How important is the viscosity of methyl cellulose ether for gypsum mortar?

How important is the viscosity of methyl cellulose ether for gypsum mortar?

Answer: Viscosity is an important parameter for the performance of methyl cellulose ether.

Generally speaking, the higher the viscosity, the better the water retention effect of gypsum mortar. However, the higher the viscosity, the higher the molecular weight of methyl cellulose ether, and the corresponding decrease in its solubility will have a negative impact on the strength and construction performance of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not directly proportional. The higher the viscosity, the more viscous the wet mortar will be. During construction, it is manifested as sticking to the scraper and high adhesion to the substrate. But it is not helpful to increase the structural strength of the wet mortar itself. In addition, during construction, the anti-sag performance of wet mortar is not obvious. On the contrary, some medium and low viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.

How important is the fineness of cellulose ether to mortar?

Answer: The fineness is also an important performance index of methyl cellulose ether. The MC used for dry powder mortar is required to be powder with low water content, and the fineness also requires 20% to 60% of the particle size to be less than 63m. The fineness affects the solubility of methyl cellulose ether. Coarse MC is usually granular, which is easy to disperse and dissolve in water without agglomeration, but the dissolution rate is very slow, so it is not suitable for use in dry powder mortar. Some domestic products are flocculent, not easy to disperse and dissolve in water, and easy to agglomerate . In dry powder mortar, MC is dispersed among cementing materials such as aggregate, fine filler and cement, and only fine enough powder can avoid methyl cellulose ether agglomeration when mixing with water. When MC is added with water to dissolve the agglomerates, it is very difficult to disperse and dissolve. Coarse MC is not only wasteful, but also reduces the local strength of the mortar. When such dry powder mortar is applied in a large area, the curing speed of the local mortar will be significantly reduced, and cracks will appear due to different curing times. For the sprayed mortar with mechanical construction, the requirement for fineness is higher due to the shorter mixing time.

The fineness of MC also has a certain impact on its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness, under the same addition amount, the finer the finer the better the water retention effect.

What is the cellulose selection method?

Answer: The amount of cellulose ether used in different applications is mainly based on the need for water retention. Suitable for all kinds of mortar. Highly absorbent substrates require higher amounts of cellulose ether. Mortars with a uniform particle size distribution and thus greater surface area also require higher amounts of cellulose ether.

The modified specifications can be selected for anti-sagging requirements. If the modification is not enough, starch ethers, usually hydroxypropyl starch ethers, can be added to prevent sags.

The total amount and particle size of fillers in the formulation must be selected to provide smoothness and good consistency.

Mixing gypsum, filler, type and quantity of cellulose ether and how to use starch ether should be combined with the following methods:

When the dry-mixed mortar is added to a certain amount of water, the amount added is based on the amount of water, and all the water is soaked without stirring the dry powder with the correct water-to-paste ratio. If the different ingredients are mixed in the correct proportion, then we can get a smooth mortar with suitable application properties after mixing.

What are the modifications of building gypsum by water retaining agent?

Answer: Building wall materials are mostly porous structures, and they all have strong water absorption. However, the gypsum building material used for wall construction is prepared by adding water to the wall, and the water is easily absorbed by the wall, resulting in the lack of water necessary for hydration of the gypsum, resulting in difficulties in plastering construction and reduced bond strength, resulting in cracks, Quality problems such as hollowing and peeling. Improving the water retention of gypsum building materials can improve the construction quality and the bonding force with the wall. Therefore, water retaining agent has become one of the important admixtures of gypsum building materials.

The commonly used water retaining agents in my country are carboxymethyl cellulose and methyl cellulose. These two water retaining agents are ether derivatives of cellulose. They all have surface activity, and there are hydrophilic and hydrophobic groups in their molecules, which have emulsification, protective colloid and phase stability. Due to the high viscosity of its aqueous solution, when it is added to the mortar to maintain a high water content, it can effectively prevent excessive absorption of water by the substrate (such as bricks, concrete, etc.) and reduce the evaporation rate of water, thereby playing a role to the water retention effect. Methyl cellulose is an ideal admixture for gypsum which integrates water retention, thickening, strengthening and thickening, but the price is relatively high. Usually, a single water-retaining agent cannot achieve the ideal water-retaining effect, and the combination of different water-retaining agents can not only improve the use effect, but also reduce the cost of gypsum-based materials.

How does water retention affect the properties of gypsum composite cementitious materials?

Answer: The water retention rate increases with the addition of methyl cellulose ether in the range of 0.05% to 0.4%. When the addition amount further increased, the trend of increasing water retention slowed down.