Effect of Boron Carbide Addition on the Physical, Mechanical and Microstructural Properties of Portland Cement Concrete
Fatin Nabilah Tajul Ariffin,
Sahrim Haji Ahmad
Concrete nuclear reactors could be improved in terms of life safety by adding boron carbide. This study presents an experimental investigation of the physical, mechanical and microstructural properties of Portland cement concrete containing boron carbide (B4C) as a neutron radiation-absorbing material for nuclear reactor applications. The boron carbide powder additions were 5 and 20% of the cement weight. The water-to-cement ratio of the concrete design mix was 0.4. The results show that the concrete density decreased as the percentage of boron carbide content increased. The results also show that concrete with a 0% content of B4C produced the highest compressive strength (32.73 MPa) and that the addition of 5 and 20% B4C produced a negligible reduction of strength (<2% compared with 0% B4C concrete). Scanning Electron Microscopy (SEM) results confirm that the addition of boron carbide to Portland cement concrete reduces the strength and density of concrete because the morphology of samples containing 5 and 20% B4C by weight (wt.) shows a more porous concrete microstructure compared with the control samples. Energy Dispersive X-ray (EDX) analysis was used to conclude that the higher content of B4C results in a lower percentage of calcium in the concrete which in turn reduces the strength. Up to 20% B4C powder by weight can be added to concrete which produces minimal strength reduction.
Received: July 24, 2011;
Accepted: November 07, 2011;
Published: December 29, 2011
A nuclear reactor is a source of energy for heat and electricity generation
(Morioka et al., 2002). Nuclear research in Malaysia
began in 1983 with the development of a nuclear research reactor named Triga
Mark II which generates 1 mW of power. Nuclear reactors are usually surrounded
by thick layers of concrete which serves as biological shielding. The concrete
protects the surroundings from the high levels of radiation emitted from the
reactor and is used to shore up the reactor and its related equipment (Yousef
et al., 2008). Therefore, a special radiation shielding concrete
is a very important component of a nuclear reactor.
Boron carbide (B4C) is the third hardest material after diamond
and cubic boron nitride (McColm, 1990). Boron carbide
has ideal characteristics for use as a radiation shielding material because
of its high neutron absorption capacity. It can sustain high temperatures, has
a low density and a high degree of chemical inertness (Unal
et al., 2006). Fast neutrons are not easy to extinguish because they
easily penetrate most materials. They should be slowed down by scattering within
materials that have a high interaction-scattering cross section (Kharita
et al., 2011). The lighter isotope of boron, boron-10, could cause
low penetration from neutron capture by hydrogen in the gamma ray test (ASTM
C 638-92, 2002). The neutron absorption cross section of boron-10 is 760
barns. The higher the neutron absorption cross section, the more stable the
nucleus capturing the neutron. Li and Qiu (2007) reported
that boron carbide is used as a reinforcement in ceramic matrix composites in
high-technology industries to increase wear and corrosion resistance properties.
Thevenot (1990) noted that nuclear reactors have taken
extensive advantage of boron carbide because of its adaptable properties, such
as its low density, low thermal conductivity and high neutron absorption area
(Rao et al., 2009). Boron carbide powder (15
μm in size) is added to concrete to produce Portland cement/B4C
concrete as a neutron-absorber concrete. Boron carbide is categorized as a non
metallic ceramic material and the hardest compound of the boron-containing materials.
The properties of boron carbide is a combination of unique characteristics,
such as strength (H = 35 Gpa), low density (ρ = 2.51 g cm-3)
and dynamic modulus of elasticity which is the highest of ceramics. Boric oxide
and petroleum coke were used as the raw materials to produce boron carbide in
a heat-resistance furnace at a temperature of up to 2600°K (Rao
et al., 2009).
A combination of calcium and aluminum silicates is used to produce Portland
cement. These two powdered crystalline minerals can be hardened with the addition
of water which produces a stone-like material. The recrystallization process
forms interlocking crystals which results in the production of cement gel. Once
hardened, this material has a high compressive strength (Nawy,
Aggregates is also a key component in the formation of concrete. The size and
quantity of aggregate influence the concrete mix design. The concrete strength
increases as the size of the aggregate decreases (Abo-Qudais,
2005). However, a small-aggregate-concrete will have a higher cost. Sand
and rock fragments are commonly used in concrete mix (Kamarudin,
In the process of forming the concrete, the water to cement ratio (w/c) is
the most direct measure of the strength of the concrete (Nawy,
2009). Cement will harden when it reacts with water. The concrete strength
is governed by its water content (Ohdaira and Masuzawa,
In this work, the main objective was to determine the maximum percentage possible of boron carbide that can be added to Portland cement concrete to improve the nuclear reactor concrete shielding capacity while at the same time maintaining the concrete strength.
MATERIALS AND METHODS
Twelve samples of concrete cube with dimensions of 100x100x100 mm3
were prepared using ordinary Portland cement (Type I) from YTL Cement Berhad.
Four cubes were prepared as control samples (0% B4C content), while
another eight cubes of concrete contained different contents of boron carbide
from Sigma-Aldrich Corporation (5 and 20%). Details of the mix proportions for
the concrete containing different weight percentages of boron carbide are provided
in Table 1.
|| Quantity of Portland cement/B4C concrete components
Samples were prepared using the Department of Environment, United Kingdom method
(Teychenne et al., 2010) for concrete grade 20
in the laboratory. Coarse aggregate with a maximum size of 10 mm was obtained
from Lafarge Aggregates Sdn Bhd.
The density of the samples is determined using the Archimedes' concept according
to BSI (1983a) to obtain more accurate density values.
The values of the compressive strength of the concrete cubes after curing for
28 days are obtained in accordance with BSI (1983b).
The morphology of Portland cement/B4C concrete is examined using
Scanning Electron Microscopy (SEM). Prior to the scan, the sample surface is
coated with a thin layer of gold. The thickness of the gold layer is between
0.01 to 0.1 μm. The elements present in the sample are analyzed by the
X-ray Energy Dispersion (EDX) method (Buhrke et al.,
1998) conducted in conjunction with Scanning Electron Microscopy (SEM).
RESULTS AND DISCUSSION
The densities of the concrete samples are shown in Table 2. The results show that the Portland cement/B4C concrete sample with 0 wt.% of boron carbide has the highest density, 2255 g cm-3. The lowest density is observed for Portland cement/B4C concrete with 20 wt.% of boron carbide, 2096 g cm-3. This indicates that the concrete cube without boron carbide is denser than Portland cement/B4C concrete.
Table 2 shows that the density of Portland cement/B4C
concrete decreases with the increase in the boron carbide percentage. This is
due to the density of boron carbide (2.51 g cm-3) which is lower
than the density of concrete. As a result, the higher the weight percentage
of boron carbide added to the concrete, the lower the density of the Portland
cement/B4C concrete. To confirm these results, Albano
et al. (2005) found that the addition of other materials, such as
scrap rubber, can lower the density of concrete. The study showed that concrete
mixes with scrap rubber can decrease the density of the concrete by 20.34% compared
with only a 2% density loss by the addition of 5 wt. % boron carbide.
Table 3 shows that the compressive strength decreases with
increasing boron carbide content. Portland cement/20% B4C concrete
had the lowest compressive strength of 29.7 MPa, while the Portland cement/0%
B4C concrete had the highest compressive strength of 32.73 MPa. The
decrease is less than 2% compared with concrete with 0% boron carbide. It can
be concluded that up to 20% B4C content can be added to Portland
cement concrete without affecting the strength significantly. Based on previous
research by Kharita et al. (2009), the improvement
of the compressive strength of shielding concrete made from hematite aggregates
can be performed by adding the optimum weight of 6% of carbon powder. Their
study showed that the addition of carbon powder can improve the mechanical properties
of the concrete. However, the addition of boron carbide gives the opposite result:
it did not enhance the concrete compressive strength.
Scanning Electron Microscopy (SEM) is capable of analyzing images and providing
a quantitative analysis of the surface microstructure of the samples. SEM observations
of Portland cement/0%B4C concrete are shown in Fig.
|| Density of Portland cement/B4C concrete
|| Compressive strength of Portland cement/B4C concrete
This Fig. 1 clearly indicates that the sample grain-size
is small and irregular. Therefore, the surface area of the sample in Fig.
1 is large and this causes the particles to become more compact, as indicated
by the higher density of Portland cement/0% B4C concrete. Figure
2 is the SEM image of the 5 % B4C concrete mixture which has
a more homogeneous structure (Unal et al., 2006).
The morphology of the boron carbide distribution was observed by Unal
et al. (2006), who studied the grain size determination of boron
carbide. The SEM test of the samples shows that the boron carbide compound contained
large particles, with smaller particles separated evenly throughout the matrix
with a low-porosity surface. According to Fig. 2, when boron
carbide was added to concrete, the SEM image shows that the microstructure has
a high porosity with large and small particles of boron carbide separated on
the concrete surface. It is clearly visible that the boron carbide particles
can be detected by their shape and size difference. This shows that the boron
carbide particles are bound to the concrete surface and influence the morphology
of the concrete.
Figure 3 shows a larger grain size and a more porous structure
than that observed in Fig. 1 and 2. This
is because of the higher content of boron carbide of 20%. The relationship between
density and porosity is an indirect proportionality. The density of 20 wt.%
B4C concrete sample is the lowest because the surface area of the
grains in the sample is the smallest. It is clearly observed that the B4C
particles can be recognized by their shape difference (Aitcin,
||SEM with 5000 times magnification of the 0 wt.% B4C
||SEM with 5000 times magnification of the 5 wt.% B4C
||SEM with 5000 times magnification of the 20 wt.% B4C
Energy Dispersive X-ray (EDX) analysis was performed to analyze the elements
qualitatively. Previous study by Asokan et al. (2010)
indicated that the EDX spectrum can show the elemental compositions of concrete.
The results of EDX show that there are four main elements detected in the concrete:
calcium, silicon, oxygen and boron. These elements are high-concentration elements
and other trace elements are also present which include iron, aluminium, magnesium
and sodium. The results confirmed the presence of boron in both spectra (Fig.
5 and 6). The relative percentage of calcium present in
the 0 wt.% B4C concrete sample (Fig. 4) is the
highest (20.93%), followed by the 5 wt. % B4C concrete sample (Fig.
5), which is 15.63%. The percentage of calcium in the 20 wt.% B4C
concrete sample shown in Fig. 6 is 12.16%.
||EDX spectrum graph of a 0 wt.% B4C concrete sample
||EDX spectrum graph of a 5 wt.% B4C concrete sample
||EDX spectrum graph of a 20 wt.% B4C concrete sample
From this analysis, it can be concluded that the higher content of B4C
results in a lower percentage of calcium in the concrete which in turn reduces
The peak of the boron element in Fig. 5 shows the presence of 20.42% boron. Meanwhile, Fig. 6 (20 wt.% B4C) shows 37.02% boron which is higher than that in Fig. 5 (5 wt.% B4C).
The density analyses prove that the density of the concrete with 0% boron carbide is higher than that of the Portland cement/B4C concrete. Portland cement/5% B4C concrete reduced the density by 2% which is significant. If strength is the major issue, 5 wt.% B4C is the optimum value for the boron carbide addition in concrete since this mix strength is at par with the normal concrete. The addition of up to 20% boron carbide particles to Portland cement concrete reduces the strength of the concrete by 2%. The morphology of SEM observations also showed that the morphology of 0% B4C is denser than the morphology of 5 and 20% B4C concrete samples. Energy dispersive X-ray (EDX) analysis concluded that the higher content of B4C results in a lower percentage of calcium in the concrete which in turn reduces the strength.
1: Abo-Qudais, S.A., 2005. Effect of concrete mixing parameters on propagation of ultrasonic waves. Construct. Build. Mater., 19: 257-263.
2: Aitcin, P.C., 2003. The durability characteristics of high performance concrete: A review. Cement Concrete Compos., 25: 409-420.
3: Albano, C., N. Camacho, J. Reyes, J.L. Feliu and M. Hernandez, 2005. Influence of scrap rubber addition to Portland I concrete composites: Destructive and non-destructive testing. Compos. Struct., 71: 439-446.
4: Asokan, P., M. Osmani and ADF Price, 2010. Improvement of the mechanical properties of glass fibre reinforced plastic waste powder filled concrete. Construction Build. Mater., 24: 448-460.
5: ASTM C 638-92, 2002. Standard Descriptive Nomenclature of Constituents of Aggregates for Radiation-Shielding Concrete. American Society for Testing and Materials, West Conshohocken, USA., pp: 338-340
6: Buhrke, V.E., R. Jenkins and D.K. Smith, 1998. A Practical Guide for the Preparation of Specimens for XRF and XRD Analysis, John Wiley and Sons, Inc., New York, USA., Pages: 334
7: BSI, 1983. Testing Concrete, Part 114: Methods for Determination of Density of Hardened Concrete. British Standard Institution, London, UK., ISBN-13: 9780580129483
8: BSI, 1983. BS 1881-116: Testing Concrete. Method for Determination of Compressive Strength of Concrete Cubes. British Standards Institution, London, UK.,
9: Kamarudin, M.Y., 1995. Introduction to Strength and Durability of Concrete. 1st Edn., Hulu Kelang, Dewan Bahasa dan Pustaka, ISBN: 983-62-4785-8
10: Kharita, M.H., S. Yousef and M. Al-Nassar, 2009. The effect of carbon powder addition on the properties of hematite radiation shielding concrete. Prog. Nucl. Energy, 51: 388-392.
CrossRef | Direct Link |
11: Kharita, M.H., S. Yousef and M. Al-Nassar, 2011. Review on the addition of boron compounds to radiation shielding concrete. Prog. Nucl. Energy, 53: 207-211.
12: Li, Y.Q. and T. Qiu, 2007. Oxidation behaviour of boron carbide powder. Mater. Sci. Eng. A, 444: 184-191.
13: McColm, I.J., 1990. Ceramic Hardness. Plenum Press, New York, USA., ISBN 0-306-43287-0, Pages: 226
14: Morioka, A., A. Sakasai, K. Masaki, S. Ishida and N. Miya et al., 2002. Evaluation of radiation shielding, nuclear heating and dose rate for JT-60 superconducting modification. Fusion Eng. Design, 63-64: 115-120.
15: Nawy, E.G., 2009. Reinforced Concrete: A Fundamental Approach. 6th Edn., Prentice Hall, USA., ISBN-13: 9780132417037, pp: 9-10
16: Ohdaira, E. and N. Masuzawa, 2000. Water content and its effect on ultrasound propagation in concrete: The possibility of NDE. Ultrasonics, 38: 546-552.
CrossRef | Direct Link |
17: Rao, M.P.L.N., G.S. Gupta, P. Manjunath, S. Kumar, A.K. Suri, N. Krishnamurthy and C. Subramanian, 2009. Temperature measurements in the boron carbide manufacturing process: A hot model study. Int. J. Refract. Met. Hard Mater., 27: 621-628.
CrossRef | Direct Link |
18: Thevenot, F., 1990. Boron carbide: A comprehensive review. J. Eur. Ceram. Soc., 6: 205-225.
Direct Link |
19: Unal, R., I.H. Sarpun, Y.A. Yalim, A. Erol, T. Ozdemir and S. Tuncel, 2006. The mean grain size determination of boron carbide (B4C)-aluminium (Al) and boron carbide (B4C)-nickel (Ni) composites by ultrasonic velocity technique. Mater. Charact., 56: 241-244.
CrossRef | Direct Link |
20: Yousef, S., M. Al-Nassar, B. Naoom, S. Alhajal and M.H. Kharita, 2008. Heat effect on the shielding and strength properties of some local concretes. Prog. Nuclear Energy, 50: 22-26.
CrossRef | Direct Link |
21: Teychenne, D.C., R.E. Franklin and H.C. Erntroy, 2010. Design of Normal Concrete Mixes. 2nd Edn., Taylor and Francis, USA., ISBN: 9781860811722, Pages: 48