The types of admixtures commonly used in building dry-mixed mortar, their performance characteristics, mechanism of action, and their influence on the performance of dry-mixed mortar products. The improvement effect of water-retaining agents such as cellulose ether and starch ether, redispersible latex powder and fiber materials on the performance of dry-mixed mortar was discussed emphatically.
Admixtures play a key role in improving the performance of building dry-mixed mortar, but the addition of dry-mixed mortar makes the material cost of dry-mixed mortar products significantly higher than that of traditional mortar, which accounts for more than 40% of the material cost in dry-mixed mortar. At present, a considerable part of the admixture is supplied by foreign manufacturers, and the reference dosage of the product is also provided by the supplier. As a result, the cost of dry-mixed mortar products remains high, and it is difficult to popularize ordinary masonry and plastering mortars with large quantities and wide areas; high-end market products are controlled by foreign companies, and dry-mixed mortar manufacturers have low profits and poor price tolerance; There is a lack of systematic and targeted research on the application of pharmaceuticals, and foreign formulas are blindly followed.
Based on the above reasons, this paper analyzes and compares some basic properties of commonly used admixtures, and on this basis, studies the performance of dry-mixed mortar products using admixtures.
1 water retaining agent
Water retaining agent is a key admixture to improve the water retention performance of dry-mixed mortar, and it is also one of the key admixtures to determine the cost of dry-mixed mortar materials.
1. Hydroxypropyl Methyl Cellulose Ether (HPMC)
Hydroxypropyl methylcellulose is a general term for a series of products formed by the reaction of alkali cellulose and etherifying agent under certain conditions. Alkali cellulose is replaced by different etherifying agents to obtain different cellulose ethers. According to the ionization properties of substituents, cellulose ethers can be divided into two categories: ionic (such as carboxymethyl cellulose) and non-ionic (such as methyl cellulose). According to the type of substituent, cellulose ether can be divided into monoether (such as methyl cellulose) and mixed ether (such as hydroxypropyl methyl cellulose). According to different solubility, it can be divided into water-soluble (such as hydroxyethyl cellulose) and organic solvent-soluble (such as ethyl cellulose), etc. Dry-mixed mortar is mainly water-soluble cellulose, and water-soluble cellulose is divided into instant type and surface treated delayed dissolution type.
The mechanism of action of cellulose ether in mortar is as follows:
(1) Hydroxypropyl methylcellulose is easily soluble in cold water, and it will encounter difficulties in dissolving in hot water. But its gelation temperature in hot water is significantly higher than that of methyl cellulose. The solubility in cold water is also greatly improved compared with methyl cellulose.
(2) The viscosity of hydroxypropyl methylcellulose is related to its molecular weight, and the larger the molecular weight, the higher the viscosity. Temperature also affects its viscosity, as temperature increases, viscosity decreases. However, its high viscosity has a lower temperature effect than methyl cellulose. Its solution is stable when stored at room temperature.
(3) The water retention of hydroxypropyl methylcellulose depends on its addition amount, viscosity, etc., and its water retention rate under the same addition amount is higher than that of methyl cellulose.
(4) Hydroxypropyl methylcellulose is stable to acid and alkali, and its aqueous solution is very stable in the range of pH=2~12. Caustic soda and lime water have little effect on its performance, but alkali can speed up its dissolution and increase its viscosity. Hydroxypropyl methylcellulose is stable to common salts, but when the concentration of salt solution is high, the viscosity of hydroxypropyl methylcellulose solution tends to increase.
(5) Hydroxypropyl methylcellulose can be mixed with water-soluble polymer compounds to form a uniform and higher viscosity solution. Such as polyvinyl alcohol, starch ether, vegetable gum, etc.
(6) Hydroxypropyl methylcellulose has better enzyme resistance than methylcellulose, and its solution is less likely to be degraded by enzymes than methylcellulose.
(7) The adhesion of hydroxypropyl methylcellulose to mortar construction is higher than that of methylcellulose.
2. Methylcellulose (MC)
After the refined cotton is treated with alkali, cellulose ether is produced through a series of reactions with methane chloride as etherification agent. Generally, the degree of substitution is 1.6~2.0, and the solubility is also different with different degrees of substitution. It belongs to non-ionic cellulose ether.
(1) Methylcellulose is soluble in cold water, and it will be difficult to dissolve in hot water. Its aqueous solution is very stable in the range of pH=3~12. It has good compatibility with starch, guar gum, etc. and many surfactants. When the temperature reaches the gelation temperature, gelation occurs.
(2) The water retention of methyl cellulose depends on its addition amount, viscosity, particle fineness and dissolution rate. Generally, if the addition amount is large, the fineness is small, and the viscosity is large, the water retention rate is high. Among them, the amount of addition has the greatest impact on the water retention rate, and the level of viscosity is not directly proportional to the level of water retention rate. The dissolution rate mainly depends on the degree of surface modification of cellulose particles and particle fineness. Among the above cellulose ethers, methyl cellulose and hydroxypropyl methyl cellulose have higher water retention rates.
(3) Changes in temperature will seriously affect the water retention rate of methyl cellulose. Generally, the higher the temperature, the worse the water retention. If the mortar temperature exceeds 40°C, the water retention of methyl cellulose will be significantly reduced, seriously affecting the construction of the mortar.
(4) Methyl cellulose has a significant effect on the construction and adhesion of mortar. The “adhesion” here refers to the adhesive force felt between the worker’s applicator tool and the wall substrate, that is, the shear resistance of the mortar. The adhesiveness is high, the shear resistance of the mortar is large, and the strength required by the workers in the process of use is also large, and the construction performance of the mortar is poor. Methyl cellulose adhesion is at a moderate level in cellulose ether products.
3. Hydroxyethylcellulose (HEC)
It is made from refined cotton treated with alkali, and reacted with ethylene oxide as etherification agent in the presence of acetone. The degree of substitution is generally 1.5~2.0. It has strong hydrophilicity and is easy to absorb moisture.
(1) Hydroxyethyl cellulose is soluble in cold water, but it is difficult to dissolve in hot water. Its solution is stable at high temperature without gelling. It can be used for a long time under high temperature in mortar, but its water retention is lower than that of methyl cellulose.
(2) Hydroxyethyl cellulose is stable to general acid and alkali. Alkali can accelerate its dissolution and slightly increase its viscosity. Its dispersibility in water is slightly worse than that of methyl cellulose and hydroxypropyl methyl cellulose. .
(3) Hydroxyethyl cellulose has good anti-sag performance for mortar, but it has a longer retarding time for cement.
(4) The performance of hydroxyethyl cellulose produced by some domestic enterprises is obviously lower than that of methyl cellulose due to its high water content and high ash content.
Starch ether
Starch ethers used in mortars are modified from natural polymers of some polysaccharides. Such as potatoes, corn, cassava, guar beans and so on.
1. Modified starch
Starch ether modified from potato, corn, cassava, etc. has significantly lower water retention than cellulose ether. Due to the different degree of modification, the stability to acid and alkali is different. Some products are suitable for use in gypsum-based mortars, while others can be used in cement-based mortars. The application of starch ether in mortar is mainly used as a thickener to improve the anti-sagging property of mortar, reduce the adhesion of wet mortar, and prolong the opening time.
Starch ethers are often used together with cellulose, so that the properties and advantages of these two products complement each other. Since starch ether products are much cheaper than cellulose ether, the application of starch ether in mortar will bring about a significant reduction in the cost of mortar formulations.
2. Guar gum ether
Guar gum ether is a kind of starch ether with special properties, which is modified from natural guar beans. Mainly by the etherification reaction of guar gum and acrylic functional group, a structure containing 2-hydroxypropyl functional group is formed, which is a polygalactomannose structure.
(1) Compared with cellulose ether, guar gum ether is more soluble in water. The properties of pH guar ethers are essentially unaffected.
(2) Under the conditions of low viscosity and low dosage, guar gum can replace cellulose ether in an equal amount, and has similar water retention. But the consistency, anti-sag, thixotropy and so on are obviously improved.
(3) Under the conditions of high viscosity and large dosage, guar gum cannot replace cellulose ether, and the mixed use of the two will produce better performance.
(4) The application of guar gum in gypsum-based mortar can significantly reduce the adhesion during construction and make the construction smoother. It has no adverse effect on the setting time and strength of gypsum mortar.
3. Modified mineral water-retaining thickener
The water-retaining thickener made of natural minerals through modification and compounding has been applied in China. The main minerals used to prepare water-retaining thickeners are: sepiolite, bentonite, montmorillonite, kaolin, etc. These minerals have certain water-retaining and thickening properties through modification such as coupling agents. This kind of water-retaining thickener applied to mortar has the following characteristics.
(1) It can significantly improve the performance of ordinary mortar, and solve the problems of poor operability of cement mortar, low strength of mixed mortar, and poor water resistance.
(2) Mortar products with different strength levels for general industrial and civil buildings can be formulated.
(3) The material cost is significantly lower than that of cellulose ether and starch ether.
(4) The water retention is lower than that of the organic water retention agent, the dry shrinkage value of the prepared mortar is larger, and the cohesiveness is reduced.
Redispersible polymer rubber powder
Redispersible rubber powder is processed by spray drying of special polymer emulsion. In the process of processing, protective colloid, anti-caking agent, etc. become indispensable additives. The dried rubber powder is some spherical particles of 80~100mm gathered together. These particles are soluble in water and form a stable dispersion slightly larger than the original emulsion particles. This dispersion will form a film after dehydration and drying. This film is as irreversible as the general emulsion film formation, and will not redisperse when it meets water. Dispersions.
Redispersible rubber powder can be divided into: styrene-butadiene copolymer, tertiary carbonic acid ethylene copolymer, ethylene-acetate acetic acid copolymer, etc., and based on this, silicone, vinyl laurate, etc. are grafted to improve performance. Different modification measures make the redispersible rubber powder have different properties such as water resistance, alkali resistance, weather resistance and flexibility. Contains vinyl laurate and silicone, which can make the rubber powder have good hydrophobicity. Highly branched vinyl tertiary carbonate with low Tg value and good flexibility.
When these kinds of rubber powders are applied to mortar, they all have a delaying effect on the setting time of cement, but the delaying effect is smaller than that of direct application of similar emulsions. In comparison, styrene-butadiene has the largest retarding effect, and ethylene-vinyl acetate has the smallest retarding effect. If the dosage is too small, the effect of improving the performance of mortar is not obvious.
Post time: Apr-03-2023