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.
1.1 Cellulose ether
Cellulose ether is a general term for a series of products produced 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) After the cellulose ether in the mortar is dissolved in water, the effective and uniform distribution of the cementitious material in the system is ensured due to the surface activity, and the cellulose ether, as a protective colloid, “wraps” the solid particles and A layer of lubricating film is formed on its outer surface, which makes the mortar system more stable, and also improves the fluidity of the mortar during the mixing process and the smoothness of construction.
(2) Due to its own molecular structure, the cellulose ether solution makes the water in the mortar not easy to lose, and gradually releases it over a long period of time, endowing the mortar with good water retention and workability.
1.1.1 Molecular formula of methyl cellulose (MC) [C6H7O2(OH)3-h(OCH3)n]x
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.
1.1.2 The molecular formula of hydroxypropyl methylcellulose (HPMC) is [C6H7O2(OH)3-m-n(OCH3)m,OCH2CH(OH)CH3]n]x
Hydroxypropyl methylcellulose is a cellulose variety whose output and consumption have been increasing rapidly in recent years. It is a non-ionic cellulose mixed ether made from refined cotton after alkalization, using propylene oxide and methyl chloride as etherification agent, through a series of reactions. The degree of substitution is generally 1.2~2.0. Its properties are different due to the different ratios of methoxyl content and hydroxypropyl content.
(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.
1.1.3 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.
1.1.4 Carboxymethylcellulose (CMC) [C6H7O2(OH)2och2COONa]n
Ionic cellulose ether is made from natural fibers (cotton, etc.) after alkali treatment, using sodium monochloroacetate as etherification agent, and undergoing a series of reaction treatments. The degree of substitution is generally 0.4~1.4, and its performance is greatly affected by the degree of substitution.
(1) Carboxymethyl cellulose is more hygroscopic, and it will contain more water when stored under general conditions.
(2) Carboxymethyl cellulose aqueous solution will not produce gel, and the viscosity will decrease with the increase of temperature. When the temperature exceeds 50°C, the viscosity is irreversible.
(3) Its stability is greatly affected by pH. Generally, it can be used in gypsum-based mortar, but not in cement-based mortar. When highly alkaline, it loses viscosity.
(4) Its water retention is far lower than that of methyl cellulose. It has a retarding effect on gypsum-based mortar and reduces its strength. However, the price of carboxymethyl cellulose is significantly lower than that of methyl cellulose.
Post time: Mar-30-2023