Elsevier

Atmospheric Environment

Volume 81, December 2013, Pages 413-420
Atmospheric Environment

Radon resistant potential of concrete manufactured using Ordinary Portland Cement blended with rice husk ash

https://doi.org/10.1016/j.atmosenv.2013.09.024Get rights and content

Highlights

  • Rice husk ash is used to reduce radon diffusion through concrete.

  • Burning temperature and time of rice husk ash are optimized.

  • Radon diffusion coefficient through RHA substituted concrete decreases.

  • The compressive strength of concrete increases up to 30% RHA addition.

  • This study provides a cost-effective method to reduce indoor radon entry through floor.

Abstract

The emission of radon from building materials and soil depends upon the radium content, porosity, moisture content and radon diffusion length of materials. Several techniques have been used to reduce the radon emission from the soil using different flooring materials. But the effectiveness of radon shielding depends upon the diffusion of radon through these materials. The present study proposes a method for producing a radon resistant material for decreasing radon diffusion through it. The method involves rice husk ash (RHA) in addition to cement for the preparation of concrete used for flooring and walls. The radon diffusion, exhalation and mechanical property of concrete prepared by rice husk ash blended cement were studied. The addition of RHA caused the reduction in radon diffusion coefficient, exhalation rates, porosity and enhanced the compressive strength of concrete. The bulk radon diffusion coefficient of cementitious concrete was reduced upto 69% by addition of rice husk ash as compare to that of control concrete.

Introduction

The contribution of various sources of radiation varies according to geographic and topographic region. Out of all radioactive sources, radon contributes a major part of radiation dose approximately 55% of the total dose (UNSCEAR, 2000). Thus choice of materials used in building construction should be made according to shielding purpose of radon. Two most important mode of transportation of radon gas entering from soil to residential building are advection and diffusion (Kendrick and Langner, 1991). The contribution of these two modes to indoor radon concentration is basically dependent on the physical structure of intermediate medium i.e concrete. The assumption of negligible contribution of radon through concrete by diffusion was adopted by most of the cement industries as most of the radon pass through cracks and gaps of concrete by advective flow. But the study of the transport phenomenon of radon diffusion concluded that diffusion is major transport phenomenon contributing to 80% of radon flow from underlying soil to building. The transport of radon through concrete can be reduced by the use of anti radon coating studied earlier by Gao et al. (2008) while the effect of depressurization of house on indoor radon and decay product was studied by Korhonen et al. (2000).

The most important physical properties of concrete that control the flow of radon are porosity, permeability and diffusion coefficient through concrete. The radon diffusion coefficient depends upon porosity and physical structure of the concrete. The choice of building materials can be done on the basis of the porosity and physical structure as far as radon shielding is concerned. The radon diffusion length of building materials is important factor regarded as one of the criterion of tightness of radon. The materials can be used as radon tight if their thickness is three times the radon diffusion length. The concrete mix design is the most important factor that affects the physical structure of concrete because the various chemical reactions occur at the time of cement hydration depends upon the silica and lime compositions of the mix design. The silica and lime present in cement react to form calcium-silica hydride linkage which is responsible for the ultimate strength of the concrete and other properties. Since silica present in cement is small compared to lime, the completion of reaction between silica and lime results in surplus lime in cement. A number of super-plastizer or pozzolona like fly ash, silica fume Metakolin, and rice husk ash etc. are used with cement in order to utilize the surplus lime present in cement.

The compressive strength of concrete is a major concern for cement industries for construction of flyover bridges and high rise buildings. However, the addition of rice husk ash with cement causes increase in the strength of concrete along with modification in radon diffusion and exhalation properties. Also the use of rice husk along with cement reduces the cost of cement production and environmental pollution produced due to open burning of rice husk. Rice husk has been widely used as fuel in the rice paddy milling process generating a large amount of rice husk ash which is disposed as landfill causing reduced fertility and enhanced environmental pollution. Now a days, the interest in rice husk ash in construction industry has increased tremendously (Saraswathy and Song, 2007, Chindaprasirt and Rukzon, 2008). Rice husk is major agricultural waste products available in many parts of the world. Rice husk consists of about 40% cellulose, 30% lignin group and 20% silica (Umeda et al., 2007). After burning the cellulose and lignin groups are lost. Hence the ash contains a large amount of silica (Stroeven et al., 1999). The silica exists in two forms: amorphous or crystalline silica depending on the temperature and duration of burning. Silica in amorphous form is reactive and suitable for use as a pozzolona to replace part of Ordinary Portland Cement (OPC). When rice husk is burnt at temperatures lower than 700 °C, rice husk ash with cellular microstructure is produced. Rice husk ash contains high silica content in the form of non-crystalline or amorphous silica (Mehta, 1975). Therefore, it is a pozzolanic material and can be used as supplementary cementitious materials (Erdogdu, 1998). The ash from open-field burning (or from non-controlled combustion in industrial furnaces) usually contains a higher proportion of non-reactive silica minerals in crystallite form such as cristobalite and tridymite, and it can be pulverized into very fine particles to develop pozzolanic activity. Such silica can react with calcium hydroxide when added to cement in the presence of water resulting in cementitious compounds (Malhotra and Mehta, 1996). Numerous researches confirm the fact that burning temperature is a critical point in the production of amorphous reactive rice husk ash. The chemical effect of rice husk ash is related to the fact that when produced by controlled combustion it is a highly pozzolanic material, which combines quickly with calcium hydroxide forming a secondary C–S–H linkage; the physical effect is linked to particle size, which produces a refinement on the pore structure, acts as nucleation point for hydration products, and restricts the growth of crystals generated in the hydration process (Mehta, 1975).

The following reaction sequences are expected to take place in RHA mixed concrete during setting process leading to development of strength and responsible for strength enhancements (James and Rao, 1986)SiO2(RHA) + H2O → C–S–H + untreated SilicaC3S/C2S (OPC) + H2O → C–S–H + Ca (OH)2OPC + RHA + H2O → C–S–H + untreated silica

The highly reactive amorphous silica reacts with calcium hydroxide released during the hydration of cement resulting in the formation of Calcium Silicates Hydrate (CSH) linkage responsible for strength.SiO2 + Ca (OH)2 → CSH + SiO2

The presence of calcium silicate hydrate (C–S–H) bond in concrete is responsible for decrease in porosity which strongly affects the diffusion length and exhalation rates of radon in concrete. A very fine particle size (10–20 μm) of rice husk ash can be obtained by burning of rice husk in controlled manner and grinding such that the C–S–H bond remain always in amorphous form in any age of concrete.

Section snippets

Preparation of rice husk ash

Rice husk was converted into ash into a concrete cylindrical pipe in a control manner such that the burning temperature ranged from 300 °C to 450 °C so that the entire cellulose and lignin group was burnt. The husk so produced was black in colour containing considerable amount of carbon which was expected to have an adverse effect upon the pozzolanic activity of rice husk ash (Columna, 1974). The rice husk and ash are shown in Fig. 1.

The ash produced by open burning was further burnt into

Results and discussion

The optimum temperature and time for preparation of rice husk ash was 700 °C for 60 min as determined by XRD analysis. Beyond this time and temperature the loss on ignition was neglected and rice husk ash exists in cristobalite and tridymite structure shown by XRD study and in Fig. 3.

The percentage of SiO2, CaO and Al2O3 in rice husk ash estimated by XRF analysis was 90% which ensures all the requirement of pozzolona used as substitution of cement. The addition of rice husk ash in cement

Conclusions

  • 1.The optimum temperature and time for preparation of rice husk ash was 700 °C and 60 min. The temperature higher than that increases the crystallinity of silica. Beyond this time the loss on ignition of rice husk ash is negligible.

  • 2.The silica content of rice husk ash increases with increase in temperature of burning.

  • 3.The porosity of concrete decreases with increase in the rice husk ash content due to its fine particle size.

  • 4.

    The compressive strength of concrete made from the rice husk ash

Acknowledgements

The authors are thankful to Board of Research in Nuclear Science, Mumbai, India and Civil Engineering Department, National Institute of Technology, Kurukshetra, India for providing support and facilities during the present study.

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