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

(1) Gypsum

According to the raw materials used, it is divided into type II anhydrite and α-hemihydrate gypsum. The materials they use are:

① Type II anhydrous gypsum

The transparent gypsum or alabaster with high grade and soft texture should be selected. The calcination temperature is between 650 and 800°C, and the hydration is carried out under the action of an activator.

②-Gypsum hemihydrate

-The production technology of hemihydrate gypsum mainly includes dry conversion process and wet conversion process mainly integrating dehydration and drying.

(2) Cement

When preparing self-leveling gypsum, a small amount of cement can be added, and its main functions are:

①Provide an alkaline environment for certain admixtures;

② Improve the softening coefficient of gypsum hardened body;

③ Improve slurry fluidity;

④Adjust the setting time of type Ⅱ anhydrous gypsum self-leveling gypsum.

The cement used is 42.5R Portland cement. When preparing colored self-leveling gypsum, white Portland cement can be used. The amount of cement added is not allowed to exceed 15%.

(3) Setting time regulator

In self-leveling gypsum mortar, if type II anhydrous gypsum is used, a setting accelerator should be used, and if -hemihydrate gypsum is used, a setting retarder should generally be used.

① Coagulant: It is composed of various sulfates and their double salts, such as calcium sulfate, ammonium sulfate, potassium sulfate, sodium sulfate and various alums, such as alum (aluminum potassium sulfate), red alum (potassium dichromate) , bile alum (copper sulfate), etc.:

②Retarder:

Citric acid or trisodium citrate is a commonly used gypsum retarder. It is easily soluble in water, has obvious retarding effect and low price, but it will also reduce the strength of gypsum hardened body. Other gypsum retarders that can be used include: glue, casein glue, starch residue, tannic acid, tartaric acid, etc.

(4) Water reducing agent

The fluidity of self-leveling gypsum is a key issue. In order to obtain a gypsum slurry with good fluidity, increasing the water consumption alone will inevitably lead to a decrease in the strength of the gypsum hardened body, and even bleeding, which will make the surface soft, lose powder, and cannot be used. Therefore, gypsum water reducer must be introduced to increase the fluidity of gypsum slurry. The superplasticizers suitable for the preparation of self-leveling gypsum include naphthalene-based superplasticizers, polycarboxylate high-efficiency superplasticizers, etc.

(5) Water retaining agent

When the self-leveling gypsum slurry is self-leveling, the fluidity of the slurry is reduced due to the water absorption of the base. In order to obtain an ideal self-leveling gypsum slurry, in addition to its own fluidity to meet the requirements, the slurry must also have good water retention. And because the fineness and specific gravity of the gypsum and cement in the base material are quite different, the slurry is prone to delamination during the flow process and the static hardening process. In order to avoid the above phenomena, it is necessary to add a small amount of water retaining agent. Water-retaining agents generally use cellulose substances, such as methyl cellulose, hydroxyethyl cellulose and carboxypropyl cellulose.

(6) polymer

Improve abrasion, crack and water resistance of self-leveling materials using redispersible powdered polymers

(7) Defoamer To eliminate the air bubbles generated during the mixing process of materials, tributyl phosphate is generally used.

(8) filler

It is used to avoid the separation of self-leveling material components in order to have better fluidity. Fillers that can be used, such as dolomite, calcium carbonate, ground fly ash, ground water-quenched slag, fine sand, etc.

(9) Fine aggregate

The purpose of adding fine aggregate is to reduce the drying shrinkage of the self-leveling gypsum hardened body, increase the surface strength and wear resistance of the hardened body, and generally use quartz sand.

What are the material requirements for gypsum self-leveling mortar?

The β-type hemihydrate gypsum obtained by calcining first-grade dihydrate gypsum with a purity of more than 90% or the α-type hemihydrate gypsum obtained by autoclaving or hydrothermal synthesis.

Active admixture: 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 material hardened body. Slag powder undergoes 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). The sulphoaluminate cement is used as the early-strength cementing material. The sulphoaluminate cement has a fast hydration speed and high early strength, which can meet the requirements of the early strength of the material.

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

Coagulant: The setting time is an important performance index of self-leveling materials. Too short or too long time is not conducive to construction. 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 self-leveling materials within a reasonable range.

Water-reducing agent: In order to improve the compactness and strength of self-leveling materials, it is necessary to reduce the water-binder ratio. Under the condition of maintaining good fluidity of self-leveling materials, it is necessary to add water-reducing agents. The naphthalene-based water reducer is used, and its water-reducing mechanism is that the sulfonate group in the naphthalene-based water-reducer molecule and the water molecule are associated with hydrogen bonds, forming a stable water film on the surface of the gelled material, making it easy to produce water between the material particles. Sliding, thereby reducing the amount of mixing water required and improving the structure of the hardened body of the material.

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. In this test, methyl cellulose (MC) was selected as the water-retaining agent. MC has good wettability, water retention and film-forming properties, so that the self-leveling material does not bleed and is fully hydrated.

Redispersible latex powder (hereinafter referred to as latex powder): latex powder can increase the elastic modulus of self-leveling materials, improve crack resistance, bond strength and water resistance.

Defoamer: The defoamer can improve the apparent properties of the self-leveling material, reduce the bubbles when the material is formed, and have a certain effect on improving the strength of the material.