Gypsum Retarder for Pumped Self-Levelling Compounds

Pumped self-levelling changes the job of a retarder. In a bagged compound mixed on site, the clock starts when the installer adds water. In a pumped system, the material may sit in a hose, a hopper and a pump for minutes before it reaches the floor, and every one of those minutes is open time the retarder has to hold without letting the mix stiffen in the line. This article is for producers and contractors running pumped self-levelling and screeds, and for anyone specifying a retarder that has to perform after transit as well as at the nozzle.

The pump adds a fourth clock

A pumped pour runs on the same three clocks as any self-levelling job: pot life, flow retention and set, plus a fourth that only pumped work has, which is line residence time. The material can spend several minutes in the hopper and hose before it is laid, and if the retarder is set only for the floor it can begin to thicken in the line. That fourth clock is why pumped systems need a retarder tuned for transit as well as for placement, and why a compound that flows well in a bucket can still choke a pump.

Flow time versus set time

Flow time is how long the material keeps self-levelling after placement; set time is when it is firm enough to walk on. A retarder that extends set but not flow leaves a floor that will not flatten; one that extends flow but not set leaves a soft skin over a moving base. Pumped compounds need both held, and held together, because the transit time eats into the flow window before the crew ever sees the material. The retarder is judged by whether the floor still levels after the hose, not only by the lab flow test.

What stiffening in the line costs

When the mix thickens in the hose it raises pump pressure, cuts output and can stall a pour mid-room, leaving a cold joint that shows through the finish. Worse, a partially set material laid under pressure can trap air and give a pocked surface. The retarder therefore has to keep the rheology stable through the line, not merely delay the final set, which is why the residence time and the temperature of the hose are part of the specification rather than an afterthought.

Matching the retarder to the run length

Longer hose runs and slower lay rates need more open time, so the retarder dose and grade are matched to the worst-case run, not the average. A site that pumps across a large slab from a single point needs more margin than one that moves the pump often. Temperature compounds this: a hot hose accelerates set, so summer runs often need a higher grade or dose than winter ones, and the schedule is written against the hottest part of the day rather than the mean.

Grade, form and metering

Pumped systems generally use a liquid amino-acid retarder metered at the plant or the pump, because it blends into the slurry without dust and tracks the output. The grade is chosen for a stable flow window rather than the longest possible set, since over-retardation softens early strength and slows the walk-on time the crew needs. Confirm that the retarder is compatible with the superplasticiser that supplies the flow, because the two share the rheology and have to be balanced as a pair.

Keeping the dose consistent

Consistency comes from metering the retarder against the output and the slurry temperature, not from a fixed scoop. Log the dose with the run length, the hose temperature and the ambient, because any of those can move the result between pours. A small set-time check on the first bay of each shift catches a drift before a full floor is laid, and that habit matters more in pumped work than in hand mixing, where the operator feels the stiffening directly.

Questions to put to a supplier

Ask for the flow and set response at the residence time and temperature of your runs, and for the compatibility data with your superplasticiser. Ask how the grade behaves after freeze-thaw if it arrives as a liquid in winter, and request a residue and heavy-metal profile that suits a floor coating later. A supplier who can run a pumped simulation, not only a bench flow test, gives you a result you can qualify with confidence.

Running a pumped trial

Trial the full loop, not a bucket. Pump a short run at two retarder doses across the day's temperature range, then measure flow, open time and walking time at the nozzle and inspect the surface for air and cold joints. Compare against your longest realistic run so the worst case still levels. Record the hose length, the lay rate and the ambient with the dose, because the combination is what decides whether the next pour repeats.

Frequently asked questions

Does the retarder stop the mix setting in the hose? It holds the flow window through the line residence time, but the grade and dose are matched to the run length and hose temperature, not guessed.

Can I use a bagged-compound retarder in a pump? Many can be adapted, but pumped work needs a grade tuned for transit as well as placement, so confirm the flow stability after residence time.

What stalls a pump? Premature stiffening in the line raises pressure and cuts output; the retarder and the superplasticiser are balanced to keep rheology stable through the hose.

Does hotter weather need more retarder? Usually yes, because a warm hose accelerates set; the dose is set against the hottest part of the run.

Yuanlian Chemical supplies an amino-acid based gypsum retarder for pumped self-levelling and screed systems. Request a sample and specification through [email protected] or call +86-537-3739818. Product details are at https://www.yuanlianchem.com.