Pre-mixed mortar can be divided into wet mixed mortar and dry mixed mortar according to the production mode. The wet mixed mortar formed by adding water is called wet mixed mortar, and the solid mixture formed by mixing 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 should be added to improve its plasticity, water retention, consistency and other properties. There are many additives in ready-mixed mortar, which can be divided into cellulose ether, starch ether, redispersible latex powder, bentonite and so on from the chemical composition. It can be divided into air entraining agent, stabilizer, anti-cracking fiber, retarding agent, accelerating agent, water reducing agent, dispersant and so on. This paper reviews the research progress of several commonly used admixtures in ready-mixed mortar.

1.1 air-entraining agent

Air entraining agent is a kind of active agent, common kinds of rosin resin, alkyl and alkyl aromatic sulfonic acid. Air-entraining agent molecules with hydrophilic group and hydrophobic groups, when mortar adding air-entraining agent, air-entraining agent molecules of hydrophilic group with cement particle adsorption, and the hydrophobic groups and tiny air bubbles connected and evenly distributed in the slurry, in order to delay the early hydration of cement process, improve the water retention performance of mortar, reduce the loss rate of consistency, tiny air bubbles can lubricate the role at the same time, Improve the pumping and sprayability of mortar.

The results show that the air entraining agent introduces a large number of tiny bubbles into the mortar, which improves the workability of the mortar, reduces the resistance in the pumping and spraying process, and reduces the blockage phenomenon. The addition of air entraining agent reduces the tensile bond strength of mortar, and the loss of tensile bond strength increases with the increase of the dosage. Air entraining agent can improve the mortar’s consistency, 2h consistency loss rate and water retention rate, and improve the spraying and pumping performance of mechanical spraying mortar. On the other hand, it causes the loss of mortar’s compressive strength and bond strength.

The results show that without considering the effect of cellulose ether, the increase of air entraining agent can effectively reduce the wet density of ready-mixed mortar, and the air content and consistency of mortar are greatly increased, while the water retention rate and compressive strength are decreased. By studying the performance index changes of mortar mixed with cellulose ether and air entraining agent, it is found that the adaptability of air entraining agent and cellulose ether should be considered after mixing. Cellulose ether may lead to the failure of some air entraining agent, so that the water retention rate of mortar decreases.

The performance of mortar is influenced by the mixture of air entraining agent and shrinkage reducing agent. Wang Quanlei found that the incorporation of air entraining agent increased the shrinkage rate of mortar, and the incorporation of shrinkage reducing agent significantly decreased the shrinkage rate of mortar, both of which could delay the cracking of mortar ring. When the two are mixed, the shrinkage rate of 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 made of polymer emulsion by 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 can form polymer film structure inside the mortar, which can improve the ability of cement mortar to resist damage.

The RESULTS SHOW THAT THE redispersible LATEX POWDER CAN IMPROVE THE elasticity and toughness of the material, improve the flow performance of the fresh mortar, and has a certain water reduction effect. His team explored the effect of curing system on the tensile bond strength of mortars and reached the same conclusion as Yoshihiko Ohama, that the latex powder can be dispersed to make mortars that are exposed to natural environment resistant to temperature and humidity changes. Wang Peiming also used XCT to study the influence of different kinds of adhesive powders in modified mortar on the pore structure, and believed that the modified mortar was larger than the ordinary mortar regardless of the number of holes and the volume of holes.

The RESULTS SHOW THAT WHEN THE DOSAGE OF MODIFIED RUBBER POWDER IS 1.0% ~ 1.5%, THE PROPERTIES OF DIFFERENT GRADES OF RUBBER POWDER ARE MORE BALANCED. After adding redispersible latex powder to cement, the initial hydration rate of cement slows down, the cement particles are wrapped by polymer film, and the cement is fully hydrated, and the properties of cement are improved.

Through the study, it is found that the redispersible latex powder mixed into cement mortar can play a role in reducing water, and the latex powder and cement can form a network structure to enhance the bond strength of mortar, reduce the gap of mortar, and improve the performance of mortar.

In the study, the fixed cement-sand ratio was 1:2.5, the consistency was (70±5) mm, and the mixing amount of rubber powder was selected as 0-3% of the mass of cement-sand. The changes in the 28d microscopic properties of modified mortar were analyzed by SEM. The results showed that the higher the mixing amount of redispersible latex powder, the more continuous the polymer film formed on the surface of hydration products of mortar, and the better the performance of mortar was improved.

The research shows that after mixing with cement mortar, the polymer particles and cement set, forming a layer between each other, forming a complete network structure in the hydration process, so as to greatly improve the bond tensile strength and construction performance of thermal insulation mortar.

1.3 thickening powder

The function of thickening powder is to improve the comprehensive performance of mortar, which is made of a variety of inorganic materials, organic polymers, surfactants and other special materials. The thickening powder includes redispersible latex powder, bentonite, inorganic mineral powder, water-holding thickener, etc., which has a certain adsorption effect on physical water molecules, not only can increase the consistency and water retention of mortar, but also has good compatibility with all kinds of cement, which can significantly improve the properties of mortar. Cao Chun et al.] studied the effect of HJ-C2 thickening powder on the properties of dry-mixed ordinary mortar, and the results showed that the thickening powder had little effect on the consistency and 28D compressive strength of dry-mixed ordinary mortar, but had a better effect on the stratification degree of mortar. Renowned thickening powder was studied under different dosage of components and the physical and mechanical indexes, the durability of new mixing mortar, the influence of the research results show that the new workability of mortar because of the addition of thickening powder had the very big improvement, to disperse, the mixed latex powder can improve the flexural strength of mortar, reduced the compressive strength of mortar, The incorporation of cellulose ether and inorganic mineral materials makes the compressive and flexural strength of mortar decrease. All the components have influence on the durability of dry mortar, and make the shrinkage of mortar increase. Wang jun, such as bentonite and cellulose ether was studied after every performance index of ready-mixed mortar admixture effect, in the case of guarantee mortar performance good, get better content of bentonite is about 10 kg/m3, the cellulose ether better content for 0.05% of the total gelled material, proportion on this, the thickening of the powder of the mixed influence on mortar comprehensive performance is better.

1.4 Cellulose ether

Cellulose ethers are derived from the definition of the plant cell wall by the French agrarian Anselme Payon in the 1830s. It is made by reacting cellulose from wood and cotton with caustic soda and then adding etherifying agents. Because cellulose ether has good water retention, thickening effect, so after adding a small amount of cellulose ether in cement, it can play a role in improving the performance of new mortar. In cement-based materials, cellulose ether commonly used varieties include methyl cellulose ether (MC), hydroxyethyl cellulose ether (HEC), hydroxyethyl methyl cellulose ether (HEMC), hydroxypropyl methyl cellulose ether (HPMC), among which hydroxypropyl methyl cellulose ether and hydroxyethyl methyl cellulose ether are the most commonly used.

It is found that hydroxypropyl methylcellulose ether (HPMC) has great influence on the fluidity, water retention and bond 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 methyl cellulose ether is between 0.02% and 0.04%, the strength of mortar is obviously reduced. Based on the change of the content of propyl methyl cellulose ether, the effect of propyl methyl cellulose ether on the properties of ready-mixed mortar was discussed. The results show that cellulose ether has the effect of air entraining and improves the working performance of mortar, and its water retention reduces the stratification degree of mortar and prolongates the operating time of mortar. It is an effective admixture to improve the performance of mortar. The study also found that the content of cellulose ether should not be too high, which will lead to a significant increase in the gas content of mortar, resulting in a decrease in density, strength loss and impact on the quality of mortar. Zhan Zhenfeng et al. studied the influence of cellulose ether on the properties of pre-mixed mortar. The study showed that the addition of cellulose ether significantly improved the water retention of mortar, and had obvious water reduction effect on mortar. Cellulose ether also reduced the density of mortar mix, prolonged the setting time, and reduced the flexural and compressive strength. Cellulose ether and starch ether are two kinds of admixtures commonly used in building mortars. The effect of their compound incorporation into dry mixed mortars on the properties of mortars has been studied. The results show that their compound incorporation can significantly improve the bond strength of mortars.

Many scholars have studied the influence of cellulose ether on the strength of cement mortar. However, due to the variety of cellulose ether and the different molecular parameters, the performance of modified cement mortar varies greatly. Ou zhihua cellulose ether were studied such as viscosity and dosage on the mechanical behavior of the slurry, according to the results, with the viscosity of cellulose ether modified cement mortar strength rather low, cellulose ether dosage increased, the compressive strength of cement slurry to show to reduce and ultimately a stable trend, flexural strength, increase, decrease, stability, and slightly increase the change process.

Conclusion 2

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

(2) The use effect of admixture should be highlighted in engineering applications, many of the current analysis is still limited to laboratory analysis. The different kinds of wall substrate, the roughness degree of the surface and the water absorption rate have different requirements on the physical indexes of the ready-mixed mortar. Different seasons, temperature, wind speed, the power of the used machinery and the operation method directly affect the use effect of the ready-mixed mortar. In order to achieve a good use effect in engineering, the ready-mixed mortar should be fully diversified and personalized design, and fully consider the requirements of enterprise production line configuration and cost, production verification of laboratory formula, so as to achieve the maximum optimization.