This definitely looks like a typographical error that gets some 800 google hits.

Each one I have checked is about rice husk and occasionally has one or more uses of risk husk which look like a slip up.

HERE ARE SOME EXAMPLES

Rice Husk Board
http://iocg.ignou.ac.in/wiki/images/5/56/MITI-023_B6CS7.pdf
yet it comes up on the google search
Exhibit 1: List of Some Firms Licensed to Manufacture Risk Husk Particle Boards. 1. B.R. Agro Boards Pvt. Ltd., Hyderabad ...

Rice Husk Ash (Rha) - Amorphous Silica:-
We exports organic amoruphous silica from rice husk ash( rha ), that is used In concrete mixes to make light weight concrete,it is High performance concrete andAlso used as a carrier for insecticides. it also offers for used in steel plants.
http://srimksdistributir.tradeindia.com/Exporters_Suppliers/...
also comes up in google under risk husk which reads as a typo in the contect, which you can find if you search the PDF document

One of the ingredients that can be used in cement is ash. By mixing rice husk ash into the cement, there is the possibility of greener concrete. Discovery News offers this on the process of creating carbon neutral rice ash:

Now, Rajan Vempati of ChK Group, Inc. in Plano, Texas, and a team of researchers have figured out a way to make nearly carbon-free rice husk ash. Heating husks to 800 degrees centigrade (1,472 degrees Fahrenheit) in a furnace drives off carbon, leaving fine particles of nearly pure silica behind.
http://www.physorg.com/news167405443.html

The last sentence is the article has this obvious typo:
However, when one considers that up and coming economic powers, such as India and (especially) China, will be likely ramping up production of concrete for use in buildings and roads, the prospect of reducing carbon emissions and pollution through the use of risk husk ash in concrete becomes even more interesting.

Otherwise it is all about rice husk