There’s nothing quite like a screed that sets too fast, cracks, or just doesn’t hold up like it should. Yet, this kind of failure still catches construction teams off guard, often because the mix was missing a little extra help. That’s where screed additives and admixtures step in.
Used well, these materials can improve everything from how easy the mix is to pour to how quickly it sets to its service lifetime. Whether it’s a retail floor or a residential block, these tweaks can be the difference between a call-back and a completed job.
So, this guide will walk you through what they are, how they work, and why more project managers are factoring them in right from the start.
What is Screed?
Screed tends to fly under the radar until it goes wrong. But in truth, it plays a crucial role in how a finished floor looks, feels and performs. Put simply, screed is a layer of material – usually cementitious – that’s applied over a structural base like concrete. Its main job is to create a smooth, level surface for the final floor covering, whether that’s tile, wood, carpet or resin.
There are several variations on the traditional sand-cement screeds, such as flowing screeds and fast-drying variants, but they all serve the same purpose: to create a strong, even base that stands the test of time.
When laid correctly, a screed can help improve load distribution, reduce floor defects, and protect underlying materials. When it’s not, well – expect costly delays, failed finishes, and, in some cases, structural problems.
What are Admixtures and Additives?
Note on Terminology: In screed applications, the terms admixtures and additives are sometimes used interchangeably. Technically speaking, admixtures are usually liquid materials added during mixing, while additives are typically powdered substances added during batching.
In this article, we’ll mostly use admixtures to refer to performance-enhancing mix components, though some examples may fall under the broader “additives” category depending on form and usage.
Sometimes, the aim is to slow things down – maybe the site’s too hot, or the team needs more time to work the mix. Other times, the goal is to speed things up, boost strength, cut shrinkage or protect against weather extremes. Whatever the challenge, there’s usually a mix-in that can help.
Used correctly, these ingredients improve workability, consistency, setting time and final durability – without requiring big changes to the core materials. In short, they’re problem-solvers that often save time, money and hassle when used right.
Types of Admixtures and Additives for Screed
Not all screeds face the same challenges, so it makes sense that different types of additives and admixtures are used to tackle specific issues. Here’s a breakdown of some of the most common categories, each offering its own benefits depending on project needs.
Plasticisers and Superplasticisers
These are all about improving flow and reducing the water demand. Plasticisers help make screed more workable, so it spreads and compacts more easily. Superplasticisers go a step further, offering high fluidity or reducing the water content further, without weakening the mix.
They’re useful on projects where consistency and return to service times matter – large pours, underfloor heating, or complex surfaces. And because they cut down on water, the final screed tends to be stronger, denser, less prone to cracking, in addition to faster drying, which leads to rapid return to service
Retarders
Retarders slow down the setting process. This might sound like a step back, but it’s actually a big help on jobs where working time is tight or conditions are tricky.
For example, if you’re pouring screed in warm weather or over a large area, a slower set gives your team time to level everything properly before it starts to harden. The result? Fewer cold joints and a more consistent finish.
Accelerators
Accelerators do the opposite – they speed up the setting. They’re a handy option for tight schedules, overnight repairs, or anything that needs a quick turnaround.
By reducing the time it takes for screed to reach workable strength, they help avoid hold-ups in follow-on trades. Less downtime means smoother timelines – and often fewer complaints from clients chasing deadlines.
Air-Entraining Agents
These additives introduce tiny air bubbles into the mix. That might sound counterintuitive, but it actually makes screed more resistant to freeze-thaw cycles, which can otherwise cause cracking and damage. With the added benefit of an improvement in the workability and often a reduction in the water demand.
Ideal for outdoor or exposed floors, air-entraining agents help the screed handle temperature shifts and moisture better over time. It’s a small tweak that makes a big difference in performance.
Rapid Setting Cements
When time’s tight and deadlines can’t budge, rapid-setting cements can be a smart choice. These specialist dry powders replace or supplement the main binder in the screed mix, dramatically reducing set and drying times.
One example is Calumex XT-20 cement – a high-performance, fast-curing cement additive designed to deliver both speed and strength. Supplied as a dry powder, it’s added during batching and works by accelerating strength gain without compromising durability.
It’s especially useful where finishes need to go down fast – think tiles, vinyl or engineered wood – or where minimal disruption is key.
Why is it Important to Use Admixtures and Additives in Screed?
Getting the screed right isn’t just about looks – it affects the build long after the job’s done. Admixtures and additives play a major part in that. They help make the screed tougher, more reliable and far less prone to issues like shrinkage or cracking.
Used properly, these materials don’t just improve performance. They cut waste, raise standards and help deliver better outcomes across the board.
Getting the screed right isn’t just about what’s underfoot – it’s what keeps timelines, finishes and reputations on track. Additives and admixtures might be just a part of the mix, but they make all the difference when performance matters.
If you’re planning a project and want a screed that delivers more from the start, it’s worth talking to a supplier that understands the detail. Rakem Group offers technical support and proven products that do exactly what they need to – without the guesswork.
Want to Improve Performance with the Right Admixtures?
If you’re working on a screed formulation or investigating performance issues, our team at Cemkem can assist. We work closely with technical teams and labs to support mix optimisation, compatibility checks, and tailored admixture selection.
