FLY ASH QUALITY CONTROL

LOI/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. 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.

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.