Study on common admixtures for ready-mixed mortar

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 effect of air-entraining agent on the performance of ready-mixed mechanical spraying mortar, the study found that: the air-entraining agent introduced a large number of tiny air bubbles into the mortar, which improved the workability of the mortar, reduced the resistance during pumping and spraying, and reduced clogging Phenomenon; the addition of air-entraining agent reduces the tensile bond strength performance of mortar, and the loss of tensile bond strength performance of mortar increases with the increase of the content; air-entraining agent improves the consistency, 2h consistency loss rate and water retention of mortar The rate and other performance indicators improve the spraying and pumping performance of the mechanical spraying mortar, on the other hand, it causes the loss of the compressive strength and bonding strength of the mortar.

The influence of three common commercially available air-entraining agents on ready-mixed mortar. The research shows that without considering the effect of cellulose ether, the increase of the amount of air-entraining agent can effectively reduce the wet density of ready-mixed mortar, and the content of mortar The air volume and consistency are greatly increased, while the water retention rate and compressive strength are reduced; and through the study of the performance index changes of the mortar mixed with cellulose ether and air-entraining agent, it is found that the adaptation of the two should be considered after the air-entraining agent and cellulose ether are mixed. The cellulose ether may cause some air-entraining agents to fail, thereby reducing the water retention rate of the mortar.

Single mixing of air-entraining agent, shrinkage reducing agent and the mixture of both have certain influence on the properties of mortar. Wang Quanlei found that the addition of air-entraining agent increases the shrinkage rate of mortar, and the addition of shrinkage reducing agent significantly reduces the shrinkage rate of mortar. Both of them can delay the cracking of the 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. Roger believes that 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.

The application research results of redispersible latex powder in cement mortar show that redispersible latex powder can improve the elasticity and toughness of materials, improve the flow performance of freshly mixed mortar, and have a certain water reducing effect. His team explored the effect of the curing system on the tensile bond strength of mortar, and came to the same conclusion that the dispersible latex powder makes the mortar exposed to the natural environment resistant to temperature and humidity changes. We applied XCT to study the effect of different types of rubber powder in modified mortar on the pore structure, and believed that compared with ordinary mortar, the number of holes and the volume of holes in modified mortar were larger.

Different grades and amounts of modified rubber powder were selected to test their influence on the performance of waterproof mortar. The research results showed that when the amount of modified rubber powder was in the range of 1.0% to 1.5%, the performance of different grades of rubber powder was 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. Through research, it is found that mixing redispersible latex powder into cement mortar can reduce water, and latex powder and cement can form a network structure to enhance the bond strength of mortar, reduce the voids of mortar, and improve the performance of mortar.

The modification effect of redispersible latex powder on the properties of ultra-fine sand cement mortar. In the research, the fixed lime-sand ratio is 1:2.5, the consistency is (70±5) mm, and the amount of rubber powder is selected as 0-3% of the mass of lime-sand , 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 redispersible 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.

The mechanism of action of redispersible latex powder in EPS insulation mortar, the research shows that after it is mixed with cement mortar, the polymer particles and cement will coagulate, forming a stacked layer with each other, and forming a complete network during the hydration process structure, thereby greatly improving the bonding tensile strength and construction performance of thermal insulation mortar.

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. We have studied the effect of HJ-C2 thickened powder on the properties of dry-mixed ordinary mortar, and the results show that the thickened powder has little effect on the consistency and 28d compressive strength of dry-mixed ordinary mortar, and has a good effect on the layering degree of mortar improvement effect. 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 and reduces the compressive strength of mortar, and the incorporation of cellulose ether and inorganic mineral materials reduces the compressive and flexural strength of mortar; The durability of the dry mix mortar has been affected, which increases the shrinkage of the mortar. The effect of compounding of bentonite and cellulose ether on the performance indicators of ready-mixed mortar, under the condition of ensuring good mortar performance, it is concluded that the optimal amount of bentonite is about 10kg/m3, and the optimal amount of cellulose ether is glue 0.05% of the total amount of cementitious materials. In this proportion, 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. Xu Fenlian discussed the influence of hydrocarbon propyl methyl cellulose ether on the performance of ready-mixed mortar by using the change of the content of hydrocarbon propyl methyl cellulose ether. The results show that 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 external additive that can effectively improve the performance of the mortar. During the research process, it was also found that the content of cellulose ether should not be too high, otherwise it will lead to a significant increase in the air content of the mortar, resulting in a decrease in density, a loss of strength and an impact on the quality of the mortar. The effect of cellulose ether on the properties of ready-mixed mortar. Studies have shown that the addition of cellulose ether can significantly improve the water retention of the mortar, and at the same time have a significant water-reducing effect on the mortar. The cellulose ether can also make the mortar mixture Decreased density, prolonged setting time, reduced flexural and compressive strength. Cellulose ether and starch ether are two kinds of admixtures commonly used in construction mortar. The effect of the two mixed into dry-mixed mortar on the performance of the mortar. The results show that the combination of the two can significantly improve the bond strength of the mortar.

Many scholars have studied the influence of cellulose ether on the strength of cement mortar, but due to the variety of cellulose ether, the molecular parameters are also different, resulting in a large difference in the performance of modified cement mortar. The effect of the viscosity and dosage of cellulose ether on the mechanical properties of cement slurry. The results show that the strength of cement mortar modified with cellulose ether with high viscosity is low, and the compressive strength of cement slurry shows a large increase in the dosage of cellulose ether. The trend of decreasing and eventually stabilizing, while the flexural strength showed a changing process of increasing, decreasing, stable and slightly increasing.

2 Epilogue

(1) The research on admixtures is still limited to experimental research, and the influence on the performance of cement-based materials lacks in-depth theoretical system support. There is still a lack of quantitative analysis of the impact of the addition of admixtures on the molecular composition of cement-based materials, the change of interface connection strength, and the hydration process.

(2) The effect of the admixture should be highlighted in the engineering application. At present, many analyzes are still limited to laboratory analysis. Different types of wall substrates, surface roughness, water absorption, etc. have different requirements on the physical indicators of ready-mixed mortar. Different seasons, temperatures, wind speeds, power of machines used and operating methods, etc. all directly affect the pre-mixed mortar. Effect of mixing mortar. In order to achieve a good use effect in engineering, the ready-mixed mortar should be fully diversified and personalized, and the production line configuration and cost requirements of the enterprise should be fully considered, and the production verification of the laboratory formula should be carried out, so as to achieve the greatest degree of optimization.