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FLY ASH QUALITY CONTROL
REGARDING ITS USE IN HIGH END PRODUCTS SUCH AS CONCRETE
CaO Total
The total CaO in fly ash is mostly present in the glassy phase and is generally not available as free lime.
Bituminous coal (hard coal) ash has lower total CaO.
Sub-bituminous/lignite coal ash has higher total CaO (some as high as 32%) Recently, in the United States, ASTM classified fly ash according to total CaO content rather than the sum of silica + alumina + iron oxide. Class F has <18% CaO and Class C has >18% CaO. Canada has three classes of fly ash based on total CaO. See specifications.
CaO free
A minor fraction of the total CaO exists as free CaO. China has limits on free CaO. European standard BS EN 450 limits this to 1.5%.
CARBON CONTENT
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.
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
SO3 Soluble: Some of the S in fly ash exists as calcium sulfate and/or alkali sulfate. This is particularly true for some Class C fly ashes. Ettringite can form in the presence of CaOH2, sulfates and aluminates. ASTM C 311 tests for this expansivity.
Water-soluble sulfates are thought to be very important , 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. There is currently no limit on soluble SO3.
S total: Sulfur is determined with chemical test method (see C 114 for acid -soluble method), atomic absorption or the LECO sulfur analyzer and calculated as SO3 equivalent. Total SO3 is limited to 5% in ASTM C 618 and in Canada. Australia limits total SO3 to 3%. European BS EN 450 limits total SO3 to 3%.
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 larger-sized amorphous carbon residue is the issue. This is especially true for fly ashes treated with powdered activated carbon (PAC). The foam index test as originally determined by Vance Dodson of WR Grace has been modified by researchers at Brown University in Providence, RI. Currently ASTM has a new specification for Foam Index.
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.)
FINENESS
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.
DENSITY
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.
COLOR
The variation of the color of fly ash affects the final color of concrete products.
OIL
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.
NH3
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.
HYDRAULICITY
Some fly ashes are self-cementing particularly from the burning of sub-bituminous or lignite coal. See fly ash chemistry. 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. 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. Some of these ashes set and harden within 15 minutes.
STRENGTH ACTIVITY INDEX (SAI)
During the late 1980's, the Pozzolanic-Activity Index was evaluated by a special round robin ASTM committee. In the early 1990's, the Pozzolanic-Activity Index based on lime- fly ash mortar mixes was replaced with fly ash+cement mortar mixes. The Strength-Activity Index (SAI) acknowledges the fact that fly ash affects the strength of cement-based pastes by its fine filler effect, its hydraulic activity (if self-cementing), 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 cement-fly ash mortar cubes in saturated lime water at 23 degrees C (replacing the lime-fly ash mortar 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.
The SAI limit is 75% of the control. Some fly ashes passed the SAI at 7 days while other ashes took 28 days to pass. Both fly ashes meet the specification C 618. However those ashes passing in 7 days probably reflect a more reactive Class C fly ash. Though not common, some ashes have greater strengths at 7 day vs 28 days because they are either hydraulic Class C ashes or because the alkalies in the paste (from the cement or fly ash) activate the finely-ground Class F.
There are some limitations to this test : 1.) The higher alkali cement could give higher strengths so two different laboratories could get very different results. 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.
2.) Fine filler effect: Finely-ground quartz might pass the Strength-Activity Index (SAI) both at 7 days and 28 day. The 28 day SAI is the same at 7 days or lower. Thus no strength gain at later ages. Finely-ground limestone might pass the SAI specification but shows slightly reduced SAI at later ages. Finely-ground bottom ash has no amorphous phase and would be expected to have reduced pozzolanic activity but may pass the SAI specification. Since fine fillers are not very pozzolanic, raising the SAI limit from 75 to 85% of control is recommended in ASTM 618.
3.) Is 28 days long enough to evaluate a pozzolan's true pozzolanic potential especially with non-accelerated temperature curing? Do we need 56 day specimens? Although not required by ASTM, It would be great to report both 7 day and 28 day SAI to show compressive strength trend.
4). Questions have been raised about the SAI test. It evaluates the physical (rheological) properties of the cement-fly ash mortars more than its chemical (pozzolanic) properties. ASTM C618 specifies SAI of mortar mixes whose water content is adjusted to constant flow. It specifies SAI of 75% of control to be achieved by 7 or 28 days. The European specification EN 450-1 specifies SAI of mortar mixes whose water content is adjusted to a set amount. It specifies SAI of 75% of control at 28 days and 85% of control at 90 Days. Which is the right approach?
PAC
If powdered activated carbon (PAC) is used to remove Mercury from air emissions, the resulting fly ash has a higher foam index. The carbon rapidly adsorbs air-entrainment agent. The native carbon in the ash has a slower rate of adsorption. See foam index test.
Trona (sodium sesquicarbonate) or sodium bicarbonate used in dry sorbent injection into flue gas stream
When these agents are used to capture acid gases such as HCL and SO2 from air emissions, the resulting residue has fly ash, sodium chloride, CaCO3, and high SO3. Calcium sulfite hemihydrate (CaSO3·0.5H2O) , the calcium sulphates hemihydrates (CaSO4 ·0.6H2O) and calcium sulphates (CaSO4) are usually formed in various proportions as products of flu-gas desulfurization process. When used alone, this kind of material has a number of limitations related to strength development and long-term performance. Ettringite materials contribute to strength loss and volume expansion.
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