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The same formulation works fine in winter but fails in summer — this is a real experience for many dry-mix mortar manufacturers. Summer temperatures often exceed 35°C, and substrate surfaces under direct sunlight can reach over 50°C. The water evaporation rate far exceeds the cement hydration rate, and the mortar dries out before it even gets on the wall.

Formulations must adapt to the season, and the core of the adjustment is cellulose ether.

Two key reasons:

• Water retention drops — The gel temperature of ordinary HPMC is generally around 60°C-65°C, but on high-temperature summer substrates, the mortar surface temperature can easily exceed 50°C or even higher, causing a sharp decline in cellulose ether water retention. The higher the temperature, the poorer the water retention.

• Thermal dilution effect — Mortar viscosity naturally decreases at high temperatures. At the same dosage, summer mortar is simply “thinner” than winter mortar, and open time shortens dramatically.

The consequences: mortar dries too fast, workers can’t keep up, insufficient strength development, cracking, and hollowing follow.

Adjustment 1: Increase Dosage

In high-temperature summer environments, water evaporates quickly, and HPMC dosage needs to be increased to compensate for water retention loss. The industry generally recommends increasing by 0.1%-0.2%. In tile adhesives, total dosage can be increased by 0.02%-0.05% to maintain water retention above 93% under high-temperature conditions.

Caution: Dosage cannot be increased indefinitely. When cellulose ether dosage exceeds 0.3%, the water retention trend plateaus. Excessive dosage increases water demand, makes the mortar sticky and difficult to spread.

Adjustment 2: Switch Grades

Not all HPMC is heat-resistant. In summer, choose cellulose ether grades with a high gel temperature (＞75°C) , or use a combination of modified HEMC and HPMC. The higher the degree of etherification, the better the high-temperature water retention. High-viscosity HPMC (30,000-100,000 mPa·s) offers stronger water retention and is suitable for high-temperature dry environments.

Adjustment 3: Combine with Retarders

Increasing HPMC dosage alone is not enough in summer — retarders are also needed. High temperatures accelerate cement hydration, and while HPMC can slow hydration, its effect is limited. Combining with sodium gluconate or tartaric acid more effectively controls open time.

• HPMC dosage: Increase by 0.1%-0.2% over normal-temperature formulations (general plastering mortar total dosage is typically 0.2%-0.4%)

• HPMC grade: Choose high gel temperature (＞75°C) or HEMC-blended types

• Retarder: Combine with sodium gluconate or tartaric acid for precise open time control

• Superplasticizer: Add a small amount of polycarboxylate superplasticizer to reduce water demand and maintain flowability at low water-cement ratios

When adjusting summer formulations, cellulose ether is the top priority. Remember three rules:

• Increase dosage in summer — raise by 0.1%-0.2% .

• Switch grades in summer — choose high gel temperature (＞75°C) types.

• Combine in summer — HPMC + retarder + superplasticizer work best as a team.

Don’t wait for problems to show up on site. Adjust the formulation in advance, and summer construction will be worry-free.