Shandong chuangyao analyzes performance of fresh polymer modified cement mortar

Shandong chuangyao Biotechnology Co., Ltd. analyzes and introduces the properties of fresh polymer modified cement mortar, modification mechanism, selection of polymer redispersible latex powder for repair, etc

1、 Properties of fresh polymer modified cement mortar

1. Working polymer

The type and content of polymer have significant influence on the workability of fresh mortar. Studies have shown that the water reducing rate of different polymer emulsions can reach more than 20%, and the water reducing effect is obvious. SBR has better water reducing effect. Even polymers of the same kind, due to the different properties of polymer emulsion, have different effects on the fluidity of modified mortar. Generally, with the increase of the ratio of polymer to cement (the mass ratio of polymer to cement), the mobility of emulsion modified mortar increases and the workability is improved. The incorporation of polymer emulsion can improve the workability of the new mortar. This is because the surfactant and stabilizer in the emulsion have introduced more bubbles into the modified mortar, and the accumulation state of cement particles in the mortar has been improved, and the dispersion effect of the cement particles has been improved. The hydrophobicity and colloidal properties of the emulsion make the new modified mortar have good water retention, thus reducing the need for long-term wet curing. The water retention rate of fresh mortar can be further improved by adding hydroxypropyl methyl cellulose ether and modified inorganic mineral powder into polymer modified mortar.

2. Gas content

Studies have shown that the air content of polymer emulsion modified mortar is higher than that of ordinary cement mortar. This is because the surfactant and stabilizer in the polymer emulsion have introduced more bubbles into the new mortar. Proper air entrainment can improve the fluidity, impermeability and freeze-thaw resistance of fresh cement mortar, but excessive air bubbles will reduce the strength of mortar. The general polymer emulsion modified mortar has a gas content of 5% to 20%, and some even up to 30%. To control the air content of modified mortar, the usual method is to mix proper amount of defoamer into the emulsion. Studies have shown that the air content of cement mortar modified with polyacrylate emulsion without defoamer is 43.6%, but when the 0.5% defoamer is added, the gas content is reduced to 8%. Considering that defoamer may affect the bonding between cement and reinforced materials, some other methods of reducing gas content are studied in some literatures. For example, the temperature of epoxy emulsion can be reduced by constant temperature water bath before mixing, and the air content of modified mortar can be reduced.

3. Setting time and working time

When the polymer emulsion is added, the setting time of cement mortar is prolonged, and the effect of emulsion dosage is more significant. There are great differences in the research of different students. Some studies have found that the setting time of polymer modified mortar is longer than that of ordinary cement mortar, and increases with the increase of polymer cement ratio. But some studies have found the opposite. The working time of polymer modified mortar is not directly related to the setting time, but mainly related to the surface drying conditions (temperature, humidity, wind speed, etc.) during construction. If the surface of modified mortar is dried too fast, the "hard skin" will be formed earlier, which will affect the final finishing work. Generally speaking, the polymer modified mortar needs 15-30 minutes of working time after mixing and exposed to the air.

4. Plastic cracking

Before setting and hardening (plastic stage), fresh mortar is prone to plastic cracking due to the rapid evaporation of surface moisture. The main reason is that there is a difference between the internal bleeding speed of mortar and the evaporation rate of surface moisture. The results show that the addition of polymer limits the formation of surface and internal micro cracks caused by plastic shrinkage of mortar. Some scholars believe that polymer can slow down the heat release rate of cement hydration and improve the cracking resistance of mortar, but the shrinkage deformation will increase. In this regard, some scholars found that adding appropriate amount of polypropylene fiber can effectively inhibit the plastic cracking of modified mortar. Further study found that the type and length of fiber, the geometric shape of polypropylene fiber, different mixing methods and particle size of sand all have certain influence on the early water loss of modified mortar. Taking into account the large shrinkage deformation of emulsion modified mortar, special attention should be paid to the evaporation rate exceeding 0.5kg/ (m2. H) in site construction. After construction, it is best to take short-term moisture conservation measures for mortar.

2、 Properties of hardened polymer modified cement mortar

1. Bond properties

The adhesion of polymer modified mortar on various substrates is better than that of ordinary cement mortar. The reason is that the polymer has a good bonding effect with the adhesive matrix material. Different kinds of polymers have great influence on the bonding properties of modified mortar. Some scholars have found that the long-term bonding performance of acrylic mortar to old concrete matrix is better than that of pure acrylic mortar and chloroprene mortar. The amount of polymer and water cement ratio also have important influence on the bond strength of modified mortar. The test shows that the addition of 5% to 20% polymer emulsion can increase the bond strength of the benchmark mortar by 1~4 times. Some scholars also believe that the bond strength of modified mortar is directly proportional to the content of polymer under the condition of low polymer content (no more than 3.5%); when the polymer content is more than 3.5%, the bonding strength will be reduced by 40% when the content of polymer is more than 3.5%. In the research of literature, the influence of different water cement ratio on the bonding performance of polymer modified mortar was studied. The results showed that the bonding strength of two kinds of modified mortar with different water cement ratio (0.35, 0.4) was greater than 4MPa at 90d age, and the bond strength of modified mortar with water cement ratio of 0.3 was increased by more than one time, and it was much higher than that of blank cement mortar. Different polymer modified mortars have their own suitable bond curing systems.

2. Compressive strength and flexural strength

Generally, the addition of polymer will reduce the compressive strength of cement mortar and improve its flexural strength. At the same water binder ratio, the compressive strength of polymer modified mortar is lower than that of unmodified ordinary cement mortar. The strength of modified mortar mixed with SBR emulsion and PAE emulsion is compared. The results show that the compressive strength of the two modified mortar decreases compared with the blank cement mortar, but the compressive strength of the 0.2 mortar is higher than that of the 0.1 mortar.

The mixing ratio of polymer mortar (polymer cement ratio, water cement ratio, lime sand ratio, etc.) is the main factor affecting the mortar strength. Literature studies show that when the polymer cement ratio is less than 7.5%, the addition of SBR can significantly reduce the compressive strength of mortar. The influence of 3 different types of polymer emulsion on the strength of self leveling mortar is compared. The results show that the compressive strength of the 3 modified cement mortar decreases with the increase of polymer content. In this regard, some scholars believe that the incorporation of hydroxypropyl methyl cellulose ether slows down the decreasing trend of polymer on mortar compressive strength. It is found that the water cement ratio has a little influence on the compressive and flexural strength of the modified mortar. When the ratio of mortar to sand is different, the modification effect of polymer emulsion on compressive and flexural strength is also different.

The results show that the strength of modified mortar is significantly affected by the addition of admixtures, and the research results of different scholars are quite different. Some scholars have found that the strength of modified cement mortar is improved after adding silica fume or slag, and the modification effect of 10% silica fume is better than that of 40% slag. Some studies show that the 28d compressive and flexural strength of polymer modified mortar is 15% - 25% higher than that of ordinary cement mortar. In the literature research, the influence of different kinds of mineral admixtures on the performance of polymer modified mortar is studied. The results show that adding fly ash can improve the flexural strength of mortar, adding slag powder can improve the compressive strength of mortar, the addition of fly ash will reduce the strength of modified mortar, and the fineness of fly ash will also affect the strength of modified mortar.

Adding polypropylene fiber can improve the flexural strength of modified mortar, polypropylene fiber can greatly improve the flexural strength of cement mortar, but the effect of short fiber is not obvious. Some mineral wastes such as iron tailings are used to replace quartz sand to prepare modified cement mortar or blast furnace slag. Water glass, metakaolin or calcined bentonite can also improve the compressive and flexural strength of polymer modified mortar, and the effect of metakaolin is better than that of calcined bentonite. The strength of modified cement mortar will also be affected by changing the order of water reducing agent and emulsion, the type and size of fine aggregate.

The curing conditions also have some influence on the strength of modified mortar. Some scholars have found that the compressive and flexural strength of styrene butadiene epoxy (no curing agent) modified mortar after steam curing and heating curing is three times of that of unmodified mortar. Literature research shows that with the increase of emulsion dosage, the time for wet curing of specimens reaches a certain intensity. However, some scholars believe that short-term wet curing is still necessary for polymer modified mortar. Generally speaking, early water curing and later drying curing are ideal curing conditions.

Some scholars have found that the compressive strength of polymer modified mortar increases after freeze-thaw cycle. The reason is that the polymer film is destroyed by freeze-thaw, and the cement particles wrapped by it are released and continue to participate in hydration, thus increasing the strength. In addition, different freeze-thaw environments have different effects on the strength of polymer mortar. It has been found that the compressive strength and flexural strength of the modified mortar will decrease under the conditions of air freeze-thaw and water freezing cycles, and the influence of "air freezing" is greater.

3. Toughness

There are many characterization indexes of toughness of polymer modified mortar, such as compression ratio, impact resistance, lateral deformation, etc. the commonly used characterization index is compression ratio, which is commonly used to characterize the impact resistance when used in pavement repair. It has been found that when the toughness is low, the bending of the modified mortar is obvious, and the transverse deformation is most obvious when the toughness is high, and its impact resistance is obvious in any case. At the same fluidity, the toughness of polymer modified cement mortar is better than that of ordinary cement mortar. Some studies show that when the polymer cement ratio is in a certain range (< 10%), the toughness of the modified mortar increases with the increase of the polymer cement ratio. The same polymer emulsion with different performance indexes has different effects on the toughness of mortar. It has been found that adding 30% - 40% fly ash into the modified mortar can significantly reduce the compression ratio and improve the toughness of the mortar. Further research shows that the increase of fineness of fly ash can reduce the compression ratio of modified mortar and improve its toughness. The toughness of polymer modified mortar can also be improved by adding fiber. It has been shown that compared with short polypropylene fiber, long polypropylene fiber has more obvious effect on improving toughness of modified mortar

4. Dry shrinkage

The drying shrinkage of polymer modified mortar is mainly affected by polymer type and polymer cement ratio, which increases with the extension of time. Generally, the 28d dry shrinkage of mortar decreases with the increase of polymer cement ratio. In the literature, different amounts of styrene butadiene emulsion were mixed into cement mortar, and the emulsion content was 3%, 6%, 9% and 12% respectively. The study showed that when the emulsion dosage was 6%, the 90d shrinkage deformation of the modified mortar decreased the most (9.4%). When the emulsion content was greater than 3%, the shrinkage of the modified mortar could be inhibited before 28d age, and the shrinkage deformation of the modified mortar decreased by more than 20% when mixed with 12% emulsion. Regarding the mechanism of shrinkage reduction of polymer emulsion mortar, it is believed that the incorporation of emulsion slows down the rate of hydration heat release of mortar at early stage, and reduces the loss of moisture in mortar during later curing, resulting in the reduction effect. In order to further reduce the shrinkage, the usual method is to mix fiber. It has been found that the shrinkage of modified mortar mixed with 1.5% polypropylene fiber is 40.5% less than that of ordinary cement mortar, and 28.6% less than that of modified mortar with the same amount of emulsion. In addition, the modification of polymer emulsion with modifiers such as polymer monomers with special groups can also reduce the shrinkage.

5. Durability

The durability of polymer modified mortar generally includes permeability resistance, corrosion resistance, frost resistance, etc. Some foreign scholars believe that the durability of most cement-based materials can be attributed to the permeability and dimensional stability of cement-based materials. Generally, the durability of mortar modified by polymer will be significantly improved. The experimental results show that the pore size decreases and the macropores decrease with the addition of polymer emulsion in mortar. The macropores and connected pores are filled by the polymer itself or sealed by the polymer film, so the water absorption of the modified mortar is reduced and the impermeability is improved. As a result, the penetration resistance of the polymer mortar is improved by adding chloride ions, which makes the mortar have better resistance to chloride ion penetration. The results showed that compared with the blank cement mortar, the chloride penetration resistance of the SAE emulsion, VAE emulsion, SBR emulsion and PAE emulsion modified mortar increased by 69%, 27%, 75% and 42%, respectively, and the SBR emulsion improved the effect more than 20%. Some scholars believe that a layer of 10 mm thick high performance polymer modified mortar can protect steel from seawater corrosion for at least 25 years. Further experimental studies show that the incorporation of high-performance fiber into cement-based repair materials can effectively inhibit the penetration of chloride ions and prevent corrosion of reinforcement. By modifying the varieties of cement, the modified mortar has special salt tolerance. For example, the Modified Sulphoaluminate Cement Mortar with polymer emulsion has excellent sulfate corrosion resistance.

It has been found that the acid resistance of modified mortar modified with styrene acrylic emulsion is much better than that of blank mortar, and the degree of improvement is related to the type and concentration of acid, but the overall improvement effect is still poor. This may be because the cement hydration product itself is not acid resistant, so the modified mortar is not acid resistant. Some scholars believe that the acid resistance of polymer modified mortar can be improved by adding sodium silicate.

It has been reported that the frost resistance of polymer modified mortar is better than that of ordinary cement mortar, because of the low water cement ratio and the existence of polymer film in hardened mortar and its reasonable pore structure. It has been found that the frost resistance of cement mortar is improved after incorporation of polymer emulsion. Some scholars have compared the effects of freeze-thaw cycles on the blank cement mortar and polymer modified mortar. The research results show that after 100 freeze-thaw cycles, the strength loss of the modified mortar is less than 6%, the mass loss is less than 2%, and the appearance damage is also slight.

3、 Mechanism of polymer modification

1. Effect of polymer on cement hydration

The modification effect of polymer on cement mortar is related to the influence of polymer on cement hydration. A soft X ray microscope was used to study the effect of VAE emulsion on the early hydration of pure calcium silicate (C3A). The results showed that VAE copolymers hydrolyzed and released CH3C00-1 in the alkaline environment containing C3A, which reacted with Ca2+ to form organic salts in the solution, changed the Ca/Si ratio in C-S-H, reduced the Ca (OH) 2 content, and increased the incorporation of the C3A into the gel layer. Layer spacing. In addition, VAE particles were adsorbed on the surface of C3A particles

4、 Selection of polymer redispersible latex powder for repair in practical engineering, the appropriate polymer redispersible latex powder modified cement mortar should be selected according to the specific technical requirements and cost performance ratio. The performance of various polymer mortars is compared from several aspects.

(1) cost control is a very important link in the repair and reinforcement works. The cost of polymer emulsion also accounts for a large part of the repair material. The price range of several polymer emulsions mentioned in the introduction is generally EE > PAE > SAE > CR > SBR > VAVEOEA > VAE > PVAC.

(2) If the requirements of impermeability and bond strength are high, but the color requirements are not high, SBR is better. For the above three items, PAE should be selected especially when it is important to keep the color and white cement is used externally. VAE can be used when the requirements of bond strength are high, impermeability and color retention are general. PVAC is only required for bonding strength and no water requirement. This emulsion is most commonly used to repair bridge deck, car park panels and floor. Styrene butadiene emulsion (SB) can be used for wet environment or external exposure rather than exposure to ultraviolet radiation. PAE should be used for occasions exposed to ultraviolet radiation. For chemical industry with high corrosion resistance, chloroprene emulsion cement mortar, polyacrylate emulsion cement mortar and epoxy emulsion cement mortar should be selected in the GB50046-2008 code for anticorrosion design of industrial buildings.

(3) since the properties of mortar modified by polymer emulsion are more stable than that of the mortar modified by re dispersible latex powder, the relevant regulations provide that the emulsion should be used for repairing and strengthening important structures. The price of re dispersible latex powder is much higher than that of the corresponding emulsion, and the performance of the emulsion is not better than that of the corresponding emulsion modified mortar. Therefore, the milk redispersible latex powder is used only on occasions where the operation requirements are high and the material cost requirements are not high. Polymer modified cement mortar has the advantages of high strength, high toughness, good bonding performance and good durability. It has been widely used in the repair and reinforcement of industrial and civil buildings, roads and bridges, underground buildings, seaport buildings, etc. But there are also some deficiencies. First, the problem of price performance. The cost of polymer is 10 times or even more than 100 times of the cost of cement. Therefore, taking into account the cost problem, we should minimize the consumption of such materials. Two, it is not resistant to aging, high temperature and fire. Therefore, the rigidity and crosslinking degree of polymer chains should be increased when polymer emulsion is prepared, and the unsaturated content in the emulsion should be avoided. Thirdly, some polymers are toxic and have peculiar smell. Therefore, necessary protective measures should be taken during construction, and attention should be paid to the use environment. For the future research and application of this kind of repair materials, this paper suggests to study from the following aspects: (1) improve the cost performance: the price of polymer cement mortar is several times that of ordinary cement mortar, so it is a big problem to ensure its performance while reducing the cost. Therefore, it is necessary to systematically study and optimize the composition and proportioning of emulsion, superplasticizer, fiber, superfine mineral powder admixture, water glass and water-soluble polymer in order to further improve the performance of cement mortar. According to the performance characteristics of various polymers and specific engineering application requirements, the repair materials with more reasonable cost performance are selected.

(2) Prolonging service life: polymer cement mortar is often used for thin layer repair of concrete surface. How to ensure the durability of repair materials and structures should be further studied, especially crack resistance, fatigue resistance and aging resistance. In particular, the research on the frost resistance of polymer mortar is less at home and abroad, especially the Research Report on the frost resistance of concrete structure repaired by mortar is less, so the research is necessary.

(3) Research and development of special polymer modified mortar: in addition to repair, polymer modified mortar can also be used for durability protection of concrete surface. Research and development of special polymer modified mortar with different properties (such as carbonation resistance, permeability resistance, chloride salt resistance, sulfate resistance, etc.) according to different environmental conditions and durability damage types is a hot spot in future research.

(4) Mechanism of hydration and film formation of polymer modified mortar: in addition to the types of polymer, the performance of polymer modified cement mortar is also affected by the shape, thickness, spatial crosslinking morphology of polymer modified cement mortar and its interaction with hydration products of cement. The research in this field is not deep.

(5) Further improve the standardization work: the state and industry departments should timely issue or update the relevant standards and specifications according to the performance characteristics of various polymers, and formulate the mix design method, product standard, testing method and application technical regulations of such repair materials.

Source: Shandong chuangyao Biotechnology