According to the two applicable standards for tile adhesives—GB/T 25181-2019 (Ready-mixed Mortar) and JC/T 547-2017 (Ceramic Tile Adhesives)—the tensile bond strength must remain ≥0.5 MPa after a specified open time:

  • F-type (Fast-setting) adhesives: ≥0.5 MPa after 10 minutes
  • E-type (Extended open time) adhesives: ≥0.5 MPa after 30 minutes
  • Standard adhesives: ≥0.5 MPa after 20 minutes

(Note: Open time refers to the maximum allowable interval between adhesive application on the substrate and tile installation while still achieving the required bond strength.)

Recommended Solution

While increasing the dosage of cellulose ether (HPMC) can extend open time, we generally do not recommend:

  1. High-viscosity HPMC (e.g., >100,000 mPa·s, NDJ method) – May cause surface film formation (“skinning”), impairing workability.
  2. Excessive HPMC content – Leads to delayed setting, negatively impacting other strength tests (e.g., 28-day strength).

Optimal Approach:

Use moderate-viscosity HPMC (40,000–100,000 mPa·s, NDJ) at an appropriate dosage to balance:

  • Extended open time (meeting E-type requirements)
  • No adverse effects on setting or final strength

(Technical basis: Empirical data from adhesive formulations compliant with EN 12004 and JC/T 547, where HPMC viscosity directly correlates with open time but must avoid over-retardation.)

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