What are the functions and requirements of various materials in gypsum-based self-leveling mortar?

What are the functions and requirements of various materials in gypsum-based self-leveling mortar?

Water-retaining agent: Self-leveling materials are constructed on the ground base, and the construction thickness is relatively thin, and the water is easily absorbed by the ground base, resulting in insufficient hydration of the material, cracks on the surface, and reduced strength. Hydroxypropyl methylcellulose is widly used as the water-retaining agent. Hydroxypropyl methylcellulose has good wetting, water-retaining and film-forming properties, so that the self-leveling material does not bleed and is fully hydrated.

Redispersible polymer powder: redispersible polymer powder can improve the elastic modulus of self-leveling materials, improve crack resistance, bond strength and water resistance.

Active admixtures: self-leveling materials can use fly ash, slag powder, etc. as active admixtures, the purpose is to improve the particle gradation of the material and improve the performance of the hardened body of the material. The slag powder undergoes a hydration reaction in an alkaline environment, which can improve the compactness and later strength of the material structure.

Early-strength cementitious materials: In order to ensure the construction time, self-leveling materials have certain requirements for early strength (mainly 24h flexural and compressive strength). Sulfoaluminate cement is used as the early-strength cementitious material. The sulfoaluminate cement has fast hydration speed and high early strength, which can meet the requirements of early strength of materials.

Alkaline activator: The gypsum composite cementitious material has the highest absolute dry strength under moderately alkaline conditions, and the pH value can be adjusted with quicklime and 32.5 cement to provide an alkaline environment for the hydration of the cementitious material.

Coagulant: coagulation time is an important performance index of self-leveling materials, and it is not conducive to construction if the time is too short or too long. The coagulant stimulates the activity of gypsum, accelerates the supersaturated crystallization speed of dihydrate gypsum, shortens the setting time, and keeps the setting and hardening time of the self-leveling material within a reasonable range.

Water reducing agent: In order to improve the compactness and strength of the self-leveling material, it is necessary to reduce the water-binder ratio. Under the condition of maintaining the good fluidity of the self-leveling material, it is necessary to add a water reducing agent. The naphthalene-based water-reducing agent is used, and its water-reducing mechanism is that the sulfonate and water molecules in the naphthalene-based water-reducing agent molecules associate with hydrogen bonds, forming a stable water film on the surface of the cementitious material, which makes it easy to generate between the material particles. Sliding, thereby reducing the required amount of mixing water and improving the structure of the hardened body of the material.