Burning rice hulls at 450 for four hours produces an ash that meets requirements for an ASTM C 618 Class N pozzolan. Advocates of using rice-hull ash (called rice-husk ash outside the United States) in concrete can cite years of lab research when selling concrete producers on the pozzolan's durability-enhancing benefits. Nationwide acceptance of RHA's benefits may take several years, but research and a recent field project indicate that, when used as a partial cement replacement, RHA improves concrete durability.
While it shares some properties of other mineral admixtures, the key to RHA's effectiveness is its particle surface. RHA's average particle size is comparable to that of fly ash, and RHA's degree of reactivity is similar to silica fume's due to its comparable silica content. Research indicates a 10% replacement of the weight of cement by RHA produces higher compressive, tensile, and flexural strengths than ordinary concrete but slightly lower strengths than silica fume/cement concrete.
Reduced porosity in RHA concrete also increases resistance to chloride ion penetration, so RHA could help protect against rebar corrosion in marine environments or in structures exposed to deicers. RHA, like Class F fly ash, has proven to control alkali-silica reactivity (ASR). California's department of transportation has verbally approved RHA as an ASR reducer and is looking for a large project in which to use RHA in the near future.
What makes RHA a unique pozzolan is its multilayered, microporous surface, which differs from fly ash's and silica fume's spherical particles. The surface is thought to reduce bleeding through absorption when RHA is used in concrete. Despite producing less bleeding in fresh concrete, RHA particles' absorbency reportedly maintains the workability of fresh concrete used for flatwork finished in hot, windy conditions.
Rice producers claim they need a distributor to commit to marketing RHA if the material is to make inroads in concrete production. There's no shortage of raw material; the United States produces about 8 million metric tons of rice annually. Complete combustion of this output would produce about one-fifth the weight, or 1.6 million metric tons, of ash.
But RHA is less available than other mineral admixtures because it's not a production byproduct. Rice hulls are not a traditional fuel source to begin with; producing useable ash requires a capital investment by rice plants, one not driven by law but by potential profitability.
RHA Technology Inc., El Cerrito, Calif., markets RHA for use in concrete, owns a sublicense on a UC-Berkeley patent for RHA use in concrete, and purchases RHA from the Wadham Biomass Facility in Williams, Calif. A dam repair project in northern California utilized RHA processed at the Wadham plant and sold by RHA Technology.
KEYWORDS: durability, RHA, rice-hull ash