Typical composition of rice-husk ash can be found at: E. Stav, E.A. Burkard, R.S. Finkelstein, D.A. Winkowski, L.J. Metz, P.J. Mudd (National Gypsum Company), "Cementitious gypsum-containing binders and compositions and materials made therefrom", United States Patent 5,858,083 (Jan. 12, 1999); apud: V.M. Malhotra, Mehta, P.K. Kumar, "Pozzolanic and Cementitious Materials", Advances in Concrete Technology, Vol. 1, Gordon & Breach Science Publishers, The Netherlands, 1996; reprinted by Taylor & Francis, 2004, p. 19.
Can't you have some sample to determine it by yourself?
I have found some theoretical ranges, but for ash in general. Maybe this might be a starting point for you, just in case you have to calculate some theoretical data:
Particle density of rice husk ash should be found within 1800–2100 kg/m3 according to: M.R.B. Taib, "Production of amorphous silica from rice husk in fluidised bed system", Faculty of Chemical Engineering and Natural Resources Engineering, Universiti Teknologi Malaysia, April 2007 (Research Report), p. 163, and references quoted herein; http://eprints.utm.my/5831/2/74526.pdf