The role of redispersible latex powder in mortar
At present, as various special dry powder mortar products are gradually accepted and widely used, people in the industry pay attention to redispersible latex powder as one of the main additives of special dry powder mortar, so various attributives have gradually appeared. latex powder, multi-polymer latex powder, resin latex powder, water-based resin latex powder and so on.
The microscopic properties and macroscopic performance of redispersible latex powder in mortar are integrated, and some theoretical results are analyzed. The action mechanism of redispersible latex powder Redispersible latex powder is to prepare polymer emulsion into a mixture that can be used for spray drying by adding different additives, and then add protective colloid and anti-caking agent to make the polymer form after spray drying. Free-flowing powder redispersible in water. The redispersible latex powder is distributed in the evenly stirred dry mortar. After the mortar is stirred with water, the polymer powder is redispersed into the freshly mixed slurry and emulsified again; due to the hydration of the cement, the surface evaporation and the absorption of the base layer, the pores inside the mortar are free. The continuous consumption of water and the strong alkaline environment provided by the cement make the latex particles dry to form a water-insoluble continuous film in the mortar. This continuous film is formed by the fusion of single dispersed particles in the emulsion into a homogeneous body. It is the existence of these latex films distributed in the polymer modified mortar that enables the polymer modified mortar to obtain characteristics that rigid cement mortar cannot possess: due to the self-stretching mechanism of the latex film, it can be anchored to the base or mortar At the interface of the polymer modified mortar and the base, this effect can improve the bonding performance of the mortar and different bases, such as the adhesion of special bases such as high-density ceramic tiles and polystyrene boards; This effect inside the mortar can keep it as a whole, in other words, the cohesive strength of the mortar is improved, and as the amount of redispersible latex powder increases, the bond strength between the mortar and the concrete base is significantly improved; High The presence of flexible and highly elastic polymer domains greatly improved the bonding performance and flexibility of the mortar, while the elastic modulus of the mortar itself decreased significantly, indicating that its flexibility was improved. Latex film observed inside the mortar in polymer modified cement mortar at different ages. The film formed by the latex is distributed in different positions in the mortar, including the base-mortar interface, between the pores, around the pore wall, between the cement hydration products, around the cement particles, around the aggregate, and the aggregate-mortar interface. Some latex films distributed in the mortar modified by redispersible polymer powder make it possible to obtain properties that rigid cement mortar cannot possess: the latex film can bridge the shrinkage cracks at the base-mortar interface and allow the shrinkage cracks to heal. Improve the sealability of mortar. Improvement of the cohesive strength of the mortar: The presence of highly flexible and highly elastic polymer domains improves the flexibility and elasticity of the mortar, providing cohesion and dynamic behavior to a rigid skeleton. When force is applied, microcrack formation is delayed until higher stresses are reached due to improved flexibility and elasticity. The interwoven polymer domains also hinder the coalescence of microcracks into penetrating cracks. Therefore, the redispersible latex powder increases the failure stress and failure strain of the material. The modification of the polymer to the cement mortar makes the two obtain complementary effects, so that the polymer modified mortar can be used in many special occasions. In addition, due to the advantages of dry-mix mortar in quality control, construction operation, storage and environmental protection, redispersible latex powder provides an effective technical means for the production of special dry mortar products.
Based on the mechanism of action of redispersible polymer powder in mortar, we conducted some comparative tests to verify the performance of another material currently on the market, also known as latex powder, in mortar. 1. Raw materials and test results 1.1 Raw material cement: Conch Brand 42.5 Ordinary Portland Cement Sand: River Sand, Silicon Content 86%, Fineness 50-100 Mesh Cellulose Ether: Domestic Viscosity 30000-35000mpas (Brookfield Viscometer, Spindle 6, Speed 20) Heavy calcium powder: heavy calcium carbonate powder, fineness is 325 mesh Latex powder: VAE-based redispersible latex powder, Tg value is -7°C, here is called: redispersible latex powder Wood fiber: ZZC500 of JS company Commercially available latex powder: a commercially available latex powder, called here: commercially available latex powder 97. The mechanical test formula is: Laboratory standard test conditions: temperature (23±2)°C, relative humidity (50±5)%, test The circulating wind speed in the area is less than 0.2m/s. Molded expanded polystyrene board, bulk density is 18kg/m3, cut into 400×400×5mm. 2． Test results: 2.1 Tensile strength under different curing time: The specimens were made according to the test method of mortar tensile bond strength in JG149-2003. The curing system here is: after the sample is formed, it is cured for one day under the standard conditions of the laboratory, and then put in a 50-degree oven. The first week of testing is: put it in a 50-degree oven until the sixth day, take it out, stick the pull-out test head , On the 7th day, a set of pull-out strength was tested. The test in the second week is: put it in a 50-degree oven until the 13th day, take it out, stick the pull-out test head, and test a set of pull-out strength on the 14th day. The third week, the fourth week. . . and so on.
From the results, we can see that the strength of the redispersible latex powder in the mortar increases and maintains as the time in the high temperature environment increases, which is the same as the latex film that the redispersible latex powder will form in the mortar The theory is consistent, the longer the storage time, the latex film of the latex powder will reach a certain density, thus ensuring the adhesion of the mortar to the special base surface of the EPS board. On the contrary, the commercially available latex powder 97 has a lower strength as it is stored in a high-temperature environment for a longer period of time. The destructive power of the dispersible latex powder to the EPS board remains the same, but the destructive power of the commercially available latex powder 97 to the EPS board is getting worse and worse.
Generally speaking, commercially available latex powder and redispersible latex powder have different mechanisms of action, and redispersible latex powder, which forms a film in various parts of the mortar, acts as a second gelling material to improve the physical properties of the mortar. The mechanism of action of the performance is inconsistent.
Post time: Apr-25-2024