“The self‑leveling floor is covered with loose dust – it wipes off with a touch.”
“Flow seems good, but all the aggregates sank to the bottom, leaving a thin paste on top.”

Do these scenarios sound familiar? Many formulators blame superplasticizers or defoamers. In reality, the real culprit is often the wrong viscosity combination of cellulose ether.

Self‑leveling mortars demand a delicate balance from cellulose ethers. They need sufficient water retention and thickening to prevent settlement – but without sacrificing flowability. Choose too high a viscosity, and flow suffers while air bubbles get trapped. Choose too low, and particles settle, leaving a dusty surface. It’s a precision game.

How Do Dust and Settlement Occur?

Surface dust and aggregate settlement are actually two sides of the same coin – instability of the paste system.

Settlement happens during the resting stage. Heavy aggregates sink under gravity, while lighter binders and fines float upward, creating a distinct layering. When viscosity is too low to suspend solid particles, settlement becomes inevitable.

Dust is the aftermath of settlement. The floating fines accumulate on the surface, and after water evaporates, they form a loose, low‑strength powder. This dust not only ruins aesthetics but also severely weakens the bond with subsequent floor coverings.

Viscosity Selection: A Low‑vs‑High Trade‑off

The viscosity requirement for self‑leveling mortars is radically different from ordinary mortars.

Low viscosity is the entry ticket. For highly flowable materials like self‑leveling mortar and self‑compacting concrete, cellulose ether viscosity must be very low. Low‑viscosity grades provide good suspension to prevent sedimentation without hindering flow. In fact, the typical HPMC viscosity for self‑leveling is only 300–500 mPa·s – a stark contrast to the tens‑of‑thousands used in standard mortars.

However, low viscosity is not a universal cure. Too low, and water retention suffers, suspension weakens, and bleeding and settlement quickly appear – especially when the dosage is insufficient. On the other hand, if viscosity is too high, flowability and self‑leveling are severely compromised. Studies clearly indicate that medium‑viscosity cellulose ethers are unsuitable for self‑leveling materials; low‑viscosity products are essential, and dosage should be kept below 0.2%.

Practical Viscosity Matching Strategies

Since a single viscosity grade can rarely meet all requirements, blending and precise dosage control become the key to success.

Core principle: low viscosity as the base, with strict dosage limits. The foundation of a self‑leveling formulation should be a low‑viscosity cellulose ether in the 400–600 mPa·s range, ensuring that flowability remains unaffected. Dosage is typically kept between 0.05% and 0.1%, and should not exceed 0.2%.

Advanced approach: synergistic blending for complementary benefits. A single cellulose ether often struggles to satisfy the competing demands of flow, water retention, and suspension. Research shows that when welan gum is combined with cellulose ether, paste stability improves dramatically. For instance, with a blend of 0.02% welan gum and 0.02% cellulose ether, water demand can increase from 32% to 36% without causing bleeding – the paste remains stable for 60 minutes with a density difference between top and bottom of less than 10 g/L. After hardening, the surface is free of dust, pinholes, and colour variation.

Alternative: HEMC offers inherent advantages. Hydroxyethyl methyl cellulose (HEMC) features lower air entrainment and higher density compared to standard HPMC. It delivers greater hardened density and surface strength – naturally suppressing dust formation – while effectively suspending solid particles to prevent segregation.

Formulation Tips and On‑Site Verification

– Base selection: For self‑leveling mortars, prioritise low‑viscosity HPMC or HEMC in the 400–600 mPa·s range.
– Dosage control: Start at 0.05%–0.1% and never exceed 0.2%.
– Blending option: If a single low‑viscosity cellulose ether fails to provide enough suspension, consider combining with welan gum (0.01%–0.02%) to expand the water‑demand window and inhibit bleeding.
– Simple field test: Pour the mixed paste into a transparent cup and let it rest overnight. If you see free water on top, bleeding is severe. A 1–2 mm layer of dust indicates mild bleeding but is acceptable. A smooth, even surface with no water or dust means the system is stable.

Conclusion

Surface dust and settlement in self‑leveling mortar may appear to be workmanship issues, but they are fundamentally the result of imbalanced viscosity selection. Too high a viscosity chokes flow; too low lets particles settle. Only by combining a low‑viscosity base, precise dosage control, and smart blending can you achieve a paste that flows freely yet stands firm – delivering a self‑leveling floor that is truly dust‑free, uniform, and durable.