Frozen Solidified Cake Overnight: Breakout, Re-Handling and Acceptance

A transfer station in Alberta loaded a 20 m³ skip with solidified leachate at 15:40 on a Tuesday in January. Overnight the temperature fell to -19 °C. By Wednesday morning the surface carried a 40 mm ice crust, and when the loader broke the mass apart roughly 60 litres of amber liquid ran across the skip floor. Fourteen hours earlier the load had passed a paint filter test. It failed at the receiving facility, and the site paid a standing charge plus the return trip.

Freezing does not preserve a set gel. It dismantles the network and puts the water back where it started, which means every site that solidifies through winter is running a cold-store problem alongside a chemistry problem. This article explains what ice does inside a swollen bead, why the thaw costs more than the freeze, and how to schedule and re-handle loads when the overnight forecast sits below -5 °C.

Ice Forms Inside the Gel and the Network Does Not Recover

A swollen SAP bead holds water in three rough states: bound to the polymer chain, held in the pore structure by capillary force, and free in the void space between beads. In a typical leachate gel at 22 kg/m³, somewhere between 40 and 60 percent of the water is freezable. Below about -2 °C that fraction begins to form ice, and ice occupies roughly 9 percent more volume than the water it came from.

That expansion has to go somewhere. It goes into the pore walls, and the polymer strands that give the gel its stiffness are thin enough to rupture under that strain. The damage is not visible while the mass is frozen, because the ice holds everything rigid. It becomes visible on thaw, when the ruptured network can no longer hold what it once held and water separates out. Laboratory checks on freeze-thaw cycled SAP typically show a 30 to 50 percent loss of absorption capacity after two or three cycles, and the loss is largely permanent. Re-freezing does not restore it.

The Thaw Is the Expensive Half

A frozen load is easy to handle and hard to accept. While it is solid at -10 °C it will pass any visual inspection, and a loader can move it like rock. The failure appears at the gate, four to eight hours later, as a wet patch spreading from the base of the load and a receiving-site technician with a paint filter and a stopwatch. Nine times out of ten the argument at that point is not about whether the material is wet but about whether the wetness was there at dispatch. Without a temperature log the site cannot answer, and the load goes into quarantine.

The cost breakdown is worth writing down because it changes behaviour more than any procedure. A rejected 20 m³ load typically costs the return haul, a quarantine day, a re-test, and sometimes a re-work charge, and none of that appears in the dose budget that the operator is being measured against. Sites that track it usually find that one winter rejection costs more than four seasons of slightly higher dosing.

Scheduling Pours Around a -5 °C Threshold

Treat -5 °C as the line where the standard procedure stops working. Above it, a covered skip will hold enough heat for six to eight hours and the load is normally safe. Below it, four rules apply. Do not pour after 13:00 if the overnight minimum is forecast below -5 °C, because the mass needs six hours above freezing to shed its heat of mixing. Cover the skip with an insulated tarpaulin and, on exposed sites, 100 mm of straw or a 150 mm EPS board laid on the surface. Park loaded skips against a south-facing wall rather than in the open yard. And drop cake moisture by three to four points by raising dose 2 kg/m³, because every point of water removed is a point that cannot form ice.

Part-filled skips are the worst case and should be avoided outright. A 6 m³ layer has far less thermal mass than a 20 m³ one, and it will freeze through in a single night at -12 °C even under cover. Where a campaign cannot fill a skip in one day, keep the batch in an insulated hold tank and pour once.

Re-Handling a Load That Froze

If a load freezes, do not dispatch it and do not add water. Move it to a heated bay at 8 to 12 °C, leave it 24 hours for a 20 m³ mass and 48 hours if it is deeper than 1.5 m, and break it with a grab rather than a bucket edge, which smears the surface and hides the release. Re-test the paint filter only after the whole mass is above 5 °C, sampling from the centre and the base as well as the top.

If the thawed material fails, re-blend 3 to 5 kg of fines per tonne in a mixer with a 90-second cycle, then re-test. Record the freeze event, the thaw duration, the bay temperature and the re-work dose, and send that record with the load. Receiving sites accept a documented re-work far more readily than they accept an unexplained wet patch, and the record is also what protects the site if the same skip is questioned a second time.

None of this is complicated, but all of it depends on the polymer being able to rebuild a network after the first freeze, which is why we publish freeze-thaw retention alongside free swell for every SAP grade we supply for leachate work. If your winter campaign runs below -5 °C, ask for the retention figure before you ask for the price.