Hot Spots in the Trough: Why the Middle of a Big Batch Stays Wet

Three hundred kilos in a mortar tub, mixed by two people with paddles for eleven minutes, and the density reading from the middle was 0.68 kilos per litre against 0.44 at the edge. Same tub, same batch, same water. The floor laid from the middle went down like porridge and set hard; the floor laid from the edge was exactly right. The crew blamed the powder. The powder was fine.

Large batches fail from the inside out. The geometry of a tub, the order in which powder and water meet, and the fact that gel is a much worse mixer than water all combine to produce a batch that is uniform at the edges and starved or drowning in the middle. It is the most common cause of a floor with visible patches, and it is almost entirely preventable.

Why the Middle Is Different

When powder is added to a tub of water, the first material to land sinks and begins to swell where it sits. The material that lands later sits on top of it and swells there. A paddle turning through the middle pulls the wet mass around but does not fold the top into the bottom, so you get a layered structure: a wet dense core, a drier crust on top, and a ring at the wall that never gets touched at all.

The wall ring is the reason so many batches have a hard skin that has to be cut away. Material against the tub wall does not move when the paddle turns, because there is no shear at the boundary. It hydrates differently from everything else and it ends up in the floor as a lump.

Adding Water to Powder, Not Powder to Water

The most reliable method at scale is to put two-thirds of the water in first, add the powder in a steady stream while turning, and then add the remaining third slowly over the next few minutes. This gives the powder a chance to wet out before it is surrounded by water it cannot reach, and it avoids the sealed-core problem that comes from dumping dry powder into a full tub.

Cap batch size by tub, not by ambition. A 200 litre tub comfortably handles 40 kg of dry powder. At 60 kg the geometry stops working and the middle starts to lag. Two 40 kg batches take barely longer than one 60 kg batch and they are far more predictable.

Turn It, Do Not Stir It

Stirring moves material around the axis. Turning folds the bottom to the top and the wall to the middle, which is what actually homogenises. With a paddle, that means working from the centre outward in a spiral, then reversing direction and coming back in, and scraping the wall down once at the halfway point with a spatula or a gloved hand.

Then stop. Over-mixing is the second failure mode, and it is the one that produces snow which is dry to the touch, heavy in the jug and flat on camera. Every extra minute of paddle time collapses air out of the gel. Five minutes of mixing, twenty minutes of rest, one turn, and it is done. If you have to choose between a batch that is slightly uneven and a batch that has been worked for fifteen minutes, choose uneven: it grooms out on the floor, while collapsed structure does not come back.

Counterintuitively, the batch that looks least uniform at the end of mixing is often the better one, because a batch that looks perfectly smooth has usually been worked until the structure is gone. Judge it with the jug, not with the eye.

Before the next large build, cut a 40 kg batch in half and compare densities at the wall and the centre. If they differ by more than 0.05 kilos per litre, your mixing method is the problem, not the material. The SAP powder for snow hydrates evenly when it is given the chance, and a tub that is too full does not give it one.