Full Length Research Paper
A study on
the engineering properties of sandcrete blocks produced with
rice husk ash blended cement
G. L. Oyekan1* and O. M. Kamiyo2
of Civil and Environmental Engineering, University of Lagos,
of Mechanical Engineering, University of Lagos, Nigeria.
*Corresponding author. E-mail:
Accepted 13 January, 2011
Sandcrete blocks have been in used in many nations of the
world including Nigeria, playing a major role in the
building industry. The material constituents, their mix,
presence of admixtures and the manufacturing process are
important factors that determine the properties of sandcrete
blocks. This paper investigates the effects of a partial
replacement of cement with rice husk ash (RHA) on some
engineering properties of hollow sandcrete blocks with 1:6
cement-sand mix ratios. Single block size 225 x 225 x 450
mm, is produced with a vibrating machine. Results show that
the addition of RHA produces blocks of lower density.
Particularly, the density of the blocks decreases as the RHA
content increases. The compressive strength of the block is
also not enhanced. Results also reveal that the thermal and
hygrothermal properties of the blocks are significantly
Sandcrete blocks, rice husk ash, compressive strength,
hygrothermal properties, thermal properties.
Hollow sandcrete blocks containing a mixture of sand, cement and
water are used extensively in many countries of the world especially
in Africa. In many parts of Nigeria, sandcrete block is the major
cost component of the most common buildings. The high and increasing
cost of constituent materials of sandcrete blocks has contributed to
the non-realization of adequate housing for both urban and rural
dwellers. Hence, availability of alternatives to these materials for
construction is very desirable in both short and long terms as a
stimulant for socio-economic development. In particular, materials
that can complement cement in the short run, and especially if
cheaper, will be of great interest.
the past decade,
the presence of mineral admixtures in construction materials has
been observed to impart significant improvement on the strength,
durability and workability of cementitious products (Mental, 1994;
Falade, 1997; Oyekan, 2001). In areas prone to flood, hygrothermal
properties of the buildings’ construction materials are of
importance. Also, energy requirements for residential and commercial
buildings are known to be influenced by building design and by the
materials used. In both temperate and tropical regions, thermal
properties of building materials are of significant importance to
the determination of
the heating or cooling load within the building and hence the
capacity of the mechanical equipment required in handling the load.
This is necessary to provide a given level of thermal comfort within
the building and over the annual climatic cycle.
Substitution of any of
these admixtures is aimed at enhancing at least one of the
properties of the block. Rice husk is a residue produced
in significant quantity on a global basis. While it is utilized as
fuel in some regions, it is regarded as a waste in others thereby
causing pollution; due to problem with disposal.
Hence, it’s profitable use in an environmentally friendly manner,
will be a great solution to what would otherwise be a pollutant.
When burnt under controlled conditions, the rice husk ash (RHA) is
highly pozzolanic and very suitable for use in lime-pozzolana mixes
and for Portland cement replacement (Smith, 1984; Chandrasekhar et
al., 2003; Yogenda et al., 1988; Anwar et al., 2000; Paya et al.,
2000; Nair et al., 2006; Goncalves and Bergmann, 2007; Rodriguez et
al., 2008). Effect of RHA blended cement on the strength and
permeability properties of concrete has been investigated by Ganesan
et al. (2008). On sandcrete block, Cisse and Laquerbe (2000)
observed that the mechanical resistance of sandcrete blocks obtained
when unground ash is added increased in performance over the classic
mortar blocks. Their studies on Senegalese RHA also revealed that
the use of unground RHA enabled production of lightweight sand-crete
block with insulating properties at a reduced cost. The ash
pozzolanic reactivity was responsible for the enhanced strength
obtained. Okpala (1993) partially substituted cement with RHA in the
percentage range of 30–60% at intervals of 10% while considering the
effect on some properties of the block.
results revealed that a sandcrete mix of 1:6 (cement/sand ratio)
required up to 40% cement replacement and a mix of 1:8 ratio
required up to 30%, are adequate for sandcrete block production in
Nigeria. However, it is worthy of note that due to the high cost of
procuring the rice husk required for producing large number of
blocks needed for an average-size building, and in the light of the
diminishing agricultural activities in Nigeria, replacing cement
with such high volume of RHA could be economically counterproductive
for local sand-crete block manufacturers thereby defeating the main
purpose of the substitution which is to reduce the unit cost of the
the hygrothermal properties and some vital thermal properties are
not considered in the investigation. Therefore, to encourage
continual production and for better characterization of the
sandcrete blocks, this paper investigates experimentally the effect
of partially replacing cement with the Nigerian rice husk ash on
some structural, thermal and hygrothermal properties of the blocks.
The replacement is by volume and it is in the range of 5–20% at
intervals of 5%.
Materials and Methods
material constituents, their mix, presence of admixtures and
manufacturing process are important factors that determine the
properties of sandcrete blocks. The materials used and method of
manufacture employed in this investigation are thus presented.
Materials of the sandcrete blocks
sandcrete blocks are made of sand, cement and water.
river quartzite sand that is free of clay, loam, dirt and any
organic or chemical matter is used. It is sieved with the 3.35 mm
zone of British standard (BS) test sieves. The sand has a specific
gravity of 2.66 and an average moisture content of 0.90%. The
coefficient of uniformity of the sand is 2.95.
cement used is the ordinary portland cement (OPC) from the West
African Portland Cement Company, Ogun State, Nigeria with properties
conforming to BS 12 (British Standards Institution, 1971).
Fresh, colourless, odourless and tasteless potable water that is
free from organic matter of any kind.
Rice husk ash
husks used were collected from a threshing site at Ifo, Ogun State,
Nigeria. Open field burning method is employed. The husks are burnt
into ash and sifted in order to remove all particles that do not
pass the 45 µm test sieve (British Standards Institution, 1997).
Table 1 shows the result of the chemical analysis of the RHA and of
the Portland cement. From the chemical analysis of RHA, the sum of
SiO2, Al2O3 and Fe2O3
is 79%. This satisfies the minimum percentage requirement for
pozzolana when these constituents are added, which is 70% for ASTM
C618 (American Society for Testing and Materials, 1978).The moisture
content is less than 1.5% specified by BS 3892. It therefore implies
that the RHA used in the present study is a good pozzolana since it
meets the requirements of BS 3892 and ASTM C618 for pozzolana. The
specific gravity of the RHA is 2.17 and its density is 2170 kg/m3.
The specific gravity value is almost the same as 2.15 obtained by
Sampaio et al. (2003) but less than 2.36 reported by Cook et al.
(1977) and higher than 1.54 obtained by Okpala (1993).These values
indicate that the specific gravity of RHA is location and
Grading of aggregates
grading of an aggregate defines the properties of different sizes in
the aggregate. This grading has a considerable effect on the
workability and stability of the mix. Wet sieving analysis which is
in accordance with BS 1377 (British Standards Institution, 1990) is
used. The particle size distribution curve of the sand used in this
study is shown in Figure 1.
Manufacture of sandcrete blocks
blocks (all hollow) are manufactured with the use of a vibrating
machine. The standard mix proportion of 1:6 cement-sand ratio; that
is, one part by volume of cement to six parts by volume of coarse
sand, is used in this investigation. The size of the block produced
is 225 x 225 x 450 mm with one-third of the volume void so as to
produce the type of hollow sandcrete blocks commonly used for
construction of buildings in Nigeria. Four percentages of cement
substitution with RHA (that is, 5, 10, 15 and 20%) by volume are
used. A specific gravity of 2.17 for RHA in comparison with 3.14 for
OPC implies that replacement of cement with RHA by weight would
result in a much greater volume of the cementitious material in the
mix. Consequently, the replacement of cement with RHA is by volume,
as well as the batching of cement and sand.
the manufacture of the blocks, hand mixing is employed and the
materials are turned over a number of times until an even colour and
consistency are attained. Water is added through a fire hose and it
is further turned over to secure adhesion. It is then rammed into
the machine moulds, compacted and smoothed off with a steel face
tool. After removal from the machine moulds, the blocks are left on
pallets under cover in separate rows, one block high and with a
space between 2 blocks for the curing period. They are kept wet
during this period by watering daily. The laboratory condition is 27
± 2°C dry-bulb temperatures, 50 ± 5% relative humidity. Testing for
compressive strength is then carried out at ages 7, 14, 21 and 28
days. On the average, four specimens are tested at each age and each
percentage replacement of cement with RHA.
units immersed in liquid absorb the liquid due to their porous
nature. The volume, the rate and the dominant method of
Chemical analysis of RHA and cement (OPC).
Rice husk ash (%)
Aluminium oxide (Al2O3)
Ferrous oxide (Fe2O3)
Calcium oxide (CaO)
Magnesium oxide (MgO)
Sodium oxide (Na2O)
Potassium oxide (K2O)
Barium oxide (BaO)
Lead oxide (PbO)
Particle size distribution curve of sand.
absorbing the liquid depends on the interstitial arrangement of the
particles of the constituent materials at macro level. It is
therefore necessary to investigate the effects of replacing cement
with RHA on the hygrothermal properties of the block. Some of the
properties are determined as follows:
Presence of admixtures may increase, decrease or maintain the
porosity of the main material depending on the aggregate sizes. When
exposed to persistent flooding, a highly porous block could absorb
much water, consequently become weakened and eventually fail.
volume of liquid absorbed by a porous medium is an indication of its
pore volume and it is a good approximate measure of its porosity.
is obtained with the relation.
term “permeability” is often loosely used to cover a number of
different properties. In this paper, it is defined as the property
of a porous medium which characterizes the ease with which a fluid
will pass through it under atmospheric pressure. Darcy’s law for
fluid flow in a permeable medium expresses permeability in terms of
measurable quantities and states that the steady state rate of flow
is directly proportional to the hydraulic gradient. Thus, for uni-axial
penetration employed, permeability, K can be expressed as
using blocks for external walls in tropical humid climate,
water-resistance ability of the blocks must be considered in order
to minimize penetration of moisture or rain water into the interior
of the building. Many times, block work is used in the construction
of channels for drainage. Blocks to be used for such purposes must
have low sorptivity value. Sorptivity is a measure of the capacity
of a porous medium to absorb liquid by capillarity. The absorption
of water under capillary action is directly proportional to the
square-root of time (Hall, 1989).
Various test methods are used to determine hygrothermal properties
of a material. However, in most cases, the test method chosen for a
particular property is always the one appropriate to the predominant
transport mechanism acting on the block. After evaluation of various
methods, capillary rise method is employed in this investigation due
to its simplicity and accuracy. Basically, a sample of sandcrete
block is placed with one edge fully in contact with the water
surface. Through that area, water is absorbed. The height of
capillary rise is then measured at increasing time intervals. The
fineness of the capillary pores in sandcrete blocks promotes
absorption of water by capillary attraction; hence, a measure of the
rate of absorption provides a useful indication of the pore
structure. If water is absorbed rapidly, it shows that the pores are
either large or straight; if the absorption rate is slow, then the
pores are small or not easily accessible.
Thermal properties of most cementitious materials are found to
change with the presence of admixtures (Cisse and Laguerbe, 2000;
Okpala, 1993). The change is found to depend on the admixture’s
grain structure or interstitial arrangement within the main material
and other micro structural parameters including the volumetric
fraction of each constituent, the shape of the particles, and the
size distribution of the particles. In predicting the thermal
performance of buildings, it is necessary to consider the dynamic
effects of this variation. The thermal properties investigated in
this study are:
Thermal conductivity (k)
Thermal conductivity is a measure of the quantity of heat that flows
through a material per unit time. Thermal conductivity of most
materials is found to change with the presence of impurities
or admixtures. From the Fourier’s steady-state heat conduction
equation, thermal conductivity is determined as
Specific heat capacity (c)
a measure of the thermal storage capacity of the material. The
specific heat capacity of a sandcrete block indicates the relative
amount of heat energy the wall built with it is capable of storing
per unit mass. Walls with high specific heat capacity can store more
energy, have a larger thermal lag, and thus, generally be more
effective for thermal storage and peak load shifting. This time lag
effect contributes to shifting demand to off-peak periods and
improves overall thermal efficiency. Specific heat capacity of the
sandcrete block is determined from the classical heat capacity
Thermal diffusivity (a)
Thermal diffusivity is a measure of the material’s ability to
undergo a temperature change. It describes the heat transfer
capability of a material relative to its heat storage ability.
Materials with a low thermal diffusivity have a slow rate of heat
transfer relative to heat storage. Thermal diffusivity is obtained
through the relation
Thermal effusivity (b)
Thermal effusivity represents the capacity of a material to absorb and
release heat. The value of the thermal effusivity is useful in
calculating the heat-accumulation capacity of materials. Materials
with high thermal effusivity cannot hold heat long enough because
heat will quickly dissipate from its surface as soon as surrounding
temperature drops. On the other hand, materials with low thermal
effusivity (but with high thermal inertia) will hold heat much
longer. Thermal effusivity is calculated as
Necessary precautions were taken at every stage and for each
procedure to minimize error as much as possible. The results are
presented in graphical form in most cases.
Effect of RHA substitution on compressive strength
Figures 2a and b show the variation of compressive
strength with percentage RHA content for the mix pro-portion used.
The graphs show that the blocks decrease in strength as the RHA
percentage content in the mix increases. It shows that RHA does not
appreciably enhance the compressive strength of the conventional
sandcrete block. The reduction in strength could be attributed to
the fact that the partial replacement of cement with the RHA caused
a reduction in the quantity of cement in the mix available for the
hydration process and hence a reduction in the formation of the
stable strength- producing cementitious compounds. The difference in
the chemical composition of RHA and of cement it replaced is also a
major factor. As shown in Table 1, the major constituent of RHA is
silica (SiO2) while that of cement is quicklime (CaO). On
mixing with water, the silica produced in solution reacts with
calcium to produce calcium fly ash (C3H) which is further
hydrated in the reaction with water to produce tobermorite (CSH)
gel. The formation of CSH gel is responsible for slower rate of
hydration of cement which commences after about 14 days and extends
up to about 150 days after which the fly ash particles would almost
be completely disintegrated.
Plot of compressive strength against (a) percentage RHA
substitution; (b) age of sandcrete blocks; (c) percentage RHA
Figure 2c shows the plot of the compressive strength against
percentage RHA substitution for the 0-20% range considered in the
present study and the plot for the 30-60% range investigated by
Okpala (2003) for the 28-day-old blocks. Rather than the gradual
decrease in the compressive strength observed in Okpala (1993), the
decrease is sharp in the present study. At 20% RHA substitution, the
compressive strength has reduced drastically to 29.5% in the
strength of the control block. But in Okpala (1993), at 60% RHA
substitution, the strength is just 39% of that of the control block.
This means that the strength of the blocks produced with RHA used by
Okpala within the 5-20% range would be higher than
that obtained in the present study. This may be due to differences
in the physical properties of the RHA used. These properties depend
on the source location, harvest time, rice husks processing and
burning procedure used. The differences in block size and block
manufacturing process could also be some of the possible reasons.
Plot of density against percentage RHA substitution.
Effect on density
plot of density variation with the percentage increase in the RHA
substitution after 28 days is shown in Figure 3. The plot indicates
decrease in density with percentage increase in RHA substitution.
Other data indicate that the density decreases with curing age. This
means that partially substituting cement with RHA produces lighter
Effect on porosity
variation of porosity with percentage substitution of the RHA is
presented in Figure 4.
results show that all the blocks with the admixture are more porous
than that of the control (0%). Figure 4 shows a gradual increase in
the value of porosity when only 5% RHA is added. But further
addition of the admixture rapidly increases the porosity value. This
increase in porosity may be as a result of trapped air bubbles that
are interconnected. It can be concluded that sandcrete blocks with
RHA blended cement is more porous, absorbs more liquid and hence
could fail faster than those without admixture. This makes such type
of block not suitable for areas prone to flood or with year-round
rainfall like Niger Delta areas.
Effect on permeability
Figure 5 shows the variation of permeability with the percentage
increase in RHA content. It is observed that partially replacing
cement with RHA increases the permeability of the sandcrete block.
The value of the permeability steadily increases with percentage
increase in RHA content. The value is almost doubled for every 5%
addition of RHA. This means that the inclusion of the admixture
opens up the block in a way that encourages upflow of fluid. This
rapid flow of
indicates that the pores are either large or straight.
Effect on sorptivity
observed in Figure 6a that the value of sorptivity of the sandcrete
blocks gradually increases with the percentage content of RHA. This
implies that blocks made with cement partially replaced with RHA are
not suitable for drainage channels construction but could be useful
for partitioning of building spaces. The results of the present
study were compared with the experimental data of Sampaio et al.
(2003) on sandcrete blocks produced with Portuguese RHA (Figure 6b).
The plot shows that while the sorptivity value of the blocks
produced with cement and blended with Nigerian RHA increases, that
of the blocks produced with the Portuguese RHA blended cement decreases as the
percentage RHA content increases. The implication of this is that
the substitution of cement with the Nigerian RHA forms a material
structure that encourages liquid absorption as the RHA content
increases whereas the blocks produced with the cement partially
replaced with the Portuguese RHA forms a more compact structure that
reduces absorption of liquid.
Plot of porosity against percentage RHA substitution.
Plot of permeability against percentage RHA substitution.
Effect of temperature change on the hygrothermal properties
the temperature of the block increased above the room temperature,
it was observed that the block absorbed water at a faster rate. This
means that the higher the temperature around a building, the greater
the rate of liquid absorption.
Effect on thermal conductivity
thermal conductivity of the sandcrete blocks is determined using the
Guarded Hot Plate Box
conforming to the
requirements of ASTM C177
(American Society for
Testing and Materials, 2004). The effect of the substitution of RHA
on the thermal conductivity of the sandcrete blocks is presented in
Figure 7a. The value of the thermal conductivity increases with the
percentage RHA content. The higher k value obtained when cement was
partially substituted with RHA may be as a result of the products of
reaction of RHA with cement in the mix during the hydration process.
The products that tend to increase the k value of the block despite
its increased porosity are subject to further investigation. For a
hot climate, higher thermal conductivity block is undesirable
for construction as it increases the cooling/heating
load in the building space.
plot of the 0-20% RHA substituted range investigated in this study
and the 30-60% range of Okpala (1993) in Figure 7b, shows an
interesting variation. The value of the thermal conductivity is
found to increase with percentage RHA substitution up to a maximum
at 20% (or possibly 25%) and then decreases thereafter as the
percentage RHA increases. The initial increase in k value with
increase in the percentage RHA content may be as a result of higher
k values of the products of reaction between RHA and cement.
However, as more RHA is added to the mix and the quantity of cement
reduces, the inert nature of the excess RHA probably caused the
thermal conductivity to reduce. The complex structure of the block
material may be a contributing factor. Also, being a
non-metallic solid, heat is transferred via lattice vibrations which
is characterised by much variability.
Figure 6. Plot of sorptivity against (a) percentage RHA substitution; (b)
percentage RHA substitution for Nigeria and Portugal.
Plot of thermal conductivity against percentage RHA substitution for
(a) 0-20; (b) 0-60.
specific heat capacity
determination of the specific heat capacity of the sandcrete blocks
is carried out using the adiabatic calorimetric technique. The plot
of the specific heat capacity against percentage RHA substitution in
Figure 8, indicates that all the blocks with the admixture have
slightly lower values (except the 5% block) than that of the control
block; hence, lower heat energy storing capacity and lower thermal
mass. In tropical environment, these blocks will lose heat gained
during the day faster, thereby making the building space comfortable
for early sleep.
Plot of specific heat capacity against percentage RHA substitution.
Plot of thermal diffusivity against percentage RHA substitution.
Effect on thermal diffusivity
manner slightly consistent with the thermal conductivity variation,
Figure 9 shows that the values of the thermal diffusivity of all the
blocks is higher than that of the control. The reducing values of
both density and the specific heat capacity with RHA content caused
the thermal diffusivity value to increase with the percentage RHA
substitution. In effect, sandcrete blocks made with RHA-blended
cement will undergo a faster temperature change or allow more rapid
heat flow through them than those
without the admixture.
Effect on thermal effusivity
thermal effusivity value in Figure 10 fluctuates as the percentage
RHA content increases. The β values of most of the blocks containing
RHA are more than those of the control block. The β value of the 15%
RHA block, which is the largest, is 17.5% more than that of the
control block. In practical terms, the higher thermal effusivity of
the blocks made with RHA-blended cement increases its ability to
conduct heat away from the building space faster thereby reducing
the air-conditioning load, and consequently increasing the
period of thermal comfort.
Plot of thermal effusivity against percentage RHA substitution.
is worthy of note that, despite much effort in minimizing possible
areas of errors, the observed thermal test results may have some
influences which might likely be caused by two factors: differences
in heat transfer pathways and differences in sample homogeneity.
There may be inhomogeneous intrasample effects, contact resistance
effects and differences in the heat transfer process through the
sample which could affect the test results. Though the particle
sizes of RHA and OPC are the same, for a given volume of sample,
changes in the particle size distribution of both materials in the
mix with sand may lead to changes in the number of particle-particle
and particle-gas interfaces as well as the interstitial voids and
the volume of those voids. A wider variety of heat transfer pathways
are available to the penetrating heat wave through a sample with
broader particle size distribution. This also could, by extension,
be applied to the hygrothermal properties due to the phenomenon of
flowing water’s ‘path of least resistance’. Nevertheless, since the
test results agree to some extent with the internationally-accepted
standard values available in the literature, it would appear that
these factors did not have any significant effects.
main conclusions derived from this investigation are as follows:
the percentage RHA content in the mix increases, the compressive
strength of the sandcrete block de-creases. Hence, RHA does not
enhance the compressive strength of the conventional sandcrete block
with 1:6 cement-sand mix.
The addition of RHA into the cement-sand matrix produces sandcrete
blocks of lighter weight. The density
the blocks decreases as the percentage RHA content in the mix
sandcrete block with RHA-blended cement is more porous than a pure
sand-cement block. Its permeability increases with RHA content and
The products of the reaction between the rice husk ash and the
ordinary Portland cement tend to initially slightly increase the
thermal conductivity of the block. However, as the percentage RHA
content increases beyond 20%, the thermal conductivity reduces with
increasing RHA content.
The sandcrete blocks made with RHA-blended cement have lower heat
storage capacity and lower thermal mass than those without the
Due to the increasing values of thermal diffusivity with increasing
RHA content, blocks containing RHA tend to undergo faster
temperature change as heat flows through them more rapidly.
The increased thermal effusivity of the sandcrete block with RHA
content is an advantage over pure sandcrete block as it improves its
ability to conduct heat away from the building space faster thereby
reducing the air-conditioning load and hence increasing the period
of human thermal comfort.
practical significance of this study is that the strength,
liquid-absorption and heat transfer characteristics of the sandcrete
blocks presented will be of a great value to building professionals
engaged in the design and analysis of building structures.
Designers would be able
to properly size the heating/cooling equipment necessary to provide
a given level of thermal comfort within the building and over the
annual climatic cycle. Consequently, there is reduction
in the size of the air-conditioning system required to cool or heat
the space, reduction in the thickness of the thermal insulator and
extension of the period of human comfort without reliance on
mechanical air-conditioning. The above
qualities reduce the annual energy cost in addition to other energy
conservation and environmental effects.
work was funded by the Central Research Committee of the University
of Lagos, Nigeria. Their assistance is hereby gratefully
BS, British standard; OPC, ordinary Portland cement;
RHA, rice husk ash; A, area of material perpendicular
to heat flow (m2); A'', cumulative
infiltration; c, specific heat capacity (J/g K); H,
height of liquid rise after time t (m); h, hydraulic head
(m); k, thermal conductivity (W/m K); K, permeability
(cm/min); m, mass of the sample (kg); P, percentage
substitution of admixture (%); Q, heat flux (W); S,
temperature change (K); T, time taken for liquid to rise (s);
V, volume of material sample (m3); Vf,
volume of water absorbed (m3);
sample thickness (m);
thermal diffusivity (m2/s);
thermal effusivity (W/m2 K s1/2);
change in temperature (K);
porosity of the material (%).
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