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Pinholes in self-leveling mortar appear as small open pores after application or curing. They reduce surface smoothness and may indicate that excessive air remains in the fresh mortar.

The first reaction is often to add more defoamer. However, air may also be drawn into the mortar during mixing, released from a porous substrate, or trapped because the mortar loses flow and healing ability too quickly.

The key question is therefore:

Where does the air come from, and why can it not escape before the surface closes?

Pinholes should be treated as a root-cause problem rather than simply a defoamer-dosage problem. SidleyChem’s previous dry powder defoamer article also identifies slowly eliminated bubbles as a cause of pinholes and honeycomb-like surfaces in flowable mortar.

Self-leveling mortar usually contains a polycarboxylate superplasticizer, redispersible polymer powder, a suitable cellulose ether for self-leveling compounds, setting regulators, and other functional additives.

The performance of powder defoamer in dry-mix mortar depends on how it interacts with this complete additive package. A successful formulation must balance flow, stability, deaeration, setting time, and self-healing rather than maximizing only one property.

Defoamer-related pinholes may occur when:

• the dosage is too low;
• the defoamer acts too slowly;
• fine microbubbles remain in the slurry;
• the defoamer is incompatible with the PCE or polymer powder;
• the powder is not evenly distributed in the dry mix.

More defoamer is not automatically better. Excessive or incompatible defoamer may produce oily marks, uneven surface defects, or changes in mortar flow. The correct target is the minimum effective dosage that controls harmful air without negatively affecting the other properties of the mortar.

A suitable formulation can still develop pinholes if the mixing process draws too much air into the slurry.

Common causes include:

• an unsuitable mixing paddle;
• an oversized container;
• a small batch volume;
• excessive mixing time;
• operating the paddle too close to the surface;
• creating a strong air-drawing vortex;
• adding extra water after mixing.

The mixer, paddle, speed, batch size, water dosage, and mixing time should be standardized before the formulation is changed.

Commercial self-leveling underlayment instructions commonly require the paddle to remain immersed to avoid entraining excess air. They also warn against adding extra water after the defined mixing procedure.

Change only one mixing condition in each comparison test. If longer mixing produces more pinholes while the formulation and substrate remain unchanged, mixing-induced air is the most likely direction to investigate.

If the slurry looks relatively bubble-free in the mixing container but develops pinholes after it is poured onto concrete, the substrate may be releasing air.

Porous concrete contains air within its pore structure. When fresh self-leveling mortar covers the surface, this air can move upward through the wet layer and form bubbles.

At the same time, a highly absorbent substrate may remove water from the bottom of the mortar. The material can then lose the flow and self-healing ability needed to close the holes left by escaping bubbles.

A suitable primer helps:

• seal the pores of the substrate;
• reduce substrate outgassing;
• limit water loss from the fresh mortar;
• improve adhesion between the mortar and substrate.

Official primer documentation for self-leveling underlayments specifically identifies reduced outgassing, reduced bubble formation, and reduced water loss as important functions of the primer. It also notes that highly porous substrates may require additional coats, while primer puddles should be avoided.

Substrate-related pinholes are more likely when:

• concrete has pinholes but a sealed test plate does not;
• defects are concentrated in highly absorbent areas;
• primer coverage is incomplete or uneven;
• the primer is absorbed almost immediately;
• only certain parts of the floor are affected.

Good initial flow does not guarantee a smooth final surface.

If the mortar begins to stiffen before the bubbles rise and break, the openings cannot close. The material may pass an initial flow test but still have insufficient working or healing time.

Possible causes include:

• excessively rapid setting;
• high material or application temperature;
• excessive substrate absorption;
• an unbalanced accelerator-retarder system;
• excessive stabilizer or cellulose ether dosage;
• poor compatibility between the binder and PCE.

Initial flow, flow retention, healing time, setting behavior, and hardened performance should therefore be evaluated together.

ASTM C1708/C1708M includes comparative tests for initial flow, flow retention, healing time, setting time, compressive strength, and flexural strength in hydraulic-cement-based self-leveling mortars.

A practical way to separate these possible causes is to use one mortar batch and apply it at the same thickness onto three different surfaces:

1. a sealed, non-absorbent plate;
2. a correctly primed concrete panel;
3. an unprimed porous concrete panel used only for comparison.

Keep the following conditions unchanged:

• water-to-dry-mix ratio;
• mixing equipment;
• mixing time;
• material temperature;
• application thickness;
• observation time.

How to Interpret the Results

This comparison does not replace complete performance testing, but it can quickly separate substrate-related defects from formulation and mixing problems.

Pinholes in self-leveling mortar are not always caused by insufficient defoamer.

The root cause may be:

• overly stable foam;
• excessive air introduced during mixing;
• substrate outgassing;
• poor primer coverage;
• insufficient flow retention and healing time.

The fastest troubleshooting method is to standardize the mixing process, compare sealed and porous substrates, and change only one variable at a time.

A smooth self-leveling surface requires a balance of effective defoaming, controlled mixing, correct substrate priming, stable flow, and enough time for bubbles to escape before the surface closes.