14-12-2012, 05:30 PM
Rice Husk Ash
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INTRODUCTION
GENERAL
Through out the world, concrete is being widely used for the construction of most of the buildings, bridges etc. Hence, it has been properly labeled as the backbone to the infrastructure development of a nation. Currently, our country is taking major initiatives to improve and develop its infrastructure by constructing express highways, power projects and industrial structures to emerge as a major economic power and it has been estimated that the infrastructure segment in our country is expected to see investments to the tune of Rs.4356 billion by the year 2009. To meet out this rapid infrastructure development a huge quantity of concrete is required. Unfortunately, India is not self sufficient in the production of cement, the main ingredient of concrete and the demand for exceeds the supply and makes the construction activities very costlier. Hence currently, the entire construction industry is in search of a suitable and effective the waste product that would considerably minimize the use of cements and ultimately reduces the construction cost. Few of such products have already been identified like Rice Husk Ash (RHA), Fly Ash, Silica Fumes, Egg shell etc. Amongst these RHA and Egg shells are known to have good prospects in minimizing the usage of cement.
Rice plant is one of the plants that absorbs silica from the soil and assimilates it into its structure during the growth (Smith et al., 1986). Rice husk is the outer covering of the grain of rice plant with a high concentration of silica, generally more than 80-85% (Siddique 2008). It is responsible for approximately 30% of the gross weight of a rice kernel and normally contains 80% of organic and 20%of inorganic substances. Rice husk is produced in millions of tons per year as a waste material in agricultural and industrial processes. It can contribute about 20% of its weight to Rice Husk Ash (RHA) after incineration (Anwaret al., 2001). RHA is a highly pozzolanic material (Tashima et al., 2004). The non-crystalline silica and high specific surface area of the RHA are responsible for its high pozzolanic reactivity.
Prior to 1970, RHA was usually produced by uncontrolled combustion and the ash so produced was crystalline and possessed poor pozzolanic properties. In1973, Mehta published the first of several papers describing the effect of pyroprocessing parameters on the pozzolanic reactivity of RHA. Based on his research, Pitt designed a Fluidised bed furnace for controlled burning of rice husks. . Results show that temperature of 650 degrees centigrade and 60 minutes burning time are the best combination. In this study various experiments were carried out to determine properties of concretes incorporating optimum RHA. Tests include compressive strength, tensile strength, water permeability Ultrasonic pulse velocity and rapid chloride permeability test.
REQUIREMENTS OF DURABILITY
New code defines requirements for durability under following heads:
1. Shape and size of member – The shape and size of members should be such as to promote good drainage of water and to avoid standing pools and rundown of water.
2. Exposure Conditions - The general environment to which concrete will be exposed has been categorised into mild, moderate, severe, very severe and extreme.
3. Abrasives – Special literatures may be referred to, for durability of concrete surfaces exposed to abrasive action in case of machinery and metal tyres.
4. Freezing and Thawing action - Percentage of air entrainment has been defined to get enhanced durability for grades of concrete lower than M50 and for 20mm and 40mm nominal size of aggregates.
5. Exposure to sulphate attack – New code gives recommendations for the type of cement, maxm. Free water/cement ratio and minm. Cement content, which are required at different sulphate concentrations in near- neutral ground water having pH of 6 to 9.
6. Nominal cover has been specified for different exposure conditions. For mild, moderate, severe, very severe and extreme conditions the cover shall not be less than 20,30,45,50 and 75 mm respectively.
7. Concrete mix proportions - Approximate values for minimum cement content and the maximum free water cement ratio are given for different exposure conditions.
8. Mix constituents – For concrete to be durable, careful selection of the mix and materials is necessary, so that deleterious constituents such as chlorides, sulphates and alkali-aggregate reaction do not exceed the limits.
9. Concrete in aggressive soils and water – Where structures are partially submerged or are in contact with aggressive soils or water on one side only, the ground water shall be lowered by drainage so that it will not come into direct contact with the concrete.
10. Compaction, finishing and curing – Adequate compaction without segregation should be ensured by providing suitable workability. Overworking the surface and the addition of water/cement to aid in finishing shall be avoided.
11. Concrete in seawater- Concrete in seawater or exposed directly along the seacoast shall be at least M20 Grade in the case of plain concrete and M30 Grade in case of Reinforced concrete. Care shall be taken to protect the reinforcement from exposure to saline atmosphere during storage, fabrication and use.
LITERATURE REVIEW
1. Contribution of Rice Husk Ash to the Properties of Mortar and Concrete: A Review
Alireza Naji Givi, Suraya Abdul Rashid , Farah Nora A. Aziz , Mohamad Amran Mohd Salleh
In the last decade, the use of supplementary cementing materials has become an integral part of high strength and high performance concrete mix design. These can be natural materials, by-products or industrial wastes, or the ones requiring less energy and time to produce. Some of the commonly used supplementary cementing materials are fly ash, Silica Fume (SF), Ground Granulated Blast Furnace Slag (GGBFS) and Rice Husk Ash (RHA) etc. RHA is a by-product material obtained from the combustion of rice husk which consists of non-crystalline silicon dioxide with high specific surface area and high pozzolanic reactivity. It is used as pozzolanic material in mortar and concrete, and has demonstrated significant influence in improving the mechanical and durability properties of mortar and concrete. This paper presents an overview of the work carried out on the use of RHA as partial replacement of cement in mortar and concrete. Reported properties in this study are the mechanical, durability and fresh properties of mortar/concrete. [Journal of American Science 2010;6(3):157-165]. (ISSN: 1545-1003)RHA has been used in limepozzolana mixes and could be a suitable partly replacement for Portland cement (Smith et al., 986;Zhang et al., 1996; Nicole et al., 2000; Sakr 2006; Sata etal., 2007; etc). RHA concrete is like fly ash/slag concrete with regard to its strength development but with a higher pozzolanic activity it helps the pozzolanic reactions occur at early ages rather than later as is the case with other replacement cementing materials (Molhotra, 1993).
Rice Husk Ash — Properties and its Uses : A Review
The increasing demand for producing durable construction materials is the outcome of the fast polluting environment. Supplementary cementations materials prove to be effective to meet most of the requirements of durable concrete. Rice husk ash is found to be superior to other supplementary materials like slag, silica fume and fly ash. Due to its high pozzolanic activity, both strength and durability of concrete are enriched. Unlike other industrial by–products rice husk ash has to be produced out of the raw agricultural waste, husk. The quality of ash is greatly influenced by its method of production. To convert this ash into an active pozzolanic material, certain controlled conditions of production and processing methods have to be followed, which are yet to be fully understood and evolved. This paper presents various applications of rice husk ash in general and as pozzolana in particular. Works done on ash production methods have been critically reviewed. Possible areas of research have also been identified.