Ready-mixed mortar is divided into wet-mixed mortar and dry-mixed mortar according to the production method. The wet-mixed mixture mixed with water is called wet-mixed mortar, and the solid mixture made of dry materials is called dry-mixed mortar. There are many raw materials involved in ready-mixed mortar. In addition to cementitious materials, aggregates, and mineral admixtures, admixtures need to be added to improve its plasticity, water retention, and consistency. There are many kinds of admixtures for ready-mixed mortar, which can be divided into cellulose ether, starch ether, redispersible latex powder, bentonite, etc. from the chemical composition; can be divided into air-entraining agent, stabilizer, anti-cracking fiber, Retarder, accelerator, water reducer, dispersant, etc. This article reviews the research progress of several commonly used admixtures in ready-mixed mortar.
1 Common admixtures for ready-mixed mortar
1.1 Air-entraining agent
The air-entraining agent is an active agent, and the common types include rosin resins, alkyl and alkyl aromatic hydrocarbon sulfonic acids, etc. There are hydrophilic groups and hydrophobic groups in the air-entraining agent molecule. When the air-entraining agent is added to the mortar, the hydrophilic group of the air-entraining agent molecule is adsorbed with the cement particles, while the hydrophobic group is connected with the tiny air bubbles. And evenly distributed in the mortar, so as to delay the early hydration process of cement, improve the water retention performance of the mortar, reduce the loss rate of consistency, and at the same time, the tiny air bubbles can play a lubricating role, improving the pumpability and sprayability of the mortar.
The air-entraining agent introduces a large number of tiny bubbles into the mortar, which improves the workability of the mortar, reduces the resistance during pumping and spraying, and reduces the clogging phenomenon; the addition of the air-entraining agent reduces the tensile bond strength of the mortar Performance, as the amount of mortar increases, the loss of tensile bond strength performance increases; the air-entraining agent improves the performance indicators such as mortar consistency, 2h consistency loss rate and water retention rate, and improves the spraying and pumping performance of mechanical spraying mortar , On the other hand, it causes the loss of mortar compressive strength and bond strength.
The research shows that without considering the effect of cellulose ether, the increase of air-entraining agent content can effectively reduce the wet density of ready-mixed mortar, the air content and consistency of mortar will be greatly increased, and the water retention rate and compressive strength will decrease; The research on the performance index changes of mortar mixed with cellulose ether and air-entraining agent found that after mixing air-entraining agent and cellulose ether, the adaptability of the two should be considered. Cellulose ether may cause some air-entraining agents to fail, thus making the mortar water retention rate decreased.
Single mixing of air-entraining agent, shrinkage reducing agent and the mixture of both have certain influence on the properties of mortar. The addition of air-entraining agent can increase the shrinkage rate of mortar, and the addition of shrinkage reducing agent can significantly reduce the shrinkage rate of mortar. Both of them can delay the cracking of mortar ring. When the two are mixed, the shrinkage rate of the mortar does not change much, and the crack resistance is enhanced.
1.2 Redispersible latex powder
Redispersible latex powder is an important part of today’s prefabricated dry powder mortar. It is a water-soluble organic polymer produced by high-molecular polymer emulsion through high temperature and high pressure, spray drying, surface treatment and other processes. The emulsion formed by renewable latex powder in cement mortar forms a polymer film structure inside the mortar, which can improve the ability of cement mortar to resist damage.
Redispersible latex powder can improve the elasticity and toughness of the material, improve the flow performance of the freshly mixed mortar, and have a certain water reducing effect. His team explored the effect of curing system on the tensile bond strength of mortar.
The research results show that when the amount of modified rubber powder is in the range of 1.0% to 1.5%, the properties of different grades of rubber powder are more balanced. After the redispersible latex powder is added to the cement, the initial hydration rate of the cement slows down, the polymer film wraps the cement particles, the cement is fully hydrated, and various properties are improved. Mixing redispersible latex powder into cement mortar can reduce water, and latex powder and cement can form a network structure to enhance the bonding strength of mortar, reduce the voids of mortar, and improve the performance of mortar.
In the study, the fixed lime-sand ratio was 1:2.5, the consistency was (70±5) mm, and the amount of rubber powder was selected as 0-3% of the lime-sand mass. The changes in the microscopic properties of the modified mortar at 28 days were analyzed by SEM, and the results showed that The higher the content of redispersed latex powder, the more continuous the polymer film formed on the surface of the mortar hydration product, and the better the performance of the mortar.
Studies have shown that after it is mixed with cement mortar, the polymer particles and cement will coagulate to form a stacked layer with each other, and a complete network structure will be formed during the hydration process, thereby greatly improving the bonding tensile strength and construction of the thermal insulation mortar. performance.
1.3 Thickened powder
The function of the thickening powder is to improve the comprehensive performance of the mortar. It is a non-air-entraining powder material prepared from a variety of inorganic materials, organic polymers, surfactants and other special materials. Thickening powder includes redispersible latex powder, bentonite, inorganic mineral powder, water-retaining thickener, etc., which have a certain adsorption effect on physical water molecules, not only can increase the consistency and water retention of mortar, but also have good compatibility with various cements. The compatibility can significantly improve the performance of mortar. Cao Chun et al] studied the effect of HJ-C2 thickened powder on the performance of dry-mixed ordinary mortar, and the results showed that the thickened powder had little effect on the consistency and 28d compressive strength of dry-mixed ordinary mortar, and had little effect on the delamination of mortar There is a better improvement effect. He has studied the influence of thickening powder and various components on the physical and mechanical indexes and durability of fresh mortar under different dosages. The research results show that the workability of fresh mortar has been greatly improved due to the addition of thickening powder. The incorporation of redispersible latex powder improves the flexural strength of mortar, reduces the compressive strength of mortar, and the incorporation of cellulose ether and inorganic mineral materials makes the compressive and flexural strength of mortar decrease; The components have an impact on the durability of the dry mix mortar, which increases the shrinkage of the mortar. Wang Jun et al. studied the influence of bentonite and cellulose ether on various performance indicators of ready-mixed mortar. Under the condition of ensuring good mortar performance, it was concluded that the optimal dosage of bentonite is about 10kg/m3, and the ratio of cellulose ether is relatively high. The optimal dosage is 0.05% of the total amount of cementitious materials. In this ratio, the thickened powder mixed with the two has a better effect on the comprehensive performance of the mortar.
1.4 Cellulose Ether
Cellulose ether originated from the definition of plant cell walls by French farmer Anselme Payon in the 1830s. It is made by reacting cellulose from wood and cotton with caustic soda, and then adding etherification agent for chemical reaction. Because cellulose ether has good water retention and thickening effects, adding a small amount of cellulose ether to cement can improve the working performance of freshly mixed mortar. In cement-based materials, the commonly used varieties of cellulose ether include methyl cellulose ether (MC), hydroxyethyl cellulose ether (HEC), hydroxyethyl methyl cellulose ether (HEMC), hydroxypropyl methylcellulose Hydroxypropyl methyl cellulose ether and hydroxyethyl methyl cellulose ether are the most commonly used.
Hydroxypropyl methyl cellulose ether (HPMC) has a great influence on the fluidity, water retention and bonding strength of self-leveling mortar. The results show that cellulose ether can greatly improve the water retention of mortar, reduce the consistency of mortar, and play a good retarding effect; when the amount of hydroxypropyl methylcellulose ether is between 0.02% and 0.04%, , the strength of the mortar is significantly reduced. Cellulose ether plays an air-entraining effect and improves the working performance of the mortar. Its water retention reduces the stratification of the mortar and prolongs the operating time of the mortar. It is an admixture that can effectively improve the performance of the mortar; research During the process, it was also found that the content of cellulose ether should not be too high. If it is too high, the air content of the mortar will increase significantly, resulting in a decrease in density, loss of strength and impact on the quality of the mortar. Studies have shown that the addition of cellulose ether significantly improves the water retention of mortar, and at the same time has a significant water-reducing effect on mortar. Cellulose ether can also reduce the density of the mortar mixture, prolong the setting time, and improve the flexural and compressive strength. reduce. Cellulose ether and starch ether are two commonly used admixtures for construction mortar.
However, due to the large variety of cellulose ethers, the molecular parameters are also different, resulting in a large difference in the performance of modified cement mortar. The strength of cement mortar modified with cellulose ether with high viscosity is low instead. When the content of cellulose ether increases, the compressive strength of cement slurry shows a trend of decreasing and eventually stabilizing, while the flexural strength shows an increasing, decreasing, stable and stable trend. Slightly increased change process.
Post time: Feb-02-2023