Rice husk ash technology will sharpen concrete’s environmental edge.
By Sue McCraven
Concrete and rice make good partners and not just because each is the best at what they do. Worldwide, both products exist in great quantity: concrete is the most-used construction material and rice feeds more people than any other food. Rice and concrete will soon have more in common if ongoing domestic research and major overseas construction projects prove out the value of adding rice husk (or hull) ash to replace a portion of portland cement in mixes to make concrete stronger, more durable and better for the environment. Add to these assets the current federal tax credits and stimulus funding for sustainable projects that reduce greenhouse gas emissions, and economic incentives join with earth-ethics as a rationale for investing in low-carbon technologies like rice husk ash (RHA) as a cementitious material.
Cement production has earned a negative reputation for leaving behind a large carbon footprint. Cement manufacturing contributes nearly 5 percent of all man-made CO2 emissions, according to the Portland Cement Association. Raised awareness of global warming due to emissions such as these leads to considering emerging technologies such as low-carbon RHA production in North America and its many advantages, including greater concrete durability and green marketing potential for the precast producer.
When we consider rice, we find an all-natural CO2- absorbing plant, an edible grain produced in almost every temperate climate on earth. There is an unlimited worldwide supply of rice, including right here in North America. Rice husks are the byproduct from rice mill production; husks are the outer protective shell of the edible rice kernel (see Figure 1). When rice husks are burned, RHA is produced. Not all combustion processes, however, are equal. Rice husks contain about 20 percent silica, but in controlled combustion conditions, the resulting ash offers upwards of 90 percent silica content with negligible carbon content. In uncontrolled burning, the process currently used around the world, the resulting RHA contains much higher
carbon content (see Figure 2).
As a supplemental cementitious material, RHA added to concrete mixtures reduces the amount of cement used and, at the same time, increases the silica needed for the hydration process (see Chart A). With more silica available, a subsequently more efficient hydration process occurs that translates into stronger and more durable concrete products. Incorporating RHA in concrete mixtures represents a reduction in greenhouse gases over typical mixes that use only portland cement. RHA can produce more ecologically sound concrete products and infrastructure that can successfully compete for new federal and state funding for sustainable construction.
Rice husk ash is not new. RHA patents were first developed in the 1970s, but the quality of the material was generally poor due to uncontrolled burning conditions that produce ash with too much carbon. Uncontrolled burning of husks results in crystalline silica with a lower surface area that is not as desirable for concrete; crystalline silica is characterized by a lower pozzolanic reactivity than that amorphous silica produced in controlled combustion. Unlike developing countries, in the United States, the Environmental Protection Agency bans the uncontrolled burning of rice husks in open piles. Rice husks generated in North American rice production typically end up in landfills. In landfills, anaerobic decomposition of the rice husks can lead to subsidence problems. Other than being disposed of as waste material, a percentage of rice husks are sold for use in poultry and animal litter. In developing countries, rice husk is not wasted or landfilled because it can be used as fuel. In fact, rice husk can generate about 60 percent of the energy of coal; Indian and Chinese power plants use rice husks to generate electricity where fossil fuel is scarce or expensive.
Uncontrolled combustion of rice husks to obtain silica for concrete results in RHA that is darker and higher in carbon content (10 percent to 30 percent carbon). It should be noted that because fine particles of RHA have a very high specific surface area, the workability of concrete can be affected; RHA can lead to excessive water demand created in the mix. Concrete workability, however, can be regained by the addition of superplasticizers (high-range, waterreducing admixtures).
While other food plants can be used to generate green solutions (corn for vehicle biofuel, for example), potential nourishment is eliminated from the food supply in the process; rice husk use does not decrease the important grain supply used for food. Arkansas, California, Texas, Louisiana, Missouri and Mississippi are rice-producing states. By recycling RHA in concrete, local jobs in rural areas are created and revenues for the American rice milling industry are also increased.
How much RHA can feasibly be added to concrete? Researchers have found that RHA can be used as a replacement for up to 10 percent to 20 percent (by mass of cement) in concrete mixes. When regional employment benefits are combined with a decrease in concrete’s carbon footprint by recycling a natural waste product, we have a valid ecological and economic solution. Perhaps just as important to its viability in the concrete construction industry, RHA promises to be a lower-cost alternative than other silica-containing cementitious materials already on the market.
Who is making RHA in North America?
Rajan K. V empati of ChK G roup Inc. in Plano, Texas, has developed a patented technology to produce very low carbon (≤ 0.3 percent) RHA with up to 95 percent amorphous silica to replace up to 20 percent of cement in concrete mixes. ChK’s patented process produces consistent-quality RHA that is characterized by high pozzolanic activity, that is, very high reactivity due to the high surface area of the fine silica particles (see Figure 3). V empati uses husks from rice farms in the United States and is currently designing an aerobic furnace for mass production of RHA, with yields of up to 15,000 tons per year.
This is an important new technology for the concrete industry because until now, the worldwide production of RHA has come with undesirably high carbon content. Distribution of this patented RHA process to developing countries is in the planning; in terms of global warming, this technology offers a critical international environmental solution as China currently manufacturers about eight times the amount of concrete as does the United States.
ChK has received funding from the National Science Foundation and is seeking additional money to advance this new technology. The furnace process uses a temperature of less than 1,200 degrees F, and no NOx, SOx (nitrates or sulfates) or CO2 are detected in emissions. This anaerobic burning process currently produces 1.5 pounds of amorphous (not crystalline) silica (SiO2) per 40 minutes and is essentially carbonfree (see Figure 4). The end product is called off-white rice hull ash or OWRHA, and the material is characterized by a light color that holds promise for architectural concrete designs.
Availability and cost of pozzolans
Currently, concrete manufacturers use the well-known silica fume and metakaolin as pozzolan additives to mixtures. Like RHA, silica fume has an ultra-fine particle size, but is generated as a byproduct of silicon metal or ferrosilicon alloy production.
One of the most beneficial uses for silica fume is in concrete. Silica fume, however, is dark grey-colored with high carbon content and, due to the presence of toxic metals, is not environmentally friendly or suitable for architectural concrete. Metakaolin, also an ultra-fine powder, is white and is used in architectural concrete. Both silica fume and metakaolin are relatively expensive pozzolans for the precast manufacturer, particularly metakaolinite, as pure deposits of this material are mined only in a few countries. Low-carbon RHA, on the other hand, can be available at considerably lower cost.
In an article that appeared in Precast Solutions magazine’s Winter 2009 issue, authors R. Ferraro and A. Nanni stated: “From an economical standpoint, the current market price of competing supplementary cementitious materials (silica fume or metakaolin) is in the range of $400 to $580 per ton, while OWRHA may be sold at $275 per ton, which means that just in the first five years of production, this product could capture a significant portion of the market.” The article further states: “Based on the U.S. G reen Building Council certification practice, the LEED (Leadership in Energy and Environmental Design) Green Building Rating System for New Construction and Major Renovation (LEED-NC) considers that the reflective quality of white surfaces may help to reduce temperature fluctuations from solar radiation and improve lighting efficiency, resulting in lower heating and cooling with a reduction of related-energy costs.” (For more on the Ferraro and Nanni article, please visit www.precast.org/rha.)
The advantages of controlled-combustion RHA are clear: RHA is much lower in cost, is white in color, has very low carbon content, has no crystalline SiO2 and can be produced from natural local sources with local labor. Aside from its economic and environmental assets, RHA is a true sustainable product and is easy to use. RHA can deliver a strong and durable concrete for a number of industry applications.
RHA advantages for producers
In addition to LEED credits, here are some good reasons for using RHA in precast concrete products:
1. Increase concrete strength and durability
2. Decrease permeability and increase resistance to reinforcing corrosion
3. Resist sulfate attack and freezing and thawing cycles
4. Produce white architectural concrete and tiles
5. Lower cost than alternative pozzolanic materials
6. RHA is a carbon-neutral material (rice life cycle balances RHA production)
7. Made from local natural resources with local labor
8. Decrease use of portland cement and therefore concrete’s carbon footprint
9. Funding availability and tax incentives for green concrete
projects and products
10. Marketing opportunities for RHA concrete products as true green solutions
The current worldwide push for more environmentally friendly building materials, designs and processes that decrease the construction industry’s contribution to greenhouse gas emissions – particularly cement’s carbon footprint – will only grow stronger with time. RHA is an emerging low-carbon technology that can produce stronger and more durable concrete while protecting the environment and enhancing local economies.
Sue McCraven, NPCA senior technical consultant, is a civil engineer, technical writer and editor, and environmental scientist who has contributed numerous articles and studies to prominent scientific journals.