Burning rice hulls at 450° Ffor four hours producesan ash that meets require-ments for an ASTM C 618Class N pozzolan. Advo-

cates of using rice-hull ash (called rice-husk ash outside the United States) inconcrete can cite years of lab researchwhen selling concrete producers onthe pozzolan’s durability-enhancingbenefits. Nationwide acceptance ofRHA’s benefits may take several years,but research and a recent field project(see related article) indicate that, whenused as a partial cement replacement,RHA improves concrete durability.

While it shares some properties ofother mineral admixtures, the key toRHA’s effectiveness is its particle sur-face. RHA’s average particle size iscomparable to that of fly ash, andRHA’s silica content and degree of re-activity are comparable to silica fume’s.

Silica in pozzolans reacts with port-land cement paste’s weak and easily sol-uble calcium hydroxide to formstronger-bonding calcium silicate hy-drates. In hardened concrete, more cal-cium silicate hydrates mean less capil-lary porosity and less permeability. Thispozzolanic reaction also reduces thepresence of soluble, alkaline calciumhydrates, which can leach out andweaken concrete.

RHA used in concrete has an effectsimilar to other pozzolans. Researchindicates a 10% replacement of theweight of cement by RHA produceshigher compressive, tensile, and flex-ural strengths than ordinary concretebu t s l i g h t ly l o we r strengths thansilica fume/cement concrete. Reducedporosity in RHA concrete also increas-es resistance to chloride ion penetra-tion (Ref. 1), so RHA could help pro-

tect against rebar corro-sion in marine environ-ments or in structuresexposed to deicers.

Chris Crouch, aPleasanton, Calif.-basedresearcher for RMCLonestar, which is study-ing RHA use as an ad-mixture at its ready mixplants and in blended ce-ments, says, “Peoplewouldn’t replace their fly ash silo withone for rice-hull ash blended cements.But a good fit for RHA would be a pre-cast plant that makes components formarine structures and needs an admix-t u r e . ”

RHA has proven to control alkali-sili-ca reactivity (ASR). In California, pro-ducers whose aggregates have been clas-sified as reactive were threatened withexclusion from bidding for state work.The state’s department of transporta-tion, Caltrans, addressed the problemby drafting a provision mandating cer-tain ASR-reducing practices, includinguse of pozzolans. Caltrans has verballyapproved RHA as an ASR reducer andis looking for a large project in which touse RHA in the near future.

RHA and fly ash particle sizes aver-age 20 to 30 microns. These admix-tures handle more like cement thansilica fume, which has an average par-ticle size about 200 times smaller. ButRHA, like silica fume, has a silica con-tent of at least 90%, compared with flyash’s typical 60% to 65% silica con-tent. The more silica, the higher thedegree of reactivity with cement’s freelime and the higher the reduction inporosity of hardened concrete.

Particles hold water

What makes RHA a unique pozzolanis its multilayered, microporous sur-face (see photo), which differs from flyash’s and silica fume’s spherical parti-cles. The surface is thought to reducebleeding through absorption (Ref. 2)when RHA is used in concrete. Despiteproducing less bleeding in fresh con-crete, RHA particles’ absorbency re-portedly maintains the workability offresh concrete used for flatwork fin-ished in hot, windy conditions. In con-trast, silica fume has a higher water de-mand due to a smaller particle size,tempting finishers to increase slump byadding water, thus increasing the wa-ter-cement ratio and reducingstrengths in hardened concrete.

Partial cement replacement withRHA alone may not be applicable tofast-track or cold-weather projects,since in one study, concrete with a10% replacement set more slowly thanordinary or silica fume concrete (Ref.1). Additionally, like silica fume, RHAuse may not suit some architecturalapplications, since its relatively high

The best-kept secret to high-performance concrete?Highly reactive rice-hull ash provides durability benefits in various environments. But who will take the lead in marketing RHA?

A rice-hull ash particle’s multilayered, micro-

porous surface, shown here by electron micro-

scope, is the key to RHA’s absorptive properties.

RHA’s high silica content makes the material a

highly reactive pozzolan that enhances durability.

carbon content gives concrete a slight-ly darker color than ordinary concrete.

Some RHA proponents think RHAshould be used to augment more plen-tiful fly ash by offsetting lower earlystrength gains when fly ash is used as apartial cement replacement (Ref. 3).

P roduction, transportation c o n c e rn s

Rice producers claim they need adistributor to commit to marketingRHA if the material is to make inroadsin concrete production. There’s noshortage of raw material; the United

States produces about 8 million metrictons of rice annually. Complete com-bustion of this output would produceabout one-fifth the weight, or 1.6 mil-lion metric tons, of ash.

But RHA is less available than othermineral admixtures because it’s not aproduction byproduct that eliminatespollution. Rice hulls are not a tradition-al fuel source to begin with; producinguseable ash requires a capital invest-ment by rice plants, one not driven bylaw but by potential profitability.

RHA production requires a two-stepprocess. Jim McDaniel, RHA team

leader at Producers Rice Mill,Stuttgart, Ark., says the company’srice-combustion plant first chars therice hull, producing crystalline silica. Asecond combustion process is requiredto get the RHA’s particle size and car-bon content down to a level suitablefor concrete. For a rice plant to build arice-combustion unit would require acommitment of well over $1 million,McDaniel estimates.

RHA Technology Inc., El Cerrito,Calif., markets RHA for use in con-crete, owns a sublicense on a UC-Berkeley patent for RHA use in con-

R ice-hull ash (RHA) use in concrete has been primarilyconfined to the laboratory. But that may be changing in

California, where rice is a major crop. The state’s depart-ment of transportation has verbally approved the materialas an alkali-silica reactivity (ASR)-reducing pozzolan. AndPacific Gas & Electric Co. recently used RHA in dry-mixshotcrete to repair the Bowman Dam in northern Califor-nia’s Sierra Nevada mountains, with positive results.

The 70-year-old dam sits at a 5,500-foot elevation. Thestructure was designed to overspill when an upstreamreservoir occasionally overflows, but the downstream faceand base were cracked from freezing and thawing of over-spilled water.

RHA purchased from RHA Technology Inc., El Cerrito,Calif., and produced by the Wadham Biomass Facility inWilliams, Calif., was used as a partial cement replacement forsome repair patches on base of the dam, while downstreamface patches utilized portland cement concrete with no sup-plementary cementing materials. Welded wire mesh and key-lock strips were also used.

“We saw the need for improved durability, and we figuredif an alternative was favorable from a cost standpoint, we’dtry it,” says Richard Farley, an inspector with the powercompany. “The supplier showed us tests, and we were im-pressed. And since the material is processed nearby, thecost was acceptable.”

The design specifications required 3000-psi compressivestrength at 28 days, and mixes with 10% and 13% RHA re-placements by weight of cement were used.

Cores showed both RHA mixes exceeded the requiredstrength at 14 days (see box). Farley reports that floodingand concurrent freeze-thaw during the winter of 1995-96 didnot crack any of the RHA repair work. He says he’s im-pressed by an absence of cracking in the dam’s base, whichis subjected to many freeze-thaw cycles at that high altitude.In his report, Farley recommends additional RHA testing. “I’dlike to use it in cast-in-place concrete soon,” he says.

RHA gets a field test

Mix designs, core tests of RHA dry shotcre t eBowman Dam Restoration Pro j e c t

Mix 1Cement 517 lbs., 5.5 sackRHA 70 lbs.*Steel fibers 60 lbs.Sand 2,867 lbs.*13% by weight of cementitious material

Core test results14 Day 56 Day4760 psi 7090 psi

Mix 2Cement 677 lbs., 7.2 sackRHA 70 lbs.*Steel fibers 60 lbs.Sand 2,867 lbs.*10% by weight of cementitious material

Core test results14 Day 56 Day3730 psi 6740 psi

Though subjected to many freeze-thaw cycles, dry shotcrete repair

work using rice-hull ash as a partial cement replacement on the Bow-

man dam shows no cracks. Note cracks in the ordinary concrete re-

pair work on the face above.

crete and purchases RHA from theWadham Biomass Facility in Williams,Calif. A dam repair project in northernCalifornia (see related article) utilizedRHA processed at the Wadham plantand sold by RHA Technology. Cal-trans has also agreed to purchase ashfrom RHA Technology for a yet-to-be-determined road project.

For a dry shotcrete project like thedam restoration, it appears a 10% ce-ment replacement with RHA selling at$200 per ton would cost producersabout $10 per cubic yard of concrete,depending on quantity and packaging.This is comparable to silica fume’sper-yard cost, according to one silicafume marketer. Dean Hiyama, a part-ner at RHA Technology, says an RHAadmixture could sell in 50- and 700- to900-pound bags, like silica fume.

Since costs of producing and trans-porting RHA may discourage cementmanufacturers, who could intergrindRHA with clinker, from buying largevolumes, many say RHA has the mostpotential as an admixture. “It tookover 20 years for silica fume to becomecost-effective,” says David Smetana,who studied RHA for several years inconjunction with UC-Berkeley re-searchers. “It’s going to take somebodyto introduce RHA as a high-perfor-mance admixture and build its reputa-tion before cement companies startbuying large volumes.”

—DON TALEND

References1. Zhang, Min-Hong and Malhotra, V.Mohan, “High-Performance ConcreteIncorporating Rice Husk Ash As a

Supplementary Cementing Material,”ACI Materials Journal, November-De-cember 1996, pp. 634-636.2. Hwang, C.L. and Wu, D.S., “Proper-ties of Cement Paste Containing RiceHusk Ash,” ACI Third InternationalConference Proceedings, 1989, p. 738.3. Mehta, P.K. “Rice Husk Ash As aMineral Admixture in Concrete,”Swedish Council for Building Re-search Second International Seminar,June 1989, pp. 132-134.

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