HEAVY WEIGHT CONCRETE CONTAINING SILICA FUME USED AS

HEAVY WEIGHT CONCRETE CONTAINING SILICA FUME USED AS RADIATION SHIELDING

A.T.M. Farag1, Ihab A. Adam2, Farouq Husain2, A.M.I. Kany1 1 Al-Azhar University, Faculty of Science, Physics Department. Cairo, Egypt.

2 Construction Research Institute. National Water Research Centre, Delta Barrage, Egypt.

Rec. 31/12/2019 In final form 23/03/2020 Accept. 11/04/2020

This research was carried out in order to produce a heavy weight concrete containing silica fume with different percentages, for studying the effect silica fume on both mechanical and nuclear properties of concrete containing silica fume was investigated. The investigated concrete mixtures composed of local Barite ore (as coarse aggregate) and Serpentine ore (as fine aggregate) mixed with ordinary Portland cement and tap water. Superplasticizer was added as chemical admixture to enhance the workability of fresh concrete. Cement was replacing with Silica fume by weight as a percentage of cement ranged (0%, 5%, 10% and 15%), Slump and unit weight of fresh concrete were measured. Compressive strength, indirect tensile strength, elastic modulus, and ultrasound pulse velocity tests were carried out on hardened concrete at various ages after casting. Furthermore, nuclear radiation attenuation properties (γ-ray) of such concretes were experimentally and theoretically evaluated. A collimated beam of gamma rays emitted from Eu-152 source was used as a source of gamma rays. The experimental measurements of the attenuation of gamma rays at different energies in such concrete samples have been carried out using 3” × 3” NaI (Tl) scintillation spectrometer. The theoretical calculations of both the total mass attenuation coefficients for gamma rays were carried out using the XCOM (version 3.1) computer program and cross-section data base for elements from Z= 1 to 100, and the elemental composition of concrete mixes. The results indicate that, in general, the rubberized heavy weight concrete could be used more successfully than conventional concrete in the field of radiation shielding.

INTRODUCTION

Many kinds of concrete are used for radiation shielding of atomic research facilities, nuclear power plants, medical research unit’s equipment’s and nuclear shelters etc... Conventional concrete of sufficient thickness can be used for such purposes. It is required that the shields give protection against gamma radiation and neurons, since these are the most penetrating. Hence any material, which attenuates these radiations to acceptable level, will automatically reduce all the others to negligible extent [1, 2, and 3]. For a protection against gamma radiation, the high-density aggregate is required, while for neutrons, the light (hydrogen) nuclei are required [4]. Neutrons of high energies are the more penetrating and give the main source of thermal and secondary gamma rays behind the shield. In case of the attenuating of gamma radiation only, the density is the prime consideration, and the thickness of a concrete shield can be reduced in about the same portion as its density is increased. However, in order to attenuate fast neutrons, the concrete shield must, in addition to its high

Journal of Nuclear and Radiation Physics, Vol. 15 (2020) 11-22

density, contain a certain amount of material of low atomic weight such as hydrogen. Since requirements of maximum density and that of maximum hydrogen content are not compatible.

A compromise must be made in the design of a concrete shield that is to attenuate both gamma radiations and fast neutrons. Thermal neutrons are readily absorbed by high-density materials, but elements such as boron or cadmium are sometimes included in small amounts to reduce the formation of secondary gamma radiation [5]. The nuclear shield must satisfy many requirements. Among a lot of requirements, the nuclear installation must satisfy the requirements of resistance against dynamic loads such as the earthquakes, tornados, floods, missiles and aircraft impact. Hence, the choice of local suitable materials and the additives from which, the shield is to be constructed becomes of great importance, [4]. In order to compose a shield against radiation it must achieve requirements the protection of gamma-rays and neutrons. Any shield which is sufficient to attenuate these radiations to a certain level is quite adequate and absorbs all other types of emitted radiation (fission fragments, alpha particles, protons, electrons, etc.). Because of these types of radiation have electric charge; they lose their energy very quickly when they pass through matter, so they can stop in relatively thin layers of materials. But the problem is arising in shielding against neutrons and gamma-rays. The attenuation of neutrons and gamma-rays on passing through matters achieved by scattering and absorption relations. However, shielding of neutrons is more complex. Specific risk produced by neutrons is the activation of components and the radiation damage to structural materials that could shorten the life of nuclear reactor. In the first case, need to attenuate the thermal neutron fluxes; in the second, it must be able to slow down neutrons below the keV region. In general, a neutron shield must provide two effects: slowing down of neutrons to low energy and then absorption.

MATERIALS USED AND MIX PROPORTION

The materials used were Barite ore as coarse aggregate (supplied by El-Nasr Phosphate Company), Serpentine ore as fine aggregate (supplied by Red Sea Company of Phosphate), Ordinary Portland Cement (supplied by Tourah Cement Company), Silica fume (supplied by The Egyptian Ferroalloys Company), Adecrete PVF-5 as super plasticizing material (supplied by Chemicals of modern building) and tap water. Each of these materials was supplied as one batch such that uniformity was secured throughout the experimental work.

In this study, heavy weight concrete is prepared from naturally occurring barite and serpentine ores. Serpentine ore contains chemically bound water in its composition (about 11.6%). Thus it increases the hydrogen content in the produced concrete element [4].

Tests were carried out to assess the properties of concrete in the fresh and hardened states for both ordinary and HWC

Coarse Aggregate

Two kinds of Barite ore (Barium Sulephate) of different sizes ranging from 5- 40 mm are used in this study as a coarse aggregate. Barite has a hardness (the resistance of any material to scratch) of 3 to 3.5 and a specific gravity of 4 to 4.5 when pure. The color ranges from colorless to white to many usually pale colors. Barite is the most common barium mineral and the major barium ore, [ASTM C638].

Results of chemical analysis, crushing factor, specific gravity, and other properties are summarized in tables (3.1) & (3.2)

Table. 3.1 Physical and chemical analysis of the Barite (1) [EDX test]

Element O S Ba Specific gravity Crushing factor Weight % 83.27 8.68 8.05 3.98 43.1

12 A. T. M. Farag et al.

Table. 3.2 Physical and chemical analysis of the Barite (2) [EDX test]

Fine Aggregate

Fine aggregate was serpentine ore (Hydrous magnesium silicate). Results of chemical analysis*, crushing factor, specific gravity, and other properties are given in table (3.3). The fixed water content of Serpentine ore ranges from10 to 13 percent by weight, [ASTM C637]. The total amount of fixed water in used ore was found to be 11.59% of the total sample weight, [6]. Table. 3.3 Physical and chemical properties of Serpentine ore (fine aggregate)

No. Parameter Weight (%)

1 Silica (SiO2) 39.11 2 Magnesium oxide (MgO) 44.61 3 Loss of ignition (L.O.I) 13.25 4 Calcium oxide (CaO) 1.11 5 Radicals (R2O3) 0.86

Total 98.94

Aggregate crushing value 25%

Specific gravity 2.6

Chemical formula MgSiO2(OH)

Portland Cement Ordinary Portland cement produced by Tourah factory was used. Its properties are

given, which indicates compliance with the Egyptian Standard Specification No. 378, 1995 [7].

Table. 3.4 Properties of Ordinary Portland Cement

Property Test Result Limits of Egyptian Specification*

Fineness, cm2/g 2850 > 2250

Setting time, hours Initial 2 24 3 45' Final 4 1 10

Compressive strength,Kg/cm2

3 days 216 160 7 days 297 240

Che

mic

al

anal

ysis

Silicon Dioxide 22.77% -

Calcium oxide 61.25% - Aluminum & from oxide 9.49% - Magnesium oxide 1.57% < 4%

Element O S Ba Si Fe Specific gravity Crushing factor Weight

% 81.61 7.59 7.11 1.89 1.8 3.65 51.8

HEAVY WEIGHT CONCRETE … 13

Sulfur trioxide 1.52% < 2.5-3% Insoluble material 0.85% - Loss on Ignition 2.45% 3%

Silica Fume

Silica fume is a by-product resulting from the reduction of high-purity quartz with coal in electric furnaces during the production of silicon and ferrosilicon alloys. The fume, which has a high content of amorphous silicon dioxide and consists of very fine spherical particles, is collected from the gases escaping from the furnaces. Silica fume is also collected as a by-product in the production of other silicon alloys such as ferrochromium, ferromanganese, ferromagnesium, and calcium silicon. [8, 9]

Properties of silica fume except, that from ferrochromium, has similar properties to those obtained from ferrosilicon; the use of this silica fume avoided unless data on their performance in concrete are available.

Table. 3.5 Chemical composition of silica fume

No. Parameter Weight percent (%) 1 Silica (SiO2) 90 2 Alumina (Al2O3) 1 3 Ferric oxide (Fe2O3) 1.5 4 Carbon C 1.0 5 Calcium oxide (CaO) 1 6 Magnesium oxide (MgO) 1.5 7 Sodium oxide (Na2O) 0.5 8 Potassium oxide (K2O) 1.5 9 Chloride (as Cl) 0.05 10 Water (H2O) 0.8 11 Loss of ignition (L.O.I) 1.5 12 Carbon dioxide CO2 0.2

Total 100.55 Specific gravity 2.2 * Tested by the supplier

Table (3.5) gives the typical chemical composition*of silica fume that has been used. Silica fume was incorporated into concrete mixtures as a partial replacement for cement. The percent of silica fume is varied from 0%to 15% of cementitious materials content (cement + silica fume).

Superplasticizer A new class of water-reducing agents, sodium silicate (Adecrete PVF-5), which can

achieve water reductions up to 10%, has been used. Water reductions of this magnitude have a much greater impact on the properties of concrete than the reductions obtained with conventional water-reducing admixtures. Plasticizers are used to achieve the required slump with less water at a constant cement content which means an effective lowering of w/c ratio with a consequent general improvement in strength and durability. Most plasticizing materials belong to polymeric materials and derivatives of lignosulfonate salt [10].

Mixing Water

Tap ordinary drinking water was used throughout experimental work of this study.


Mix Design

In this study, many series of specimens were prepared and tested in the laboratory of Construction Research Institute (CRI), National Water Research Center (NWRC), Ministry of Water Resources & Irrigation.

The factors that would affect the strength of concrete may be classified into four categories; constituent materials, methods of preparation, curing procedure, and test conditions. However, this study is concerned primarily with the effects of the concrete constituents such as aggregates, superplasticizer, silica fume, and rubber on the physical, mechanical and nuclear properties of concrete. Changes in one of the constituent materials will leads to changes in the relative proportions of the others in order to maintain the same concrete quality [11]. Concrete mixtures were according to follow the absolute volume method recommended by the American Concrete Institute, (ACI) [12

TEST RESULTS AND DISCUSSIONS

Bulk Density Since the bulk density of heavy weight concrete has main concern, it is important to study the parameters, which have influence on it. Figure (1) shows that, bulk density was influence by the inclusion of silica fume, slight increase in the bulk density due to the addition of silica fume. Increasing silica fume percent, the bulk density increases to attain the peak value at 10% silica fume and then decreases again. However, the bulk density of SFHWC is still 16.7% greater than that of ordinary concrete. Consequently, SFHWC is better than ordinary concrete in the field of nuclear shielding, especially for γ radiation, for which the bulk density of concrete is considered important and main property [13].

Compressive Strength

The compressive strength is taken as a basis for specifications and quality. Moreover, it is the reference to which some of other concrete properties are related. The development of compressive strength under continuous water curing of concrete prepared with and without silica fume is presented in figures (2 and 3). Comparing the compressive strength development of the control mixture (0% SF) with those of silica fume concretes, it can be observed that silica fume concretes have compressive strength development patterns, which are different from that of the control concrete. In other words, the strength development rates of silica fume concretes are higher than the development rate of the control concrete, particularly at ages greater than 28 days after casting. Also, it can be observed that the compressive strength of SFHWC is higher than that of ordinary concrete. In addition, it is clear that partial replacement of Portland cement with silica fume improves the strength at all ages and the silica fume contributes to the strength from as early as the age of 3 days. It has been established that silica fume starts to contribute to the strength as early as the age of one day and accelerates the hydration of cement during the early stages. This is due to the fact that silica fumes are a very fine material with very high amorphous silica content [14].

Splitting Tensile Strength

The tensile strength of concrete is an important characteristic for the development of cracking, and consequently, for deformation prediction and concrete durability [14]. The following general important remarks can be concluded from figure (4). The tensile strength increases as the amount of the silica fume increases for all testing ages. The strength of SFHWC is obviously greater than that of ordinary concrete

Gamma-ray Attenuations

The total attenuation of γ-rays


In this study Eu-152 gamma-source was used as a source of gamma rays. Seven pronounced peaks are chosen to cover a wide band of incident gamma ray energies to study the γ-ray attenuation properties in both concrete typed. The measured intensity of total γ-rays transmitted through barriers of ordinary concrete and (0% - 15%) silica fume HWC function of barrier thickness. A brief description of these curves and their related parameters are given below. The total γ-ray intensity behind ordinary concrete and SFHWC barriers of different thickness values are given in figures (5) and (6). These figures present the intensity distributions of total gamma rays as a function of barrier thickness at different incident γ-ray energies. It can be noticed from these figures that; the intensity of the γ-ray decreases exponentially with increasing the barrier thickness; the slopes of these curves were used to calculate the average values of the total attenuation coefficients (μ) of γ- rays in both ordinary concrete and SFHWC. The deduced values of the total attenuation coefficients (μ) are plotted versus γ-ray energy for both concrete types as shown in figures (7). It can be concluded from this figure that at low energy of γ-ray and up to 300 KeV the values of (μ) for ordinary concrete and SFHWC are increased slowly and the value of (μ) for SFHWC is about 4.51times attenuator. While at higher γ-ray energies the SFHWC proved itself to be 1.56 times better attenuator. It can be shown from this figure that, the general behavior of the curve is the same as that of plain SFHWC, but the magnitude of (μ) differs according to the percentage of silica fume replacement of the cement. Moreover, the maximum values of (μ) obtained are at 15% silica fume among the other percentage for all values of incident γ-ray energies. The figure, also, show that there is a sudden jump of the values of (μ) for γ-energy greater than 350 Kev, then nearly saturated values of (μ) appeared. These results indicate that, the replacement of 15% cement with silica fume can be considered as an optimum and this type of concrete proved itself as an effective γ-ray attenuator at intermediate and high γ-ray energies. Figure (8) confirms the same results of the study but the coordinates of γ-ray energy had been changed with wt.% of replaced silica fume.

Theoretical calculation of mass attenuation coefficient of γ-rays

The calculations of mass attenuation coefficients are useful for the shielding design of nuclear reactors, as well as, particle accelerators. In the current study, the elemental compositions for eight concrete mixtures have been calculated and show that Oxygen, Silicon, Magnesium, Calcium, and Barium are the dominant constituents for all concrete mixtures.

The total mass attenuation coefficients (µ/ρ) for all concrete mixtures have been calculated using the XCOM (version 3.1) program at energies from 1keV to 10MeV.

Figures from (9) to (13) show the relation between the calculated values of (µ/ρ), cm2g-

1 and the photon energy MeV and compare between experimental and theoretical value. The value of (µ/ρ) generally decreases with increasing the photon energy. The (µ/ρ) values are similar for all concrete mixtures, which attributed to the closeness of composites density, and all concrete mixtures contain the same elemental composition with different concentrations. The curves shown in these figures can be divided into three regions according to the photon energy; region (I) from 0.001MeV to about 0.04MeV, region (II) from 0.04MeV to about 5MeV and the region (III) from 5MeV to 10MeV.

In region (I), the (µ/ρ) sharply decreases with increasing the photon energy for all concrete mixtures. In this region, the predominant interactions are the absorbed photons by photoelectric effect [14&15].

In region (II), the (µ/ρ) slightly decreases with increasing the photon energy for all concrete mixtures. This may be attributed to the fact that, Compton scattering is predominant at intermediate photon energies for atomic number 60. Therefore, the Compton scattering cross-section may be considered as the main interaction in this region [14]. In the region (III), the values of (µ/ρ) show appreciable increase with increasing photon energies for all concrete mixtures. The comparison of practical and theoretical results is almost identical.


Figure 1. Bulk density of silica fume HWC

Figure 2. Compressive strength development silica fume HWC

Figure 3. Compressive strength of silica fume HWC at different ages


Figure 4. Splitting tensile strength of silica fume HWC

1000

10000

100000

0 5 10 15 20 25Concrete thickness, Cm

Cou

nt ra

te

121.78 KeV244.7 KeV344.27 KeV778.9 KeV964 KeV1112 KeV1407 KeV

Figure 5. Attenuation curves of γ-ray in ordinary concrete

1000

10000

100000

0 5 10 15 20 25Concrete thickness, Cm

Cou

nt ra

te

121.78 KeV244.7 KeV344.27 KeV778.9 KeV964 KeV1112 KeV1407 KeV

Figure 6. Attenuation curves of γ-ray in HWC


Figure 7. Linear attenuation coefficient of Gamma rays in silica fume HWC

Figure 8. Linear attenuation coefficient of Gamma rays in silica fume HWC

Figure 9. Total mass attenuation coefficient of γ-ray in in OC


Figure 10. Total mass attenuation coefficient of γ-ray in 0%of silica fume in HWC

Figure 11. Total mass attenuation coefficient of γ-ray in 5%of silica fume in HWC

Figure 12. Total mass attenuation coefficient of γ-ray in10% of silica fume in HWC


Figure 13. Total mass attenuation coefficient of γ-ray in15%of silica fume in HWC

CONCLUSION According to the results obtained from the experimental and theoretical studies on

silica fume HWC and ordinary concrete as a reference, the following conclusion could be derived from the obtained results, Barite and Serpentine ores can be used for producing heavy weight concrete, Barite is considered a dense ore with specific gravity (3.98) which is the first requirements of gamma rays attenuation or shielding, while the Serpentine ore contains significant amount of crystalline water (about 11%), which is useful to attenuate neutrons. According to the results of bulk density, compressive strength, and tensile strength of silica fume HWC is more effective than the ordinary concrete.

* The best percentage of silica fume which can be replaced by cement was found to be 15%.

* According to the attenuation of gamma –rays, the silica fume HWC are more effective than ordinary concrete.

* Using the XCOM program (version 3.1) in calculating the total mass attenuation coefficient for silica fume HWC, it is found that there is adequate agreement between the theoretical calculations and experimental measurements at the intermediate γ- ray energies.

REFERENCES

[1] Glasstone S., "Principles of nuclear reactor engineering", MC Millan & Co Ltd., London, (1955).

[2] Rockwell T., "Reactor shielding design manual", McGraw Hill book Co., New York, (1956).

[3] Price B.T., "Radiation shielding", Pergamon Press, London, (1957). [4] Mindess S. and Yong J.F., "Concrete", Prentice-Hall, New Jersey, (1981). [5] Goldstein, H., "Fundamental aspects of radiation shielding", Pergamon Press, London,

(1959). [6] R.M. El Shazly, “Design of special types of concrete mixes as radiation shielding” M Sc

Thesis Al-Azhar University, Faculty of Science Cairo, (2004) [7] A.M. Neville, “Hardened Concrete: Physical and Mechanical Aspects”, American

Concrete Institute, Detroit, Mich., (1971). [8] ACI Committee 226, “Silica Fume in Concrete”, ACI Materials Journal, Vol. 84 No. 2,

(1987) pp. 158-166. [9] Malhotra, V.M., Carette, G.G., and Sivasundaram, V., “Role of Silica Fume in Concrete:

A Review”, Advances in Concrete Technology, Edited by Malhotra, V. M., CANMET Publications, Second edition, (1994) pp. 915-990.


[10] Neville A.M., “Properties of concrete” 3rd Edition, Longman Scientific & Technical, (1981).

[11] A.F. Richard, K. Trogan, “Engineering Material and Their Applications”, John Wiley and Sons, New York, (1995)

[12] Bashter I. I., "Magnetite ores steel or Basalt for Concrete radiation shielding ", ACI Journal, V. 32, (1996), pp 40

[13] Ihab ADAM, Toshiki AYANO, Kenji SAKATA, “Influence of Silica Fume on the Creep and Shrinkage of High-Strength Concrete”, JCI, Vol.22, No.2 (2000) pp 619-624

[14] M.F. Kaplan, “Concrete Radiation Shielding” Longman Scientific& Technical, (1989) [15] El-Sayed Abdo A.,Elsarraf M.A and Gaber F.A, "Utilization of Ilmenite/Epoxy

Composite for Neutrons and Gamma Rays Attenuation", Annals of Nuclear Energy, Vol. 30, (2003) pp. 175


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HEAVY WEIGHT CONCRETE CONTAINING SILICA FUME USED AS