| dc.description.abstract | The chemical composition of rice husk ash (RHA) from Mwea and Ahero was
determined using various analytical techniques: atomic absorption
spectrometry (AAS), X-ray fluorescence (XRF) , X-ray diffraction (XRD),
colori metry and gravi metry.
Silica was found to be the major constituent (82.3-89.9%). The ash also
contains the oxides of sodium (Na) , manganese (Mn), iron (Fe), aluminium
(AI), magnesium (Mg), calcium (Ca), phosphorous (P) and potassium (K)
(0.07-2.1 %). X-ray diffraction analysis indicated that the silica in field burned
RHA was mainly amorphous. Igniting the RHA to about 1000°C was found
to induce crystallinity in the amorphous silica. The amount of silica in RHA
was found to vary with the loss on ignition (LOI), with a regression equation
given by Y = 92.4 - 1.00x, where Y is the amount of silica and X is the
LOI.
The ash obtained from the controlled burning of rice husks was ball-milled
and used to replace 15,20,25 and 30% of ordinary portland cement (OPC).
The compressive strength of mortar cubes cast using these RHA/cement
blends was determined after 7, 28, 60 and 90 days of curing in water, and
compared to that of control mortar cubes cast similarly using OPC alone.
Results of the compressive strength of RHA cement mortar suggest that, RHA
may be used to replace upto 23.4 % cement without losing the short-term
strength
of cement mortar. Slightly higher amounts ( ~ 30%) of RHA may
also replace cement without losing the long-term mortar strength.
Cow pea plants (Vigna unguiculata), grown on similar plots, were treated with
RHA, RHA enriched with KN03, and, a commercial fertiliser. The rate of
growth was then compared to that of control plants, grown under similar
conditions with no additives. Plants subjected to the additives showed
increased vigour of growth in plant height and leaf size relative to the control
plants.
Overall, the results obtained indicate the possibility of utilising RHA as a
cement replacement material, and as a fertiliser. Moreover, rice husks burned
in open fields in the areas sampled (which is usually the case), yield ash,
whose silica is mainly amorphous, and is the most suitable form needed for
cement replacement. This ash is also rich in K (1.24 - 2.1 %) which is of
nutritional importance to plants. | en |
