Microstructure Characteristics of Fly Ash Concrete with Rice Husk Ash and Lime Stone Powder
International Journal of Concrete Structures and Materials
Microstructure Characteristics of Fly Ash Concrete with Rice Husk Ash and Lime Stone Powder
Industrial wastes and recycled materials are being utilized in the construction industry for preserving the environment, saving of materials, and enhancing durability of the construction material. Blending of cement with supplementary cementitious materials like fly ash, rice husk ash, and silica fume makes concrete more durable. The main objective of this study is to make use of the rice husk ash and lime powder (LP) as a replacement of Portland pozzolana cement considering various replacement levels. The engineering and durability performance in concrete with LP were performed through compressive strength and void measurement. The microstructure in the concrete with LP was characterized through XRD, SEM/EDS, and TG/DTA. Optimum replacement ratio for rice husk ash and LP were obtained through pozzolanic reaction based CSH formation.
portland pozzolana cement; fly ash; rice husk ash; limestone powder; microstructure
Addition of LP to cement causes an increase of hydration
at early ages inducing a high early strength, but it can reduce
the long-term strength due to the dilution effect
(Ghrici et al.
. RHA could be advantageously blended with OPC
without adversely affecting the strength and permeability
properties of concrete due to the pozzolanic reaction and
(Ganesan et al. 2008)
. Addition of RHA to
OPC not only improves the early strength of the concrete but
also forms a calcium silicate hydrate (CSH) gel around
cement particles, which leads to the highly dense pore
(Saraswathy and Song 2007)
. Pozzolana, FA,
GGBFS (Ground Granulated Blast Furnace Slag), LP are the
main materials permitted by the European Standards EN
(European committee 2000)
The increase in the early strength of the mortar due to LP
addition and dolomitic limestone can be attributed to their
active participation in cement hydration and filler effect of
their fine particle size
(Voglis et al. 2005; Matschei et al.
. The additional surface area by the limestone particles
may provide sites for the nucleation and growth of hydration
products, which leads to enhancement of strength and
(Matschei et al. 2007; Bentz 2008)
. LP addition
affects the pore structure of the cement paste. It is reported
that LP addition linearly increases the size of capillary pores
(20–40 nm) and due to the high silica content, RHA is said
to be an extremely reactive pozzolana
BeaziKatsioti 2009; Xu et al. 2015; Park et al. 2016)
. Up to 20%
of the cement may potentially be substituted by LP (or other
fillers) to economize on the usage of OPC clinker and to
reduce the energy/deleterious emissions associated with its
RHA addition to OPC not only improves the early strength
of concrete but also forms a CSH (Calcium Silicate Hydrate)
gel around the cement particles which is highly dense and
less porous, which can increase the cracking resistance
(Saraswathy and Song 2007)
. RHA can be produced with
varying pozzolanic activity index depending on the degree
of grinding and the burning temperature. Up to 40%
replacement of RHA can be adopted with no significant
changes in strength characteristics and its effect on volume
changes is within the limit specified in the American
(Al-Khalaf and Yousif 1984)
. The quickly cooled RHA
from burning for 12 h at 500 C has the highest amount of
silanol groups, and it is favorable to be used as pozzolanic
(Nair et al. 2008)
. Another advantage of
RHA is durability enhancement and reasonable
compatibility with OPC.
The use of a blend of equal weight portion of FA and RHA
produces a good strength and resistance to chloride ion
penetration with less requirement of super-plasticizer
(Chindaprasirt et al. 2008)
. Rice husk after heat treating at
700 C for 6 h produces the amorphous silica which can be
used as an additive in concrete as a replacement of cement
(Della et al. 2002)
. Amorphous silica presence is
concentrated on the internal and external surfaces of the
uncalcinated husk which may promote a pozzolanic action on the
surface of the husk and hence enable its use in lightweight
(Jauberthie et al. 2000)
. The pozzolanic effect is
reported to be stronger in the binary and ternary mixtures
with RHA in proportions of 25% or higher
(Isaia et al.
The present investigation aims at evaluating the FA
cement with RHA and LP as additives, and to characterize
the microstructural properties by adapting various analytical
techniques such as, X-ray Diffraction (XRD), scanning
electron microscope/energy dispersive spectroscopy (SEM/
EDS), and thermogravimetric/differential thermal analysis
(TG/DTA). In addition, strength and porosity measurements
were also carried out.
2. Experiment Program
2.1 Material Properties
2.1.1 Cement and Aggregates
PPC 53 Grade as per IS 8112-1989 with specific gravity
3.10 is used and the initial/final setting times for cement are
30 and 600 min respectively. The percentage of cement
particles passing through IS 90 l sieve is 96.4%. The
chemical composition of FA cement is given in Table 1. Fine
Aggregate used is river sand passing through 2.36 mm sieve,
falling under zone III as specified in IS 383-1978 and with
specific gravity 2.60. Coarse aggregates with specific gravity
2.60 graded with angular aggregates of varying size between
4.75–16 mm was used. The mix proportion for M30
concrete arrived by IS code is given in Table 2.
2.1.2 Rice Husk Ash
Locally available rice husk was treated at 700 C for 3 h,
and the ash is taken for the investigation. The obtained ash is
a grey color, with the mean particle size of 25 lm, and
specific gravity of 2.3. The chemical composition of RHA is
given in Table 3. The Specific gravity and surface area of
RHA are 2.23 and 85 m2/g respectively.
2.1.3 Lime Powder (LP)
LP was used for the study, and purchased from limestone
kiln industries. The particle size distribution of LP is shown
in Fig. 1. The average particle size of the LP varies from 10
to 100 lm, with about 10% less than 1 lm. This fineness of
LP makes readily react to give the early strength of the
concrete. The major frequency lies within particle size of
40 lm to 50 lm. Chemical composition of LP is CaO
93.1%, MgO 1.6%, SiO2 1.3%, MnO3 1.2%, and CaCO3
2.2 Mixing Systems
The various mix systems used are given in Table 4. The
mix systems were carefully selected based on the
preliminary studies by conducting compressive strength test
(Kathirvel et al. 2013)
. Unit weight of cement is given in
Table 3. FA cement is replaced with RHA and LP in the
following proportions as shown in Table 4.
FA cement (%)
Aluminium oxide (Al2O3)
Ferric oxide (Fe2O3)
Calcium oxide (CaO)
Magnesium oxide (MgO)
Sulphur tri oxide (SO3)
Loss on ignition
Pozzolonic material used (fly ash %)
28 days compressive strength
Specific surface area (Blaine) (cm2/g)
Relative absorption (%)
Materials/m3 of concrete kg/m3
2.3 Property Evaluation Tests
2.3.1 Compression Test
The compressive strength of the concrete is carried out as
per IS 516-1959. Concrete cube specimens of size
150 9 150 9 150 mm3 are cast with different types of mix
systems. After 24 h, the specimens are demolded and
subjected to curing for 28 days in water. After 28 days of
watersubmerged curing, the cubes are tested in the
compressiontesting machine (200T) capacity at the rate of 140 kN/min as
per IS 456-2000. The ultimate load at which the cube fails is
taken. Tests are carried out on triplicate specimens and the
average compressive strength values are noted.
2.3.2 Tests for Density and Permeable Voids
The tests are performed based on ASTM C 642-97 by an
oven-drying method. For this test, 50 mm diameter and
50 mm height cylindrical specimens are cast. After 24 h of
demolding, the specimens are kept immersed in water. At the
end of 28 days, the specimens are taken from the curing tank
and air-dried to remove the surface moisture. Then, the
specimens are dried in an oven at a temperature of 110 C
for 48 h, and allowed to cool at room temperature. At the
end of 48 h of cooling, the weights of the specimens are
measured with 1 g accuracy. After that, the specimens are
kept immersed in water continuously for 48 h. After 48 h.
The increase in weight is measured after wiping out the
surface water using the dry cloth. Then the specimens are
immersed in water and heated continuously for a period of
5 h. Then the weights of the specimens are taken after a time
gap of 14 h. The specimens are suspended in water, and its
submerged weight is determined.
To determine the percentage of total voids, the apparent
specific gravity of the specimen must be determined. From
the above data, water absorption percentage, permeability
percentage, and percentage of total voids are determined by
adopting the following procedures.
Wp ¼ ½ðB
Pp ¼ ½ðC
Vp ¼ ½ðg1
Fly ash cement
Replacement of ? 5% RHA ? 5% LP
Replacement of ? 10% RHA ? 10% LP
Replacement of ? 15% RHA ? 15% LP
Replacement of ? 20% RHA ? 20% LP
Replacement of ? 5% RHA ? 20% LP
Replacement of ? 10% RHA ? 20% LP
Replacement of ? 15% RHA ? 20% LP
where Wp is water absorption percentage (%), A is the
weight of oven dried sample in air, B is the weight of the
surface dry sample in air after immersion in water. Pp
denotes the permeability percentage (%), C and D are the
weight after 5 h heated and submerged weight in water,
respectively. Vp is the percentage of total voids. g1 and g2 are
the results of A/(C-D) and apparent specific gravity of the
2.4 Characterization Techniques
2.4.1 XRD and SEM/EDS
The samples are crushed mechanically, and mineralogical
analysis is carried out using XRD with source CuKa
radiation (k = 1.54059 A˚) generated at 40 kV and 20 A, was
used for the recording of XRD patterns. The identification of
diffraction peaks was performed by employing ‘peak search’
and ‘search match’ programs in the software (PA Analytical,
X’pert High score plus). SEM/EDS are also performed for
chemical composition measurement. At the end of the
exposure period, center core samples are taken and subjected
to SEM/EDS analysis for identification of elements present
in the concrete samples.
2.4.2 Thermogravimetry and Differential Thermal
TG/DTA are performed using a simultaneous
thermo-analyzer with alumina crucibles and holed lids. The heating
elevation is controlled from ambient temperature to 1000 C
with static air atmosphere and 20 C/min of heating rate.
3. Results and Discussion
3.1 Compression Test
Compressive strength variations in concrete with various
mixing systems are presented in Fig. 2. From the figure, it is
seen that FRL5 and FRL10 have shown better/higher
compressive strength than the control (FAC) and other systems.
The increase in strength is 8.2 and 14.75% higher than the
control specimen. Other systems (FRL15 and FRL 201)
have shown the reduction in compressive strength values
than the control specimens.
The increase in compressive strength at 5 and 10%
replacement level of RHA and LP is due to the pozzolanic
action of RHA combined with LP accelerated the strength
(Walker and Pavia 2011)
. Since Ca(OH)2 reacts in
solution with atmospheric carbon dioxide CO2 to form
calcium carbonate or calcite, CaCO3 which is significantly
stronger and less soluble than the portlandite, enhanced the
(Johannesson and Utgenannt 2001)
addition of LP and RHA results in a reduction of strength.
The strength reduction in FRL5–10, FRL10–20, and
FRL15–20 may be due to the insufficient alkali activation in
the presence of LP. The systems FAC, FRL5, FRL10,
FRL15, and FRL20 have shown the reasonable strength
development; thereby the microstructural studies have been
carried out for these five systems.
3.1.1 Tests for Density, Permeable Voids and Water Absorption
Table 5 represents the volume of permeable voids for
various systems. From the table, it is found that FRL5 and
FRL10 systems are found to have lesser permeable voids
when compared to the other three systems (FAC, FRL15,
and FRL20). This is due to the fact that the LP reacts with
RHA forming a durable CSH gel which reduces the
permeable voids in FRL5 and FRL10. As the percentage of
RHA and LP increases, the permeable voids also increase.
This is due to the precipitation of CaCO3 through reaction
with Ca(OH)2 and CO2. The formation of CaCO3 changes
the microstructure of the mortar, also affecting the pore
Fig. 3 Strength and permeable voids of various mix systems.
structure. If PPC is used as a binder, voids are expected to
decrease in RHA and LP addition due to the formation of
dense CSH which causes the pore size refinement.
The compressive strength and permeable voids are
presented in Fig. 3. From the figure, it is observed that the
percentage volume of permeable voids is found to be less for
5 and 10% blending with RHA and LP and increases with
the further replacement of 15 and 20% LP. From the studies,
it is observed that 10% blending is found to be the critical
percentage of cement replacement. Beyond 10% blending,
there is an increase in the volume of permeable voids due to
the leaching of LP (unreacted) or higher formation of
Ca(OH)2 which is confirmed by XRD studies.
3.2 Characterization Techniques
3.2.1 X-ray Diffraction
Figure 4 shows the XRD pattern of various mix systems
FAC, FRL5, FRL10, FRL15 and FRL20. All diffraction
peaks present in the figures are coinciding with JCPDFWIN
data. The XRD pattern of FAC shows diffraction peaks at
27.03 and 29.86 corresponding to C–S–H (Ca2SiO4) and
C–A–S–H (Ca[Al2Si2O8]). The other intensity peak at 50.5
is corresponding to CH (ettringite). The XRD Pattern of
FRL5 and FRL10 shows the high intensity peaks of C–S–H
(Ca2SiO4) and C–A–S–H (Ca[Al2Si2O8]) when compared to
FAC system. The XRD pattern of FRL15 and FRL20 shows
the newer peaks at 60.7 and 64.7 corresponding to the CH
and quartz (SiO2) due to the excess amount of unreacted LP
3.2.2 SEM and EDS
Figure 5 shows the SEM image of FAC in which small
and larger particles are seen along with white crystals of
unhydrated calcium hydroxide platelets are observed in the
matrix along with small CSH fibers, which cover the
anhydrous grains of calcium silicate
(Feldman and Sereda
. Figure 6 shows the SEM image of 5% LP ? 5%
RHA replaced concrete in which large crystals are loosely
packed and covered with white clusters of Ca(OH)2. In
FRL5, the hydrates are appearing as a mix of large platelets
Fig. 6 SEM analysis for FRL5.
and noncontiguous small clusters. In contrast, if the
percentage of LP increases over 10%, micro-cracks have
appeared and these cracks in turn make the matrix less
compact (Fig. 7). As the replacement of LP and RHA
increases, SEM micrographs (Figs. 8, 9) display an altered
structure, showing a mix of large platelets and small clusters.
In this case, however, the clusters are tightened, which
induces lower porosity in the matrix microstructure.
The quantitative results such as calcium, silica,
magnesium, and alumina are presented in Fig. 10. It is found that
FAC contains 11.61% of calcium, whereas LP and RHA
replaced systems show 6.71% for FRL5, 11.87% for FRL10,
23.35% FRL15, and 42.13% of calcium, respectively. Also,
FAC contains 23.54% of silica, wherein LP and RHA
replaced systems show 28.44, 28.46, 18.28 and 5.61% of
silica with increasing replacement. In contrast, alumina
percentages are gradually decreasing from 7.94 to 0.93%
with increasing LP and RHA. This is strongly evidenced by
XRD results that the calcium silicate hydrates are formed up
to 15% of LP and RHA. The major components are plotted
in Fig. 10 through EDS analysis.
In DTA curve zone one between 100 and 300 C is
attributed to the dehydration of CSH and ettringite
. At this temperature, these compounds lose water,
depends upon the available CaO/SiO2 ratio in the hydrated
cement matrix. Zone two from 290 to 350 C corresponds to
the decomposition of calcium aluminate silicate hydrate,
calcium aluminate hydrate and calcium chloroaluminate
(Ubbriaco and Calabrese 1998)
. The third zone ranging from
450 to 510 C is attributed to the dehydration of calcium
hydroxide. Decarbonation of calcium carbonate in the
hydrated compound starts over 700 C. The observed
weights of Ca(OH)2, CSH, and CaCO3 are summarized in
From the results, it is observed that Ca(OH)2 content is
found to decrease with increase in replacement of LP and
RHA. However the weight of CSH is found to increase up to
10% and beyond 10%, there is a decrease in trend observed.
The reduction in Ca(OH)2 in blended cement concrete
indicates its consumption due to the pozzolanic reaction
(Kathirvel et al. 2013)
. This is the main advantage of using
blended cement, which leads to the densification of the
microstructure. The reduction of Ca(OH)2 content is higher
in blended cement concrete than in FAC, indicating more
pozzolanic reaction due to the presence of LP.
The main hydrated phases produced during the pozzolanic
reaction at ambient temperature are CSH, C2ASH and
C4AH. From the TG/DTA results, the calcium hydrates
(CSH) content increases with a replacement of LP and RHA
up to 15% and drops at 20%. This clearly shows that in
blended cement concretes in the presence of water, both FA
in FAC and silica present in RHA react with Ca(OH)2 to
form denser CSH, thereby the porosity gets reduced. The
reduced permeability can be evidenced by water absorption
results in Sect. 3.1.2. The weights of various compounds are
plotted with strength and measured voids in Fig. 11.
The conclusions on pore structure characteristics of fly ash
concrete with rice husk ash and limestone powder are as
1. Various proportions of LP and RHA have been tested by
holding the total binder/sand ratio constant. The
experimental results have highlighted the improvement of
chemical properties and morphology with varying LP
and RHA content.
2. Addition of 10% of RHA and 10% LP is found to
exhibit excellent performance regarding the strength and
voids by the accelerated CSH formation through
effective pozzolanic reaction.
3. The substitution of LP shows Ca(OH)2 platelets along
with small CSH fibers, which covers the anhydrous
grains of calcium silicate modifying the microstructure
of the mortar matrix.
4. The different crystallization of hydrates is observed
from ordinary pozzolanic reaction of fly ash. With
increasing replacement of cement with LP substitution
induces significant modification of the microstructure,
which is demonstrated either by the presence of
microcracks in the matrix or by an alternative hydrates
development in the case of calcium hydroxide.
This research was supported by the Basic Science Research
Program through the National Research Foundation of Korea
(NRF) funded by the Ministry of Science, ICT, & Future
Planning (No. 2015R1A5A1037548). One of the authors
(V.S) thanks the Director, CECRI, and CSIR for the
permission to pursue my fellowship at Hanyang University, Korea.
This article is distributed under the terms of the Creative
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Compliance with ethical standards
Conflict of interest The authors declare that there is no
conflict of interest regarding the publication of this paper.
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