Rice-husk stoves

The Manufacture of a Cement Extender from
Rice-Husks using a Basket-Burner

by M. L. Allen
University of Auckland
New Zealand

Rice husk ash gives a range of good castable insulating and semi- refractory blocks when mixed with cement and water (12:1 to 3:1) -- ideal for making low-cost wood or charcoal stoves.

From Michael Allen
3 Feb 2003

Attached is a Project Outline I did for UNESCO some years ago which took the results of a cooperative research programme we ran in SE Asia and the Pacific and put it into a form that could be used by schools, villages and technical colleges. It is one of about six such "Project Outlines" that I put together at that time which have turned up in the most surprising of places (including a village in Cameroon).

Rice husks are probably the most under-utilised resource available on the planet today. They are very high in silica content and, if the ash is prepared in the manner described in the attachment, a high quality amorphous silica is obtained in the form of lightweight "bubbles". Using a volume ratio of between 3:1 to 12:1 ash to cement, a range of insulating (and refractory) bricks can be cast. In Indonesia and Thailand, we have made low-cost stoves from blocks of this material cast into quite complicated shapes. One useful feature is that like their more expensive cousins, the blocks are soft enough to shape with a hack-saw blade after they have hardened.

Obviously the mechanical strength of such blocks is not particularly high (although the lower-ratio RiceHuskAsh/cement blocks are definitely stronger). And because they are made of amorphous silica, they can only be used up to about 1100 deg C. They have then started to crystallize to alpha or beta quartz. (Even higher temperatures produce crystoballite and tridymite which usually make the block lose its mechanical strength).

Because of the dust, I recommend breathing through a simple mask when working with RHA. I also recommend spraying it lightly with water to keep the dust nuisance down. It's a bit of a strange material in that it holds water within the silica bubble so that a "correct" mix with cement and water looks rather dry. However, if you continue to work the material, the bubbles break and suddenly the mix is too wet! Too much working of the mix also destroys its insulating properties because too many glass bubbles get broken. (Conventional concrete mixers tend to break the bubbles more than hand mixing does).

To summarise: Rice husk ash gives a range of good castable insulating and semi- refractory blocks when mixed with cement and water (12:1 to 3:1). The practical temperature limit of such cast blocks is probably about 1100 deg C. They have little mechanical strength but are very easily shaped. Dust and water control problems can be easily overcome with a little experimentation.

Michael Allen