Cement, Fly Ash and SAP in One Paddle: Blending Order for Wet Filter Cake

Three drums of the same filter cake, three different results. One day the operator added cement, fly ash and polymer together and got a mix that set in 45 minutes but bled for two hours. Another day he added the polymer last and got a mix that never hardened. On the third day, polymer first into the water, cement and ash after, the same recipe gave a stackable material at 7 days with 1.1 MPa unconfined strength. Nothing in the recipe had changed except the order.

Blending a superabsorbent with a hydraulic binder is not a matter of tipping three bags into a mixer. This article covers why the sequence matters, the ratios that work on wet cake, and how to run the trial so the third day becomes the standard.

Why Order of Addition Decides the Result

Cement needs water to hydrate, and so does polyacrylate. They compete for the same free water, and whichever gets there first takes it. If cement and granules are dry-blended and then wetted, the cement grains hydrate first because they are in intimate contact with the mixing water, leaving the polymer to compete for what remains. The result is a mix that sets but still bleeds.

Add the granules to the water or slurry first, allow 60 to 90 seconds for swelling, then add the binder. The polymer takes its share of free water, the cement hydrates in the remaining pore solution, and the two processes finish at different times, which is exactly what you want: the gel holds water that would otherwise have migrated to the surface, while the cement builds the skeleton.

Ratios: 3 Percent Cement, 1.5 Percent SAP, Watch the Water

On a filter cake at 55 to 65 percent moisture, a workable starting recipe is 3 percent ordinary Portland cement, 4 to 6 percent fly ash and 1.2 to 1.8 percent polyacrylate granule, all by wet weight. That delivers 1.0 to 1.5 MPa at 7 days and passes a paint filter test at 24 hours with no free liquid.

Do not push the cement above 5 percent to compensate for a wet cake. Cement hydration needs a water-to-binder ratio under about 0.6, and at 60 percent moisture the cake already supplies 1.5 kg of water per kg of binder. Extra cement beyond 5 percent simply raises the temperature, cracks the set product and costs 90 to 120 dollars a tonne of cement for no strength gain.

Set Time and Strength at 7 Days: What to Record

Record four numbers per trial: initial set by Vicat needle, bleed water at 1 and 4 hours, unconfined compressive strength at 3 and 7 days, and the paint filter result at 24 hours. The set time you want is 3 to 6 hours. Under 1 hour, the mix will set in the mixer on a warm day. Over 12 hours, the truck will arrive before the load is stable.

Bleed water is the number most often skipped and the one the disposal site cares about. Put 2 kg in a 2 litre cylinder, cover it, and read the clear layer after 4 hours. Anything over 5 ml per kg will show up as a wet patch in a trailer and get the load rejected. If bleed is high, the polymer dose is short or the granules are too fine to compete with the cement for water.

Mixing in a 1 m3 Paddle Versus a Truck Mixer

A paddle mixer at 35 rpm gives good shear and a 4-minute cycle, but the blades can chop a swollen gel and reduce its hold on water. Run the paddle at 25 to 30 rpm after the granules have swollen. A truck mixer is gentler and gives 8 to 12 minutes of transport mixing, which continues the reaction but also allows the granules to settle into dense pockets at the bottom of the drum.

In a truck mixer, load water first, then granules, then cement and ash, and keep the drum turning at 6 to 8 rpm. Discharge within 90 minutes. Where the load has to wait, a retarder at 0.2 percent of binder weight buys 2 hours without hurting 7-day strength, and it is far cheaper than a rejected load.

For stabilization work, the granule needs to be coarse enough to survive a paddle and slow enough to swell to give the cement its working window. The polyacrylate granules for binder blends are graded for this kind of shear.