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Air Entraining Admixture Adsorption by Coal Fly Ash - The Direct Adsorption Isotherm[edit]


  • Safety glasses must be worn in the lab at all times.
  • Full-length pants are required in the lab.
  • Close-toed shoes are required in the lab.
  • Consult MSDS of the AEAs being evaluated for AEA-specific safety considerations.


  • Analytical balance having 0.01 g precision
  • Erlenmeyer flask, 250 ml (3)
  • Volumetric flask, 1,000 ml (3)
  • Vacuum flask, 1l (3)
  • Vacuum source
  • Buchner funnel, 90 mm or equivalent (3)
  • Filter paper, Grade 1, 90 mm diameter or equivalent
  • 5ml pipette
  • 10ml pipette
  • 20ml pipette
  • 1ml or 2ml pipettor
  • Graduated cylinder, 250 ml (3)
  • Magnetic stirrer and stir rod suitable for 250 ml Erlenmeyer flask (3)
  • Drying oven
  • Hach DR890 portable colorimeter
  • Hach DRB200 Digital Reactor Block set for COD


  • Distilled water
  • Coal fly ash and portland cement to be evaluated
  • Air entraining admixture to be evaluated
  • Hach Cat# 2125915 150 pack high range COD vials (20-1,500 mg COD/l)
  • Worksheets

Methodology Overview[edit]

The lab is equipped to perform the three different AEA concentrations simultaneously and this procedure is written accordingly. As equilibrating solids and solutions followed by determination of COD requires several hours to complete, an educated guess is made as to the initial AEA solution concentrations. This method has proven to be effective as, generally speaking, the initial guess is correct. On the rare occasion that solution concentrations must be adjusted, it is typically necessary to only adjust one solution, yielding two useful data points which can be included in the final isotherm and from which to interpolate/extrapolate the third concentration.

AEA solution concentrations need only produce filtrates (post equilibration with solids) having COD in the range of 100 - 1,300 mg COD/l. Occam's Razor[1] applies - AEA solution concentrations made with easily obtained pipettes (i.e. 2 ml, 5 ml, 10 ml, etc.) are always preferred over mix and match or fractional ml AEA volumes.

Three separate steps having only minor differences are performed during the entire procedure. Many are repeated; this procedure is written such that the repeated steps are, for the most part, only presented once.

The entire procedure is typically performed in triplicate with a given AEA, portland cement and fly ash. Final results are presented based upon careful analysis of the results from the three individual tests.

Blank Solutions[edit]

Three AEA standard solutions will be prepared in concentrations of 5 ml/l, 10 ml/l and 20 ml/l:

  1. Fill out the top of the DAI paper worksheet (fly ash, cement, AEA, etc.)
  2. Figure out IDs (start with 1, 2, 3, then add ten to subsequent sets, i.e. 11, 12, 13 then 21, 22, 23, etc.) and record them on the sheet.
  3. Record the initial AEA concentrations (5, 10 and 20 ml/l) on the paper worksheet.
  4. Partially fill the 1 l volumetric flask labelled #1 with distilled water, add 5 ml AEA solution and dilute to the mark with distilled water. Mix thoroughly.
  5. Repeat with volumetric flasks #2 and #3, adding 10 ml AEA to #2 and 20 ml AEA to #3, so producing the three AEA solutions.
  6. Follow the Common Procedure using 20 g of the portland cement for the solid phase.

Evaluation of Blank Solution Results[edit]

If the current test is the first run with a new AEA, it is advisable to perform this step, including digestion and COD measurement, prior to proceeding with collection of isotherm data to insure that the solution concentrations produce the desired range of COD values.

The COD of the three filtrates should fall within the 100-1,300 mg COD/l range, recalling that the high range COD digestion vials have a stated range of 20 to 1,500 mg/l. A relatively wide range is desired, while usable results have been produced with a range of only 100-700 mg COD/l. If one or more of the filtrates have COD far outside the range or violating the stated range of the digestion vials, repeat the above procedure for the solution(s) producing the out-of-range value(s). For example, if the 20 ml/l AEA solution yields filtrate having out-of-range COD, but the 10 ml/l is in range, smartly reduce the concentration (i.e. try 15 ml AEA/l) and rerun with that new solution.

Isotherm Data Points[edit]

  • This procedure uses the solutions produced in the Blank Solutions procedure. Since a full liter of each solution is produced, two complete procedures can be completed with a single solution mix.

Follow the Common Procedure using 20 g of the portland cement and 40 g of the fly ash being evaluated.

Cement and Fly Ash COD Contributions[edit]

  • This procedure quantifies the COD contributed by the individual solid phases in distilled water - the AEA solutions are not used for this part of the procedure.
  1. Weigh 80 g of the portland cement into Erlenmeyer flask #1, recording this weight to the nearest 0.1 g on the paper worksheet.
  2. Weigh 80 g of the fly ash into Erlenmeyer flask #2, recording this weight to the nearest 0.1 g on the paper worksheet.
  3. Dispense 200 ml of distilled water into each of graduated cylinders #1 and #2.
  4. Add the contents of graduated cylinder #1 to Erlenmeyer flask #1; repeat process with #2.
  5. Add a magnetic stir rod to each flask and place each flask on its own magnetic stirrer.
  6. Stir at a rate that keeps the solids suspended. Use a glass rod to initiate stirring if necessary.
  7. After stirring for 60 minutes, filter the slurries and determine the COD following the procedure outlined in Common Procedure.

Digestion and COD Measurement[edit]

It has been found sufficient to digest a complete set of eight vials after equilibrating solids and solutions, it is not necessary to digest vials as they are produced.

After equilibrating all the solutions:

  1. Place vials in the DRB200 after rechecking that the lids are tight.
  2. Start the DRB200 digestion cycle.
  3. At the conclusion of the 120 minute digestion cycle, remove the vials from the DRB200 and store upright in beakers until they cool to room temperature.
  4. NOTE that there are solids in the bottom of the COD vials and should they be suspended while measuring in the DR890, vastly inaccurate results will be produced. Once the vials have cooled to room temperature, turn on the DR890 colorimeter and zero with the appropriate standard.
  5. Determine the COD of each vial by collecting three readings from each in the DRB890 and averaging those three readings. Record the COD in the appropriate cell of the paper worksheet. If significant variation is apparent in the three readings, then it is likely that solids were suspended during measurement. Return that vial to the beaker in an upright position and allow the solids to settle, then repeat the measurement.

Common Procedure[edit]

Use the weight of solids specified in the calling step.

  1. Dispense 200 ml of AEA solution #1 into graduated cylinder #1 and repeat the process with solutions #2 and #3 in graduated cylinders #2 and #3, respectively.
  2. In the 250 ml Erlenmeyer flask labelled #1, weigh out the solid phase(s) and record the weight(s) to the nearest 0.1 g in their respective cells on the paper worksheet. Repeat the process with Erlenmeyer flasks labelled #2 and #3.
  3. Add the contents of graduated cylinder #1 to Erlenmeyer flask #1. Repeat with the other two solutions and solids.
  4. Place the flasks each on their own magnetic stirrer, add magnetic stir rods and stopper the flasks.
  5. Stir the slurries for 60 minutes. Use a glass rod to initiate stirring if necessary. Since carbon (the chief adsorbant) tends to be hydrophobic, it will likely occupy the surface of the slurries and stick to the glass rod; sparingly rinse the glass rod with distilled water so that the carbon stays with its slurry.
  6. While stirring and before anything dries, rinse the volumetric cylinders thoroughly, first in tap water and then distilled water: Three rinses with fresh tap water, then three rinses with fresh distilled water.
  7. At the conclusion of the mix period, filter the slurry. If cracks form in the filter cake and vacuum is lost, use a spatula to seal the cracks. Filter until filtrate is produced at a rate of about one drop every ten seconds.
  8. While filtering, prepare COD vials, labeling them appropriately
  9. Transfer filtrate to their respective graduated cylinder (#1, #2 and #3), record the volume of each to the nearest ml in their respective cells on the paper worksheet.
  10. Rinse the pipettor with distilled water three times then pipette 2 ml of filtrate #1 into it's respective COD vial. Seal the vial. Repeat with filtrates #2 and #3, rinsing the pipettor three times with the next solution between each collection.

Isotherm Generation[edit]

Use the worksheet to generate isotherms from the data gathered.