 0 0 9 5
3 . 78 4
7 4 . 7
8 5 . 3
7 2 . 2
lots for normal concrete beam.
9 4 . 9
8 9
8 5 . 7
8 6 . 9
8 2 . 88 4 . 3
plots for RHA concrete beam.
Table 6 e Compressive strength results for different mixtures under normal and aggressive curing conditions (MPa).
Mixture Age (days)
28 90 180 270 360
Water Salt Water Salt Water Salt Water Salt Water Salt
NC 35 33.3 43.5 40.5 45.5 41.7 46 41.4 47 41.6
CR15 32.1 30.5 42.5 39.8 44.5 41.2 46.4 42.2 47.5 42.6
CR20 31 29.4 41.7 39.3 44 41.2 46.7 43.1 47.8 43.7
CR25 29.7 28.3 38.5 36.2 42.5 39.8 45.6 42.5 47.3 43.7
0.5
1
1.5
2
2.5
3
3.5
10 20 30 40Specimen depth; mm
Chl
orid
e C
once
ntra
tion
(% b
inde
r) NC
CR15
CR20
CR25
Fig. 15 e Chloride concentrations at 0e40 mm depth for
different specimens after 360 days.
b i o s y s t em s e n g i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2150
Compressive strength and chloride tests were carried out
to obtain any possible decreasing on the strength and the
depth of chloride penetration. For this purpose concrete
mixtures in Table 3 were used by emphasising on those RHA
concretes with 15e25% replacement of cement by RHA.
3.3.1. Compressive strengthTable 6 shows the results (mean value of three specimens) of
compressive strength in the environments of normal curing
and durability tank. Because of the pozzolanic reaction, the
paste incorporating RHA had lower Ca(OH)2 content than the
normal cement paste. It reduced the porosity, the amount of
Ca(OH)2 and width of the interfacial zone between the paste
and the aggregate. As it can be seen from the results, there
was a difference in the reduction of compressive strength
between specimens made from different mixtures. The
reduction for normal concrete is higher than the specimens
containing RHA. Fig. 14 shows the ratio of compressive
strength of mixtures in aggressive to normal curing condi-
tions. The lower compressive strength of mixes which cured
in salt/wetting regime may be due to salt crystallisation
formation. Probability, internal pressure caused by the crys-
tallisation of salts inside of the mortar pores resulted in
a micro-crack generation and subsequent destruction of pore
structure of the concrete matrix. This phenomenon could be
coupled with the difference in curing regimes which led to
a difference in compressive strength. From Fig. 14, the positive
effect of RHA concrete was more pronounced at the older
ages. Fig. 14 also confirms that the 20% RHA replacement
might be beneficial in overall.
0.88
0.90
0.92
0.94
0.96
0.98
28 90 180 270 360
Age; days
Rat
io o
f Com
pres
sive
Str
engt
h NC
CR15
CR20
CR25
Fig. 14 e Ratio of compressive strength results of
aggressive to normal curing condition.
3.3.2. Chloride test analysisFig. 15 shows depth of chloride ion penetration for different
concrete mixes to the aggressive environment (5% NaCl with
wet and dry cycling). After eleven months exposure in the
aggressive environments, sample location were selected for
testing at various depths 0e10mm, 10e20mm, 20e30mmand
30e40 mm. The acid soluble chloride technique (ASTM C1152)
has been used to measure chloride contents at different
Fig. 16 e SEM view of floc-like phases formed in the matrix
of RHA after 360 days.
Fig. 17 e SEM view of tinsel-like phases formed inside the
ordinary mortar after 360 days.
b i o s y s t em s e ng i n e e r i n g 1 1 0 ( 2 0 1 1 ) 1 4 4e1 5 2 151
depths of specimens. It entailed dry drilling the specimens to
obtain powder which was subsequently analysed chemically.
A comparison of the chloride profile for four mixtures is pre-
sented in Fig. 15.
A considerable amount of chloride had penetrated deeply
into the specimens. As it can be seen from these results, it is
clearly shown the effect of RHA. The higher the RHA content,
the lower the chloride concentration was appeared across the
specimens. The control specimens have significantly higher
chloride concentrations at any particular depth compared
with specimens containing RHA. The results also show that
the rate of chloride penetration decreases with the depth.
3.4. Microstructure of mortar phase containing RHA
The reaction of the RHA with water and Ca(OH)2 formed
during the hydration of cement that caused the formation of
calcium silicate hydrate (CeSeH) which is responsible for the
strength in cement-based materials. To verify this mecha-
nism, the mortar specimens containing RHA, which had 360
days of moist curing, were examined by scanning electron
microscopy (SEM). The microstructure of the mortar mixture
containing RHA (Fig. 16) revealed that the formation of floc-
like hydration products which lapped and jointed together
by many fibrous-like hydrates was denser and more compact
and the tensile-like Ca(OH)2 crystals were reduced (Fig. 17).
4. Conclusions
On the basis of this study, the following conclusions can be
drawn:
1. The partial replacement of cement by RHA (20%) indicates
that at short term ages, the growth in strength is lower for
RHA concrete compared to the control specimens.
However, in the long term, the results indicate a pozzo-
lanic activity in RHA concrete and the strength develop-
ment is comparable to normal concrete at the age of 270
days.
2. The relationship between compressive and tensile
strengths is independent of the type of the concrete. The
rate of increase in compressive strength is higher than
tensile strength.
3. The differences between the uniformity of the properties of
RHA concrete and the corresponding control mix indicate
a strength variation across the depth of concrete beams
following the general pattern of a reasonably uniform
distribution from top to bottom,with the top region being of
lower strength than the bottom region. The magnitude of
this variation varies according to the concrete type, with
least variation for RHA concrete.
4. In terms of chloride penetration, it is evident that blending
of Portland cement with RHA is beneficial from the stand-
point of the prevention of diffusion of Cl�. This blending
leads to lower porosity and finer pore structures, thereby
inhibiting penetration of chlorides.
5. There was good agreement between compression test,
chloride analysis and SEM study of the specimens.
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