Rice-hull ash's high silica content stimulates the production of calcium silica hydrates in plastic concrete, yielding enhanced durability over that of ordinary concrete, The Concrete Producer magazine, by Don Talend, brand storytelling, content management, and content strategy expert. Construction industry

1. B
urning rice hulls at 450° F
for four hours produces
an ash that meets require-
ments for an ASTM C 618
Class N pozzolan. Advo-
cates 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
(see related article) 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 sur-
face. RHA’s average particle size is
comparable to that of fly ash, and
RHA’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 form
stronger-bonding calcium silicate hy-
drates. In hardened concrete, more cal-
cium silicate hydrates mean less capil-
lary porosity and less permeability. This
pozzolanic reaction also reduces the
presence of soluble, alkaline calcium
hydrates, which can leach out and
weaken concrete.
RHA used in concrete has an effect
similar to other pozzolans. Research
indicates a 10% replacement of the
weight of cement by RHA produces
higher compressive, tensile, and flex-
ural strengths than ordinary concrete
but slightly lower strengths than
silica fume/cement concrete. Reduced
porosity 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 structures
exposed to deicers.
Chris Crouch, a
Pleasanton, Calif.-based
researcher for RMC
Lonestar, which is study-
ing RHA use as an ad-
mixture at its ready mix
plants and in blended ce-
ments, says, “People
wouldn’t replace their fly ash silo with
one for rice-hull ash blended cements.
But a good fit for RHA would be a pre-
cast plant that makes components for
marine structures and needs an admix-
ture.”
RHA has proven to control alkali-sili-
ca reactivity (ASR). In California, pro-
ducers whose aggregates have been clas-
sified as reactive were threatened with
exclusion from bidding for state work.
The state’s department of transporta-
tion, Caltrans, addressed the problem
by drafting a provision mandating cer-
tain ASR-reducing practices, including
use of pozzolans. Caltrans has verbally
approved RHA as an ASR reducer and
is looking for a large project in which to
use RHA in the near future.
RHA and fly ash particle sizes aver-
age 20 to 30 microns. These admix-
tures handle more like cement than
silica fume, which has an average par-
ticle size about 200 times smaller. But
RHA, like silica fume, has a silica con-
tent of at least 90%, compared with fly
ash’s typical 60% to 65% silica con-
tent. The more silica, the higher the
degree of reactivity with cement’s free
lime and the higher the reduction in
porosity of hardened concrete.
Particles hold water
What makes RHA a unique pozzolan
is its multilayered, microporous sur-
face (see photo), which differs from fly
ash’s and silica fume’s spherical parti-
cles. The surface is thought to reduce
bleeding through absorption (Ref. 2)
when RHA is used in concrete. Despite
producing less bleeding in fresh con-
crete, RHA particles’ absorbency re-
portedly maintains the workability of
fresh 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 by
adding water, thus increasing the wa-
ter-cement ratio and reducing
strengths in hardened concrete.
Partial cement replacement with
RHA alone may not be applicable to
fast-track or cold-weather projects,
since in one study, concrete with a
10% replacement set more slowly than
ordinary or silica fume concrete (Ref.
1). Additionally, like silica fume, RHA
use may not suit some architectural
applications, 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.

2. carbon content gives concrete a slight-
ly darker color than ordinary concrete.
Some RHA proponents think RHA
should be used to augment more plen-
tiful fly ash by offsetting lower early
strength gains when fly ash is used as a
partial cement replacement (Ref. 3).
Production, transportation
concerns
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 com-
bustion of this output would produce
about one-fifth the weight, or 1.6 mil-
lion metric tons, of ash.
But RHA is less available than other
mineral admixtures because it’s not a
production byproduct that eliminates
pollution. Rice hulls are not a tradition-
al fuel source to begin with; producing
useable ash requires a capital invest-
ment by rice plants, one not driven by
law but by potential profitability.
RHA production requires a two-step
process. Jim McDaniel, RHA team
leader at Producers Rice Mill,
Stuttgart, Ark., says the company’s
rice-combustion plant first chars the
rice hull, producing crystalline silica. A
second combustion process is required
to get the RHA’s particle size and car-
bon content down to a level suitable
for concrete. For a rice plant to build a
rice-combustion unit would require a
commitment 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-
Rice-hull ash (RHA) use in concrete has been primarily
confined 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 material
as an alkali-silica reactivity (ASR)-reducing pozzolan. And
Pacific Gas & Electric Co. recently used RHA in dry-mix
shotcrete 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. The
structure was designed to overspill when an upstream
reservoir occasionally overflows, but the downstream face
and 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 in
Williams, Calif., was used as a partial cement replacement for
some repair patches on base of the dam, while downstream
face 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 figured
if an alternative was favorable from a cost standpoint, we’d
try it,” says Richard Farley, an inspector with the power
company. “The supplier showed us tests, and we were im-
pressed. And since the material is processed nearby, the
cost was acceptable.”
The design specifications required 3000-psi compressive
strength 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 required
strength at 14 days (see box). Farley reports that flooding
and concurrent freeze-thaw during the winter of 1995-96 did
not crack any of the RHA repair work. He says he’s im-
pressed by an absence of cracking in the dam’s base, which
is subjected to many freeze-thaw cycles at that high altitude.
In his report, Farley recommends additional RHA testing. “I’d
like to use it in cast-in-place concrete soon,” he says.
RHA gets a field test
Mix designs, core tests of RHA dry shotcrete
Bowman Dam Restoration Project
Mix 1
Cement 517 lbs., 5.5 sack
RHA 70 lbs.*
Steel fibers 60 lbs.
Sand 2,867 lbs.
*13% by weight of cementitious material
Core test results
14 Day 56 Day
4760 psi 7090 psi
Mix 2
Cement 677 lbs., 7.2 sack
RHA 70 lbs.*
Steel fibers 60 lbs.
Sand 2,867 lbs.
*10% by weight of cementitious material
Core test results
14 Day 56 Day
3730 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.

3. crete and purchases RHA from the
Wadham Biomass Facility in Williams,
Calif. A dam repair project in northern
California (see related article) utilized
RHA processed at the Wadham plant
and sold by RHA Technology. Cal-
trans has also agreed to purchase ash
from RHA Technology for a yet-to-be-
determined road project.
For a dry shotcrete project like the
dam restoration, it appears a 10% ce-
ment replacement with RHA selling at
$200 per ton would cost producers
about $10 per cubic yard of concrete,
depending on quantity and packaging.
This is comparable to silica fume’s
per-yard cost, according to one silica
fume marketer. Dean Hiyama, a part-
ner at RHA Technology, says an RHA
admixture could sell in 50- and 700- to
900-pound bags, like silica fume.
Since costs of producing and trans-
porting RHA may discourage cement
manufacturers, who could intergrind
RHA with clinker, from buying large
volumes, many say RHA has the most
potential as an admixture. “It took
over 20 years for silica fume to become
cost-effective,” says David Smetana,
who studied RHA for several years in
conjunction with UC-Berkeley re-
searchers. “It’s going to take somebody
to introduce RHA as a high-perfor-
mance admixture and build its reputa-
tion before cement companies start
buying large volumes.”
—DON TALEND
References
1. Zhang, Min-Hong and Malhotra, V.
Mohan, “High-Performance Concrete
Incorporating 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 Rice
Husk Ash,” ACI Third International
Conference Proceedings, 1989, p. 738.
3. Mehta, P.K. “Rice Husk Ash As a
Mineral Admixture in Concrete,”
Swedish Council for Building Re-
search Second International Seminar,
June 1989, pp. 132-134.
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