Self-Levelling Underlayments with CSA Cement: Formulation Tips for Pumpable, Crack-Free Floors

Self-levelling underlayments (SLUs) have become the flagship application of calcium sulfoaluminate (CSA) cement technology. Walk onto any large flooring project and the grey flowable compound being pumped across the slab is very likely CSA-based. The reasons are structural: SLU products live or die by the combination of very high flow and very low shrinkage — two properties CSA binders deliver better than any other cement family.

Why CSA Is the Natural Binder for SLU

A self-levelling compound must satisfy a paradoxical set of demands:

• Flow freely and self-smooth under its own weight, without segregation
• Set and harden quickly enough to accept flooring within hours or one day
• Shrink so little that a thin (3–10 mm) layer bonded to a large slab does not crack or delaminate

Portland-based SLUs need substantial drying time before floor covering; gypsum-based ones cannot be used in wet areas and lack strength. CSA binders solve both problems: ettringite-dominated hydration provides early strength with minimal drying shrinkage, and formulated expansive versions can achieve near-zero shrinkage. This is why our self-levelling application programme is built around CSA technology.

The Formulation Stack

A typical CSA-based SLU contains six functional components. Understanding each one's role is what separates a stable, pumpable formulation from a sticky, bleeding mess.

1. Binder system

Most commercial SLUs use a binary blend of CSA cement and Portland cement, commonly in the range of 40–60% CSA + 40–60% OPC, plus anhydrite or hemihydrate gypsum to balance the sulfate supply. The blend controls set time and strength development: more CSA means faster strength gain; more OPC improves long-term surface hardness and economy. For premium fast-track products (foot traffic in 2–3 hours), higher CSA contents or grades such as our CSA Cement 62.5/72.5 are used.

2. Superplasticizer

Polycarboxylate ether (PCE) superplasticizers at 0.4–0.9% of binder weight create the flowing behaviour. Two cautions:

• PCE compatibility varies between CSA sources — verify slump-flow retention over 30 minutes at your actual water dosage.
• Overdosing PCE causes bleeding and surface fines segregation, which shows up later as weak, dusty patches.

3. Aggregates and fillers

Graded silica sand (typically 0.1–0.8 mm) at 30–45% of the dry mix provides dimensional stability. Limestone filler or fine calcium carbonate rounds out particle packing and reduces binder demand. The goal is a maximum packing density with enough fine material to keep the paste stable during flow.

4. Set and strength control

Citric acid (0.03–0.15%) or commercial retarders adjust open time; accelerators such as lithium carbonate push set earlier for ultra-fast products. In CSA/OPC blends, the sulfate source (anhydrite vs. gypsum) strongly influences both set and expansion — this is a tuning variable, not just a raw material.

5. Air and defoamer control

Entrained air bubbles ruin the mirror-like self-smoothed surface. A defoamer (0.05–0.2%) is standard; some formulators also add a small air-entraining dose in exterior or freeze-exposed grades.

6. Polymer additives

Redispersible polymer powders (1–3%) improve adhesion to the substrate, flexural toughness and abrasion resistance of the hardened layer.

Application Practice: Where Failures Actually Happen

Formulation is half the story; most SLU failures on site trace back to application errors:

• Substrate preparation: the slab must be sound, vacuum-cleaned and primed with the specified primer. Contaminated or unprimed concrete is the leading cause of debonding.
• Water discipline: the mixing water is calibrated to the formulation. Adding "a little extra for flow" destroys viscosity balance and causes segregation — use the specified ratio and achieve extra flow with the formulation's own plasticizer, not water.
• Mixing: use a high-shear mixer for the specified 2–3 minutes; SLU chemistry is designed for full dispersion.
• Pour and spread: pour in strips, spread with a pin rake or squeegee, then release air with a spiked roller. Work continuously — cold joints between batches are visible and permanent.
• Cure and cover: protect from strong drafts and direct sun during the first hours; CSA layers gain strength quickly but thin sections dry from the surface down.

Typical Performance Targets

For projects needing shrinkage compensation in thicker pours, our CSA expansive additive can be integrated into the blend.

Getting Started

Whether you are formulating a commercial SLU product or specifying one for a project, the fastest route to a reliable result is a systematic trial matrix: three binder ratios × two plasticizer doses × your substrate temperature. Our technical team provides grade selection, reference formulations and trial quantities — contact Oreworld to start.