In dry-mix mortar production, air content is sometimes overlooked until a formulation suddenly behaves differently.

A manufacturer may notice that the fresh mortar:

• Looks unusually fluffy

• Feels lighter than before

• Contains many visible bubbles

• Shows a lower wet density

• Becomes more difficult to finish

• Produces pinholes or small voids after application

• Requires a different amount of defoamer

When this happens, HPMC is often suspected first.

That is understandable because hydroxypropyl methylcellulose can influence rheology, water retention and stabilization of the aqueous phase.

However, excessive air in mortar is rarely controlled by HPMC alone.

The final air content may be influenced by:

HPMC + mixing energy + water dosage + sand grading + RDP + starch ether + surfactant-like components + defoamer + mixer design.

Therefore, the more useful question is not simply:

“Does HPMC create bubbles?”

but:

“Why is this mortar system entraining or retaining more air than expected?”

1.Why Is Air Present in Fresh Mortar？

Air can enter mortar during:

• Dry blending

• Water addition

• Mechanical mixing

• Pumping

• Manual stirring

Some air escapes quickly.

Some remains trapped inside the mortar.

The amount that remains depends on:

• Mortar viscosity

• Surface tension

• Bubble size

• Mixing intensity

• Stabilizing additives

In a highly viscous system, small bubbles may be more difficult to release.

That is one reason cellulose-ether-modified mortar can show different air behavior from a simple cement–sand mixture.

2. Does HPMC Directly Cause Air Entrainment?

HPMC can contribute to the stabilization of air introduced during mixing.

This does not mean that HPMC alone is always the source of air.

The practical effect depends on:

• HPMC type

• Dosage

• Viscosity

• Degree of modification

• Mixing conditions

• Other formulation ingredients

In some mortar systems, HPMC helps stabilize small air bubbles that would otherwise escape more quickly.

As a result, measured air content may increase.

But the magnitude of the change should be evaluated in the actual formulation.

3. Why Does HPMC Help Stabilize Bubbles?

Once HPMC hydrates, it increases the viscosity of the aqueous phase.

A more viscous liquid phase can slow:

• Bubble movement

• Bubble coalescence

• Bubble escape

Some cellulose ether systems may also influence interfaces within the mortar.

The result can be a more stable population of fine air bubbles.

This is why two mortars with similar flow can still have different:

• Wet density

• Air content

• Internal structure

4. Why Is Excessive Air a Problem?

A certain amount of entrained air is not automatically undesirable.

In some formulations, controlled air can improve:

• Workability

• Spreadability

• Application feel

However, if air becomes excessive, possible effects may include:

• Lower wet density

• Reduced compactness

• Lower strength in some systems

• Surface pinholes

• Increased porosity

• Changes in water absorption

• Different adhesive behavior

The acceptable air level depends on the specific mortar product.

Therefore, the objective is not necessarily “zero air,” but a controlled and repeatable air content.

5. Why Does HPMC Dosage Affect Air Content?

When HPMC dosage increases, the mortar may become:

• More viscous

• More cohesive

• More capable of stabilizing small bubbles

This can make air release more difficult.

If a formulation already contains sufficient HPMC, further increasing dosage may cause:

• Higher apparent viscosity

• Lower wet density

• More retained air

Therefore, when troubleshooting excessive air, the HPMC dosage should be checked rather than assuming that more cellulose ether will improve every property.

6. Does Higher HPMC Viscosity Always Mean More Air?

Not necessarily.

Nominal viscosity is only one characteristic.

The final air content can also depend on:

• Molecular structure

• Product modification

• Dissolution behavior

• Mortar composition

• Mixing process

Two HPMC grades with the same nominal viscosity may still produce different air content in the same mortar formulation.

That is one reason why supplier replacement should include an application test rather than only a viscosity comparison.

7. Why Can Two “100,000 mPa·s” HPMC Samples Behave Differently?

The label “100,000 mPa·s” does not fully describe a cellulose ether.

Two samples may differ in:

• Methoxy substitution

• Hydroxypropoxy substitution

• Particle size

• Surface treatment

• Gel temperature

• Modification technology

• Air-stabilizing behavior

Therefore, equal viscosity does not guarantee equal:

• Water retention

• Workability

• Open time

• Air content

For dry-mix mortar manufacturers, the finished mortar remains the more relevant comparison system.

8. Why Is Mixing Time Important?

Longer or more intensive mixing can introduce more air.

For example, a high-speed laboratory mixer may incorporate more air than a lower-energy production mixer.

If mixing continues after the mortar has already reached a uniform state, additional bubbles may be generated and stabilized.

This means a laboratory formulation can appear unusually aerated simply because:

• Mixing speed is too high

• Mixing time is too long

When comparing HPMC samples, mixing conditions should therefore remain identical.

9. Why Can Laboratory and Factory Air Content Be Different?

Laboratory equipment and industrial mixers differ in:

• Mixer geometry

• Shear rate

• Batch size

• Mixing time

• Filling level

A small laboratory mixer may create strong local turbulence.

A large dry-mix or wet-mortar mixer may behave differently.

Therefore, a laboratory air-content result should be treated as a screening result.

Production-scale confirmation may still be necessary.

10. Why Does Water Addition Affect Bubble Formation?

Water content strongly influences mortar rheology.

If water dosage is too low:

• Mortar may become very viscous

• Bubbles may have difficulty escaping

If water dosage is too high:

• Mortar may become unstable

• Bubble distribution may also change

Therefore, when comparing HPMC samples, the water dosage should be controlled carefully.

A small change in water can make one mortar appear “more aerated” even when the HPMC itself has not changed.

11. Why Should Water Be Added the Same Way During Testing?

The method of water addition can affect how quickly the dry materials wet and how much air is incorporated.

For example:

• Adding all water immediately

• Adding water gradually

• Adding powder into water

may produce different mixing behavior.

For comparative testing, it is useful to keep:

• Water temperature

• Water dosage

• Addition sequence

• Mixing time

consistent.

Otherwise, differences may be incorrectly attributed to the HPMC sample.

12. Can Sand Grading Affect Air Content?

Yes.

Sand grading influences the void structure of mortar.

If the sand becomes finer, the formulation may require more paste or water.

This can alter:

• Viscosity

• Bubble stability

• Wet density

A change in sand source may therefore change measured air content even if the HPMC remains unchanged.

13. Why Does Very Fine Sand Sometimes Make Mortar Feel “Fluffy”?

Fine particles increase total surface area.

A higher fine-particle content can make the mortar:

• More cohesive

• More viscous

• More resistant to bubble escape

The fresh mortar may then feel:

• Softer

• Lighter

• More aerated

This is another reason why HPMC should not be blamed immediately when mortar density changes.

14. Does Cement Type Influence Air Retention?

It can.

Different cements may differ in:

• Fineness

• Mineral composition

• Sulfate system

• Supplementary materials

These differences can change:

• Water demand

• Paste viscosity

• Interaction with additives

Therefore, changing cement source may indirectly change the air behavior of an HPMC-containing mortar.

15. Can RDP Affect Air Content?

Redispersible polymer powder can change the fresh and hardened behavior of mortar.

Depending on the product and formulation, it may affect:

• Rheology

• Surface properties

• Bubble stability

If RDP dosage or grade changes, the measured air content may also change.

For this reason, when testing a new HPMC sample, the RDP should remain unchanged.

16. Why Should RDP and HPMC Not Be Changed at the Same Time?

If both raw materials are replaced simultaneously and the mortar suddenly becomes more aerated, it becomes difficult to identify the cause.

A clearer test sequence is:

keep RDP constant → change HPMC → compare.

Then, if required:

keep HPMC constant → change RDP → compare.

This one-variable approach produces more useful formulation data.

17. Can Starch Ether Influence Air and Rheology?

Yes.

Starch ether is used in some dry-mix mortar formulations to adjust:

• Sag resistance

• Tack

• Application rheology

Because it changes rheology, it may also indirectly influence bubble release.

If starch ether dosage becomes too high, mortar may feel heavier or more structured.

This can make small bubbles more difficult to escape.

So excessive air should be investigated across the full additive package.

18. What Is the Role of Defoamer?

Defoamer is used in some mortar formulations to help control unwanted air.

Its effectiveness depends on:

• Defoamer chemistry

• Dosage

• Powder form

• Compatibility with HPMC

• RDP

• Cement system

A suitable defoamer may reduce:

• Large bubbles

• Persistent foam

• Excess air content

But it should be selected through testing.

19. Does More Defoamer Always Mean Less Air?

Not necessarily.

Adding too much defoamer may create other formulation issues.

Depending on the system, excessive defoamer may influence:

• Surface appearance

• Wetting

• Dispersion

• Local defects

Therefore, defoamer dosage should be optimized rather than continuously increased.

A dosage gradient is often more informative.

20. How Should a Defoamer Gradient Be Tested?

Keep the mortar formulation unchanged.

Prepare several samples with:

• Lower defoamer dosage

• Current dosage

• Higher dosage

Then compare:

• Wet density

• Air content

• Flow

• Application feel

• Surface pinholes

• Hardened appearance

This helps identify whether the defoamer level is actually responsible for the problem.

21. Why Is Wet Density Useful?

Wet density is a practical indirect indicator of air content.

If two mortar samples have similar formulation and water dosage but one shows much lower wet density, it may contain more entrained air.

For example, during supplier comparison, wet density can help identify whether one HPMC sample is stabilizing more air than another.

However, wet density should be interpreted together with:

• Flow

• Water content

• Raw-material density

22. Why Should Air Content Be Measured Directly Where Possible?

Visual observation is useful, but it is subjective.

One operator may describe mortar as “fluffy,” while another may consider it normal.

A more controlled comparison can include:

• Wet density

• Air-content measurement

• Flow test

• Timed observation

This creates numerical data instead of relying only on hand feel.

23. Why Do Small Bubbles and Large Bubbles Matter Differently?

Bubble size distribution can influence mortar behavior.

Large bubbles are easier to observe and may create:

• Surface voids

• Visible pinholes

Fine bubbles may be less visible but still reduce density.

A formulation can therefore look smooth while still containing relatively high air content.

This is another reason why appearance alone does not fully describe the system.

24. Why Can Mortar Show More Pinholes After Application?

Pinholes may result from air moving toward the surface during:

• Troweling

• Smoothing

• Drying

The problem may be influenced by:

• Air content

• Mortar viscosity

• Substrate absorption

• Application thickness

• Finishing method

Therefore, surface pinholes should not automatically be blamed on HPMC.

A formulation and application review is more useful.

25. Why Is Tile Adhesive Sensitive to Air Content?

Tile adhesive requires a balance of:

• Water retention

• Workability

• Open time

• Sag resistance

• Adhesive performance

Excessive air may alter:

• Contact area

• Mortar density

• Troweling behavior

Therefore, air content is one of the parameters worth checking when an HPMC change produces different tile-adhesive behavior.

26. Why Is Skim Coat or Putty Also Sensitive?

Thin-layer products can make bubbles more visible.

During application, trapped air may appear as:

• Pinholes

• Small craters

• Uneven surface texture

Because these products are applied thinly, surface appearance may become more sensitive to bubble stability.

HPMC selection should therefore consider not only viscosity but also the finished application behavior.

27. Why Can Surface-Treated HPMC Behave Differently?

Surface-treated HPMC is designed to delay hydration under certain conditions, helping dispersion in cold water or specific processing systems.

Because hydration timing changes, the early mixing process may also differ.

This can influence:

• Viscosity development

• Bubble release

• Mixing response

Therefore, surface-treated and non-surface-treated HPMC should not be assumed to behave identically in mortar.

28. Can HPMC Dissolution Rate Affect Air Retention?

Yes.

If viscosity develops very quickly during mixing, air introduced early in the process may become trapped before it can escape.

If viscosity builds more gradually, some air may escape before the mortar reaches its final structure.

This is one reason why hydration behavior can matter in addition to final viscosity.

29. Why Can Storage of Dry Mix Affect the Result?

Dry-mix mortar is a multi-component powder.

During storage, factors such as:

• Moisture

• Segregation

• Additive distribution

may affect the condition of the final mixed mortar.

If a bag absorbs moisture, some additives may not disperse exactly as before.

Therefore, when unusual air behavior appears, it may be useful to compare:

• Fresh production

• Stored material

under the same test conditions.

30. How Should a New HPMC Supplier Be Compared?

Keep the base formulation fixed.

Use the same:

• Cement

• Sand

• RDP

• Starch ether

• Defoamer

• Water

• Mixing procedure

Then test:

Current HPMC

versus

New HPMC sample.

Measure:

• Flow

• Wet density

• Air content

• Water retention

• Open time where relevant

• Application feel

This makes the comparison more meaningful.

31. Why Should the First Trial Use the Same Dosage?

If the dosage changes during the first comparison, it becomes difficult to know whether the result comes from:

• Product difference

• Dosage difference

Therefore, the first screening is usually clearer when the samples are tested at the same dosage.

After that, dosage optimization can be performed separately.

32. What If the New HPMC Produces More Air at the Same Dosage?

Possible next steps include:

1. Confirm mixing conditions.

2. Repeat the test.

3. Measure wet density.

4. Check whether water demand differs.

5. Test a small dosage adjustment.

6. Evaluate defoamer response.

7. Confirm water retention and application behavior.

This helps determine whether the new product can be adapted to the formulation or whether another grade is more suitable.

33. Why Should Defoamer Not Be Adjusted Immediately?

If a new HPMC sample produces more air, immediately increasing defoamer may hide the original difference.

For technical evaluation, it is often more useful to first establish:

how the HPMC behaves in the existing formulation.

After the baseline is understood, the formulation can be optimized.

This prevents unnecessary changes to multiple raw materials.

34. What Should Be Recorded During a Mortar Trial?

Useful records may include:

• HPMC grade

• HPMC dosage

• Water dosage

• Mixing speed

• Mixing time

• Wet density

• Flow

• Air content

• Temperature

• Application observations

For tile adhesive, additional observations may include:

• Open time

• Sag

• Troweling behavior

Consistent records make supplier comparison easier.

35. Why Is Temperature Worth Recording?

Temperature can change:

• HPMC hydration

• Mortar viscosity

• Water evaporation

• Workability

Two tests performed at very different temperatures may not be directly comparable.

Therefore, laboratory comparison should ideally use similar temperature conditions.

36. Can the Mixer Itself Cause a Sudden Air-Content Change?

Yes.

If production equipment changes, so can the amount of air introduced during mixing.

Possible changes include:

• New mixer

• Different blade design

• Higher speed

• Different batch size

• Longer mixing cycle

If air content changes after an equipment adjustment, the mixer should be investigated before changing the HPMC specification.

37. Why Does Batch Size Matter?

A mixer operating at very low filling level may create different shear and turbulence than the same mixer at normal production capacity.

This can alter:

• Powder distribution

• Air incorporation

• Mixing efficiency

For reliable formulation comparison, batch size should remain consistent.

38. How Can Excess Air Be Troubleshot Systematically?

A practical sequence is:

Step 1: Check mixing

Confirm speed, time and batch size.

Step 2: Check water

Confirm actual water dosage.

Step 3: Check HPMC

Confirm grade, dosage and batch.

Step 4: Check other additives

Review RDP, starch ether and defoamer.

Step 5: Check sand and cement

Look for recent raw-material changes.

Step 6: Measure wet density and air

Avoid relying only on appearance.

Step 7: Change one variable at a time

Use controlled trials.

39. What Should Buyers Tell an HPMC Supplier?

For a mortar with excessive air, useful information includes:

• Mortar application

• HPMC viscosity grade

• HPMC dosage

• Cement type

• Sand type

• RDP dosage

• Defoamer type and dosage

• Water dosage

• Mixing procedure

• Measured wet density

• Main observed problem

This is more useful than simply asking:

“Do you have an HPMC that does not make bubbles?”

40. Why Should HPMC Be Evaluated as Part of the Complete Mortar System?

HPMC is only one part of a dry-mix mortar formulation.

The final behavior results from interactions among:

cellulose ether + binder + aggregate + polymer powder + rheology modifiers + defoamer + water + mixing.

Therefore, a product that works well in one formulation may need adjustment in another.

The purpose of formulation testing is not to identify one “good” or “bad” raw material in isolation, but to determine whether the complete system meets the required application conditions.

Frequently Asked Questions

Does HPMC increase air content in mortar?

It can contribute to the stabilization of air introduced during mixing. The actual effect depends on HPMC type, dosage, viscosity, mixing conditions and other formulation components.

Is higher-viscosity HPMC more likely to create bubbles?

Not necessarily. Air content depends on more than nominal viscosity, including product structure, dosage and mortar formulation.

Can excessive HPMC dosage increase retained air?

In some mortar systems, a higher dosage can increase viscosity and make bubble release more difficult.

Can defoamer completely solve high air content?

Not always. The original cause may come from mixing, HPMC dosage, RDP, sand grading or other factors.

Why does mortar have lower wet density after changing HPMC?

The new HPMC may stabilize more air, but water dosage, mixing and raw-material changes should also be checked.

Should HPMC samples be compared at the same dosage?

For the first controlled comparison, using the same dosage is usually more useful. Dosage optimization can follow afterward.

Can mixing time change the apparent performance of HPMC?

Yes. Longer or higher-energy mixing may incorporate more air and change fresh-mortar behavior.

Conclusion

When HPMC-modified mortar contains more air than expected, the issue should not be diagnosed from the HPMC specification alone.

The result is usually influenced by several variables:

HPMC dosage + viscosity development + mixing energy + water + sand + RDP + starch ether + defoamer.

A more reliable troubleshooting method is:

fix the base formulation → control mixing → measure wet density and air content → compare HPMC samples → adjust one variable at a time.

This approach helps distinguish an HPMC-related difference from a broader formulation or production issue.

Shenyang Xingzhenghe Chemical Co., Ltd. supplies construction-grade HPMC for tile adhesive, dry-mix mortar and other cement-based formulations. Product specifications, samples and batch information can be provided according to customer requirements. Final grade and dosage should be confirmed through testing in the customer's actual mortar formulation.