Some RHAs studied in this research were from the
Spanish enterprise AgrocõÂtrica (Alzira, Valencia). These
were obtained from rice husks burnt in an open chamber
and from burning them as fuel in a furnace. In both cases,
the combustion process occurred under uncontrolled conditions;
but while in the open chamber, burning lasted
some weeks, in the furnace it took only a few hours.
Different samples were taken at random points from piles
of ashes. These ashes were ground in a laboratory ball
mill for 5 min until the mean particle diameter ranged
between 10 and 17 mm. Then, they were dried at 120°C
for at least 24 h. Additionally, RHA samples containing a
high content of amorphous silica were obtained in a
laboratory furnace at 600°C.
Chemical reagents, barium hydroxide, glycerol (1,2,3-
propanetriol), phenolphthalein, were analytical grade
reagents. To prepare standard amorphous silica, sodium
silicate and sulfuric acid were used; the silica gel obtained
was washed with a 5% solution of nitric acid containing a
few drops of hydrogen peroxide. After washing, the product
was dried at 120°C for at least 24 h. For the application of
the standard method, hydrochloric acid, potassium hydroxide
and silver nitrate solutions were used as reagents. Fine
powdered quartz was also used as reference material.