FLY ASH QUALITY CONTROL
LOI: After drying the sample
in a drying oven, the ash is ignited at 750 C in a muffle furnace (see
ASTM C 311). The loss in weight from 110 C to 750 C is considered a good
estimate of carbon content. Carbon is important in air-entrained concrete
mixtures. Carbon will adsorb air-entraining surfactants thereby making
less available to entrain tiny air bubbles in the concrete which are required
to lend the concrete its protection against freeze-thaw conditions. The
degree of adsorption is dependent on the surface area, type of carbon(very
coarse particles or soot), and the polarity of the carbon.
LOI: Loss on ignition can
also be determined with the hot foil instrument. A 100 mg ash sample is
dryed and then heated using an electric current. Determinations can be
made in 5-10 minutes. See Fossil
Energy Research Corp.
Carbon: Carbon and sulfur
can be quickly determined with the LECO
carbon and sulfur analyzer. Carbon can be removed through a fly ash beneficiation process.
SO3: Most of the SO3 in
fly ash exists as calcium sulfate and/or alkali sulfate. Total sulfates
(acid-soluble) are determined with ASTM C 311 and calculating the result
as SO3. Total SO3 is limited in ASTM C 618 because of concerns about expansive
calcium sulfoaluminate reactions.However water-soluble sulfates are thought
to be very important ,as well , since these sulfates interact with chemical
admixtures and affect their performance and the setting time of concrete
or mortar. When sulfates are quite soluble (alkali sulfates), efflorescence
may occur on the surface of concrete products.
S: Sulfur is determined
with the LECO sulfur analyzer and calculated as SO3.
FOAM INDEX (FI)
One of the most important quality
control tests is to determine the amount of air-entraining agent required
to achieve a stable foam. Some fly ashes, though their LOI is acceptible,
have a high demand for air-entraining agent and may cause greater air loss
in the plastic concrete. The foam index test as originally determined by
Vance Dodson of WR Grace has been modified by researchers at Brown University
in Providence, RI.
The procedure is as follows: 2 g
of fly ash is placed in a 70 ml cylindrical weighing bottle with i.d. of
40 mm x 80 mm along with 25 cc of distilled water. The sample is ultrasonically
dispersed for 5 minutes, after which time 8 g of Portland cement is added.
The weighing bottle is then capped and thoroughly shaken for 1 minute to
completely wet the cement and fly ash. A 10 vol % aqueous solution of Darex
II (of coarse one can select their own air-entraining admixture) is then
added dropwise from a 2 cc microburet. After each titration, the bottle
is capped and shaken vigorously for 15 seconds, after which time the lid
is removed and the liquid surface observed. Prior to the endpoint of the
test, the foam on the liquid surface is extremely unstable, the bubbles
bursting within a few seconds. The endpoint is realized when a constant
foam is maintained on the surface for at least 45 seconds. The volume of
diluted Darex II required to produce this stable foam is referred to as
the foam index of the fly ash/cement mixture. The entire procedure is repeated
using 8 g of the cement only thereby yielding a foam index value for the
cement. Subtraction of the two values yields an effective foam index for
the fly ash. This serves as a measure of the degree to which any given
fly ash adsorbs AEAs.(Yu-Ming Gao, Hong-Shig Shim,
Robert Hurt, and Eric Suuberg and Nancy Yang.. Effects of Carbon on
Air Entrainment in Fly Ash Concrete: The Role of Soot and Carbon Black.
Energy & Fuels. Vol. 11, No. 2, pp 457-462, 1996.)
45 um: In the United States,
the amount of fly ash passing a 45 um sieve( a maximum of 34 % retained
) is an indication of its fineness. Other countries require an additional
fineness test at 20 um.
Particle Size: A better
indication of the fineness is to determine the particle size distribution.
For example, one can determine the mass percentage below 10 um or determine
the mean particle diameter. The particle size of fly ash varies from below
1 um to 200 um or more. Thus a fly ash might have the following distribution
(on a mass basis): 0.3-2 % below 1 um, 30-70 % finer than 10 um, 0.5-7
% above 100 um and 0-2 % above 200 um. It should be noted that to increase the Strength Activity Index (ASTM C 618) one can air-classify or grind the fly ash to improve its fineness (see fly ash beneficiation).
On a numerical basis: 40-60%
of total number of particles are from 0-1 um. This is more significant
with regards to greater surface area for pozzolanic reactions and leaching
potential of trace metals.
The density of the fly ash, which
ranges from 2-2.8, determines the volume it will occupy for a given mass.
Density changes may indicate a different coal source.
The variation of the color of fly
ash affects the final color of concrete products.
Oil is sometimes added to the pulverized
coal boiler at startup and may produce a soot-like carbon. Startup ash
should not be used. Test for oil by mixing 100 g of fly ash with tap water
and noting the presence, if any, of an oily film on the surface.
Ammonia is sometimes added to the
pulverized coal boiler. It is used with deNOx technologies or for flue
gas conditioning. It adsorbs onto the fly ash particles and exists as free
ammonia or as ammonium sulfate. Ashes that are basic in nature with very
low sulfur content adsorbs much less ammonia than high sulfur Eastern bituminous
coal ashes. Typically one observes 30 to several hundred ppm. Testing for
the presence of ammonia is important as the alkaline medium of concrete
releases ammonia. Test the fly ash by mixing 2 g of fly ash with 8 g of
Portland cement and note the presence of ammonia odor.
Some Class C fly ashes are self-cementing.
The quickness of the set and strength of fly ash cubes can be observed
in the pozzolan laboratory. Mix fly ash and tap water at a w/solids ratio
of 0.35. Make 6 cubes. Test for compressive strength at 1 day and 7 days.
Class F fly ash will not harden. A moderately self-cementing ash might
have a strength of 100-500 psi at 7 days. A very self-cementing fly ash
would exceed 500 psi in 7 days with significant strength development at
1 day. All cubes should be cured in saturated lime water or moist cabinet.
Many class C fly ashes set and harden within 15 minutes.
During the 1980's the Pozzolanic-Activity Index was replaced by ASTM. The Strength-Activity Index acknowledges the fact that fly ash affects the strength of cement-based pastes by its fine filler effect, its hydraulic activity (if a Class C fly ash), its subsequent pozzolanic activity , its ability to reduce water demand and is ability to affect the fractal properties of the calcium silicate gel. The new test cured the mortar cubes in saturated lime water at 23 degrees C rather than sealed cubes at 38 degrees C. The new test replaced fly ash by mass rather than volume and the water requirement of the fly ash pastes were held to +/- 5 of the cement control flow. Some fly ashes passed the Strength -Activity Index at 7 days while other ashes took 28 days to pass. Both fly ashes met the specification C 618. However those ashes passing in 7 days probably reflect a more reactive Class C fly ash.
There are some limitations to this test : 1.) The higher alkali cement could give higher 7 day strengths so two different laboratories (using different cements) testing the same fly ash could get very different 7 day compressive strengths. 2.) Finely-ground quartz could pass the Strength-Activity Index but would be considered by most observers to be marginally pozzolanic. 3.) Is 28 days long enough to evaluate a fly ash's true pozzolanic potential especially with non-accelerated temperature curing? 4.) Since the cement is so important in this test, should we not include more than one Portland Cement to qualify the fly ash? Perhaps test the pozzolan with a low, medium and high alkali cement.