Carbonization and decarbonation are two different phases in
Carbonization and decarbonation are two different phases in the decomposition of rice husk. Silica in RHA melts around 1440°C . Burning of rice husk at temperature lower than 800°C produces reactive amorphous silica which contains approximately 90% silica, whereas above this temperature some crystalline form of silica can also be obtained , , , . Silica in amorphous form is useful as a pozzolan to produce durable good quality concrete .Crystalline and amorphous forms of silica in rice husk depend upon burning temperature and its duration. Rice husk ash should be produced with suitable specifications for a specific use as crystalline and amorphous forms of silica have different properties , , . Nair et al.  stated that the samples burnt for more than 12h, at 500–700°C produced high reactivity ashes, whereas short durations burning (15–360min) resulted in high carbon content in RHA. Mehta  reviewed physical and chemical properties of RHA, and its use as a supplementary cementing material. Use of RHA in cement based materials reduces heat of hydration, improves strength and durability parameters besides reduction in cement cost and other environmental benefits.
Effect of RHA on fresh properties of SCC
Strength properties of SCC
Effect of RHA on durability properties of SCC
Summary and conclusions Following conclusions can be drawn from the published literature.
Introduction Rapid industrialization has lead to increased disposal of heavy metal into the environment. Wastewater disposed by the industries contain heavy metal pollutants that cause direct toxicity both to humans as well as other living organisms due to their presence beyond specified limits. One such heavy metal is nickel which is a naturally occurring IOX2 in and the 5th most abundant element in the world. It is found in rocks and soils, and may be locked into the structure of the rock or soil or sediment. It is largely present in the wastewaters of electroplating, motor vehicle and aircraft industries and produces undesirable effects even in low concentrations. World Health Organization, (WHO) has suggested that the maximum amount of nickel in drinking water should not exceed 0.1mg/L. But in many electroplating effluent water, it is as high as 50mg/L . The safe and effective disposal of Ni (II) containing waste water is a challenging objective for industries because cost effective treatment alternatives are not readily available. The main goal today is to adopt appropriate methods and to develop suitable techniques either to prevent metal pollution or to reduce it to very low levels. Among the various cleanup methods available for removal of metal ions, some of them are electro dialysis , solvent extraction , electrochemical precipitation  Fertilization  reverse osmosis , adsorption , , , , , , membrane separation  and ion exchange . These methods differ with respect to cost, complexity and efficiency. Among these technologies, adsorption is a user-friendly technique for removal of heavy metal. It is an effective method for removal of heavy metal and solves the problem of sludge disposal and renders the system more viable if low cost adsorbents are used. Rice husk ash (RHA) , , ,  has a high degree of porosity and large surface area and finds use for the removal of toxic, biodegradable and non-biodegradable substances from wastewaters. Literature  reports use of RHA for Ni [II] from aqueous solution with adsorption capacity in the range of 10–15mg/g at optimum pH of 6.0 after 5h of contact time. In the present paper RHA is modified to extract mesoporous silica present and used for removal of Ni [II] from aqueous solution. Rice milling generates a byproduct known as husk. This surrounds the paddy grain. During milling of paddy about 78% of weight is received as rice, broken rice and bran. Rest 22% of the weight of paddy is received as husk. This husk is used as fuel in the rice mills for steam generation process. It contains about 75% organic volatile matter and the balance 25% of the weight of this husk is converted into ash during the firing process, which is known as rice husk ash (RHA). This RHA in turn contains around 85–90% amorphous silica. In the present study a new material is synthesized using RHA to achieve high surface area. Silica present in RHA is precipitated allowing unbunt carbon to embed over it. The synthesized material, carbon embedded silica (CES) has high surface area (100m2/g) as compared to RHA (44m2/g) and has chemically active surface.