VALORISATION OF RICE HUSK ASH AS A SUPPLEMENTARY CEMENTITIOUS MATERIAL

Abstract

The study focuses on production and valorisation of Rice Husk Ashes (RHAs) obtained from combustion of rice husks in an industrial setting. In contrast to the more common practice of obtaining RHAs from laboratory combustion, an attempt has been made in collaboration with an industrial partner using rice husks as a boiler fuel, to optimise combustion conditions so as to obtain RHAs which after minimum valorisation would fulfil standard requirements of pozzolanic materials in the context of physical and chemical properties. In the valorisation exercise, grinding conditions in a Los Angeles abrasion testing machine were optimised in terms of grinding time and material-to-charge ratio. In the first phase of investigation, Rice Husk Ash (RHA) characterisation was carried out on ash samples classified into following 3 categories: RHAs of Category-I were the ashes of unknown provenance obtained from representative locations in four rice growing regions of India and RHAs of Category-II and Category-III were the ashes obtained after controlled combustion of rice husks in the industry. The 4 RHA samples of unknown provenance showed a wide variation in their physical and chemical properties, especially the content of unburnt carbon. Since these results were attributed to improper combustion, the following combustion related trials were carried out: In the first set of trials (Category-II RHA), rice husks were burnt at average combustion temperatures of 600±5oC, 650±5oC, 700±5oC, and 750±5oC. In the next phase of combustion trials (Category-III RHA), the following 2 temperature ranges were selected for husk combustion: (a) A relatively broad range of 600-700oC and (b) A relatively narrower range of 650-700oC. After controlled combustion, irrespective of source and burning temperature the unburnt carbon content of 5% - 9% in ashes of the Category-II and the Category-III was below the upper bound value of 10% for Class-N raw or calcined natural pozzolanas in the ASTM C 618. RHAs obtained at burning temperatures between 600–750oC showed amorphous mineralogy. Valorisation of RHAs of the Category-II and the Category-III by mechanical grinding for 60 minutes in a Los Angeles abrasion testing machine at 1:3 material-to-charge ratio reduced RHA particle size to less than 25 μm and achieved the minimum fineness required for Class-N raw or calcined natural pozzolanas in the ASTM C 618.