Characterizing pozzolanic activity of rice husk ash by impedance spectroscopy

Rice husk ash (RHA) has long been known to possess a pozzolanic property. The abundance of rice husk as agricultural waste makes RHA the most promising candidate to be used as a supplementary cementitious material (SCM) in many rice-exporting countries. The use of RHA as an SCM helps reduce the use and thus the production of cement that involves great energy consumption and CO(2) emission. To promote the use of RHA as an SCM, a method to assess its pozzolanic activity is needed for the process of optimizing the burning conditions and/or selecting RHA from uncontrolled burning of rice husk as biomass fuel. The present work aims to use impedance spectroscopy to characterize pozzolanic activity of RHAs prepared on a pilot scale. The method is based on the rate of the normalized conductivity change of the Ca(OH)(2) + RHA paste, d(sigma/sigma(0))/dt, during the first 24 h of hydration. The measurement was found to be sensitive to the unburnt carbon content in the 6-8 wt.% range and the amorphous SiO(2) content (regardless of the unburnt carbon content). When used to evaluate two separate groups of RHAs, each with comparable unburnt carbon contents, the method gives very high correlation coefficients to the strength activity index at 3, 7, and 28 days. However, the correlation coefficients fall significantly when RHAs with vast difference in the unburnt carbon contents are considered together. The method thus proves to be powerful for evaluation of the pozzolanic activity of RHAs with comparable carbon contents. (C) 2010 Elsevier Ltd. All rights reserved.