Gelling wash water from a concrete truck washout

The pH meter read 12.4 in the washout pit, the conductivity was off the top of the handheld’s range, and the pit was 300 mm from the top of the bund. Two readymix trucks were still to come back that afternoon. The plant manager wanted to know whether a bag of powder would let him pump the pit out to the sewer. The honest answer was: not that pit, not at that pH, and not without letting the solids settle first.

Concrete washout is one of the most demanding liquids anyone tries to gel. It is not dirty water with a bit of cement in it; it is a strongly alkaline suspension of calcium hydroxide, cement fines and sand, and every one of those components works against a polyacrylate.

Why the chemistry fights back

Three things happen at once. The high pH, typically 12 to 13, hydrolyses the polymer network over hours to days, so a gel that looks firm at noon can be weeping by evening. The calcium at 500 to 1,500 mg/L crosslinks the carboxylate groups on the outside of the granule and forms a hard skin, which slows water uptake dramatically — the classic “skin before core” problem. And the suspended solids, often 3 to 8 percent by weight, coat the granules and physically block the wicking.

None of that means the job is impossible. It means the dose is two to three times the clean-water figure, the grade has to be coarse, and the sequence has to include a settling step.

Settle first, always

Let the pit stand for at least four hours, or overnight if the schedule allows. The sand and much of the cement fines drop out, and the liquid you decant off the top is far easier to treat — perhaps 0.5 percent solids instead of 6. That one step can halve the powder consumption and produce a gel that holds together instead of a gritty paste.

If the pit has no quiet zone, make one: a baffle or a second compartment with a weir. Plants that skip this end up dosing the whole slurry and then wondering why 25 kg produced a mass the consistency of wet mortar that would not pass a tilt test.

Dose, and check the pH afterwards

Start at 2 percent by weight of the decanted liquid and work up in a bucket trial. You are looking for a gel that holds at 45 degrees, does not weep clear water within thirty minutes, and can be cut with a spade without falling apart. If it weeps, the pH is winning and you need to bring it down before dosing — a slow addition of a dilute acid to pH 9 or below, with the usual precautions about heat and fumes.

Do not adjust pH after gelling. Once the water is bound, mixing acid into a gel is a slow, uneven process and the result is a patchy mass with liquid pockets. The order is settle, adjust, then dose, and the whole thing takes half a day for a pit of 10 cubic metres.

The treated material goes to a licensed facility as a solid, not to the sewer. Gelling does not remove the alkalinity or the metals; it only removes the free liquid. Any disposal route that assumes otherwise will fail on the first analysis.

The failure pattern is easiest to see in a readymix site near a river installed a washout bay with a 12 m³ pit and a paddle mixer. They dosed straight from the bag into the full pit, at 1 percent, on a Friday afternoon. By Monday the surface was firm enough to walk on and the layer below was still liquid. They pumped the liquid layer to the tanker, which rejected it on pH, and then had to dig out 9 tonnes of gel by hand because the excavator could not get into the bay.

The second attempt used a settling compartment, a pH check, and 2.4 percent dose on the decanted water. The same volume produced a consistent mass that loaded into a skip in two hours. The difference was not the product; it was four hours of patience and a probe.

Keep the records the disposal site will want

Note the date, the pit volume, the settled solids, the pH before and after adjustment, the dose, the batch, and the result of the tilt test. Take a photo of the gelled mass in the skip. When the facility asks what the material is, that sheet is the answer, and it takes five minutes to fill in while the pit is settling anyway.

If your washout is smaller — a single truck per day, a 2 m³ bay — the same sequence applies at a smaller scale, and a coarse grade at 2 percent will handle it. The grade data for super absorbent powder lists the calcium tolerance of each product, which is the number that decides whether a trial is worth starting.