What do you think of the quality of cellulose that determines the quality of mortar?
In ready-mixed mortars, the amount of cellulose ether added is very low. But it can significantly improve the performance of wet mortar, which is a major additive that affects the construction performance of mortar.
Reasonably select cellulose ethers of different varieties, different viscosities, different particle sizes, different degrees of viscosity and added amounts.
Water retention is an important property of methyl cellulose ether, and it is also a lot of dry powder mortar manufacturers in China. In particular, manufacturers in areas with higher temperatures in the south pay attention to performance.
Factors affecting the water retention effect of dry powder mortar include the amount of MC added, the viscosity of MC, the fineness of particles, and the temperature of the use environment.
First, the concept
Cellulose ether is a synthetic polymer made from natural cellulose as a raw material and chemically modified.
Its most basic material is cellulose, a natural polymer compound.After the swelling agent treatment, the strong hydrogen bonds between and within the molecular chains are destroyed.
The active release of the hydroxyl group becomes alkali cellulose with a reactive ability,and the cellulose ether is obtained by converting the -OH group to the -OR group through the etherification agent reaction.
The properties of cellulose ether depend on the kind, number and distribution of the substituents. The classification of cellulose ethers is also based on the types of substituents, the degree of etherification, dissolution performance and related application types.
According to the type of substituents on the molecular chain, it can be divid into monoethers and mixed ethers. We usually use MC as the monoether and HPMC as the mix ether.
From the dissolution performance, it can be divided into ionic and non-ionic.
Water-soluble non-ionic cellulose ether is mainly composed of two series of alkyl ethers and hydroxyalkyl ethers.
Ionic CMC is mainly used in synthetic detergent textile printing and dyeing food and petroleum extraction.
Non-ionic MC, HPMC, HEMC, etc. are mainly used in building materials, latex coatings, medicine, daily chemistry, etc.
Used as a thickener, water-retaining agent, stabilizer, dispersant, and film-forming agent.
Second,Water retention of cellulose ether
Water retention of cellulose ether: In the production of building materials, especially dry powder mortar, cellulose ether plays an irreplaceable role. Especially in the production of special mortar (modified mortar), it is an indispensable important component .
The important role of water-soluble cellulose ether in mortar is mainly three aspects,
The first is excellent water retention, the second is the effect on mortar consistency and thixotropy, and the third is the interaction with cement.
The water retention effect of cellulose ether depends on the water absorption of the base layer, the composition of the mortar, the layer thickness of the mortar, the water demand of the mortar, and the setting time of the setting material.
The water retention of cellulose ether itself comes from the solubility and dehydration of cellulose ether itself.
The larger the substituent, the greater the distance between the molecules. The greater the effect of breaking the hydrogen bond, the solution enters after the cellulose lattice expands.
Cellulose ether becomes water-soluble and forms a high-viscosity solution.
When the temperature rises, the hydration of the polymer weakens and the water between the chains is expelle.
When the dehydration effect is sufficient, the molecules begin to aggregate, forming a three-dimensional network structure gel and folding it out.
Factors affecting the water retention of the mortar include cellulose ether viscosity, added amount, particle fineness and use temperature.
Generally, the higher the viscosity, the better the water retention effect. However, the higher the viscosity, the higher the molecular weight of MC, and its solubility will decrease accordingly.
This has a negative impact on the strength and workability of the mortar. The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not a direct relationship. The higher the viscosity, the more sticky the wet mortar will be.
When it is apply, it is characterize by high adhesion to the substrate and scraper. But it does not help to increase the structural strength of the wet mortar itself.
During the construction, the anti-sag performance is not obvious.
In contrast, some low-viscosity but modified methyl cellulose ethers have excellent performance in improving the structural strength of wet mortar.
The greater the amount of cellulose ether added to the mortar, the better the water retention performance, the higher the viscosity, and the better the water retention performance.
For particle size, the finer the particles, the better the water retention. After the large particles of cellulose ether contact with water, the surface immediately dissolves and forms a gel to wrap the material to prevent water molecules from continuing to penetrate.
Sometimes even after a long time of stirring, no uniform dispersion and dissolution can be obtained, forming a cloudy flocculent solution or agglomeration.
It greatly affects the water retention of its cellulose ether, and solubility is one of the factors in selecting cellulose ether.
The coarser MC is not only wasteful, but also reduces the local strength of the mortar.
When such a dry powder mortar is applied in a large area, the curing speed of the local dry powder mortar is significantly reduced. Cracks due to different curing times occur.
For sprayed mortar using mechanical construction, the finer requirements are higher due to the shorter mixing time.
The fineness of MC also has a certain effect on its water retention. Generally speaking, for methyl cellulose ethers with the same viscosity but different fineness.
With the same addition amount, the finer the finer the water retention effect, the better.
Although methyl hydroxyethyl cellulose ether additive is currently consider to be at the forefront of technological development, its temperature dependence will still lead to weaken dry mortar performance.
Although the amount of methyl hydroxyethyl cellulose is increase (summer formula), the workability and crack resistance still cannot meet the needs of use.
Through some special treatments for MC, such as increasing the degree of etherification, it can keep its water retention effect better under higher temperature conditions, and make it provide better performance under severe conditions.
Third,Thickening and thixotropy of cellulose ether
Thickening and thixotropy of cellulose ether: the second effect of cellulose ether-thickening effect.
Depends on: polymerization degree of cellulose ether, solution concentration, shear rate, temperature and other conditions.
The gelation properties of the solution are unique to alkyl cellulose and its modified derivatives.
Gelation properties are related to the degree of substitution, solution concentration and additives. For hydroxyalkyl modified derivatives, the gel properties are also related to the degree of modification of the hydroxyalkyl group.
For solution and low viscosity MC and HPMC, 10% -15% concentration solution can prepare, and medium viscosity MC and HPMC can be prepared 5% -10% solution.
High viscosity MC and HPMC can only prepare 2% -3% solution, and usually the viscosity classification of cellulose ether is also classified by 1% -2% solution.
Relationship between consistency and modification: The addition of cellulose ether affects the water consumption of the mortar. Changing the water-cement ratio of water to cement is the thickening effect. The higher the amount, the more water is using.
Cellulose ether used in powdery building materials must be quickly dissolve in cold water and can provide the system with a suitable consistency. If given a certain shear rate, it still becomes flocculent and colloidal. This is an unqualify or poor quality product.
When the temperature is increase, a structural gel is forme and a high thixotropic flow occurs.
High-concentration, low-viscosity cellulose ethers exhibit thixotropy even below the gel temperature.
This property is very useful for the construction mortar to adjust its leveling and sagging.
It should be note here that the higher the viscosity of cellulose ether, the better the water retention, but the higher the viscosity.
The higher the relative molecular weight of cellulose ether, the lower its dissolution performance. This has a negative impact on the mortar concentration and workability.
The higher the viscosity, the more obvious the thickening effect on the mortar, but it is not completely proportional.
Some medium and low viscosities, but modified cellulose ethers have more excellent performance in improving the structural strength of wet mortar. As the viscosity increases, the water retention of cellulose ethers improves.
Fourth, the retardation of cellulose ether
The retardation of cellulose ether: The third role of cellulose ether is to delay the hydration process of cement.
Cellulose ether imparts various beneficial properties to the mortar, and will also reduce the early heat of hydration of cement. It will delay the hydration dynamic process of cement.
This is detrimental to the use of mortar in cold areas. This retardation is cause by the adsorption of cellulose ether molecules on hydration products such as C-S-H and ca (OH) 2.
As the viscosity of the pore solution increases, cellulose ether reduces the mobility of the ions in the solution, thereby delaying the hydration process.
With the increase of cellulose ether content, the setting time of the mortar increased significantly.
There is a good non-linear correlation between the initial setting time of the mortar and the amount of cellulose ether.
The final setting time has a good linear correlation with the amount of cellulose ether.
We can control the operation time of the mortar by changing the amount of cellulose ether.
In summary,
In the ready-mixed mortar, cellulose ether plays a role in water retention, thickening, delaying the hydration power of cement, and improving construction performance.
Good water-holding capacity makes the cement hydration more complete, which can improve the wet viscosity of wet mortar. It can improve the bond strength of the mortar and can adjust the time.
Adding cellulose ether to mechanical spraying mortar can improve the spraying or pumping performance of the mortar, as well as the structural strength. Therefore, cellulose ether is widely used as an important additive in ready-mixed mortar.