Precautions when using polycarboxylate superplasticizer(PCE) in concrete
Introduction
Polycarboxylic acid-based high-performance water-reducing agent is a general term for a series of polymers with specific molecular structures and properties. It is generally obtained by polymerizing different monomers through free radical reactions. The structure of polycarboxylate superplasticizer is a comb-type copolymer with a linear main chain connected to multiple branches. The hydrophobic molecular main chain segment contains hydrophilic groups such as carboxylic acid groups, sulfonic acid groups, and amino groups, and the side chains are Hydrophilic polyoxyethylene segments with different degrees of polymerization.
Polycarboxylic acid-based high-performance water-reducing agent are considered to be the latest generation of high-performance water-reducing agents. People always expect them to be safer, more efficient, and more adaptable than traditional naphthalene-based water-reducing agents in their applications. However, in engineering practice, we always encounter various problems, and some of them are not encountered when using other types of water reducing agents, such as the concrete mixture is abnormally dry, cannot be unloaded, and cannot even be pumped. Pouring, or the concrete mixture is seriously stratified, etc. Now, we will share these issues with you based on some of our experiences and insights in the actual production control process.
1 Advantages of Polycarboxylic acid-based high-performance water-reducing agent Low dosage: Generally, its converted solid content is about 1/4 of the naphthalene series . High water reduction rate: The water reduction rate of concrete is generally 25% to 35% , and the extreme value can be as high as 40% to 45% . Good slump retention: slump loss can be controlled within 2 to 3 hours with basically no loss. High compressive strength ratio: The contrast strength at each age has been greatly improved, and the compressive strength ratio in the early stage has been significantly improved. Green and environmentally friendly: Formaldehyde and other harmful raw materials are not used in the synthetic production process, which will not cause health hazards to the human body and will not cause pollution to the environment. High durability: The low water-to-cement ratio due to high water reduction greatly improves the anti-permeability, anti-corrosion, and anti-chloride ion diffusion and migration coefficient properties of concrete. Cost saving: Because concrete using polycarboxylate water-reducing agent has good workability and low water-cement ratio, and a small amount can have significant effects, it can reduce costs. Compared with the current naphthalene-based water-reducing agent, the comprehensive cost of concrete per unit can be saved by 5 to 10 yuan. Other advantages: excellent working performance, relatively low carbonization, and good molding appearance smoothness, etc.
2 Application of Polycarboxylic acid-based high-performance water-reducing agent
a. On-site inspection requirements Polycarboxylic acid admixtures are different from naphthalene, aliphatic, and amino series admixtures. Simple pure slurry testing can no longer reasonably reflect its working performance in concrete. It is recommended that qualified laboratories can compare clean slurry tests. The test is used as a reference, and finally the concrete trial mix is used as the basis for testing.
During the compounding process of most polycarboxylic acids, the slump-retaining properties of concrete are required to be considered. For example, the high-slump-retaining polycarboxylic acid has a slow-release carboxylic acid in the masterbatch as a post-release water-reducing component. The initial fluidity of the clean slurry is It may be smaller, but the slump retention performance of concrete is good. If the adaptability of polycarboxylic acid and glue is measured by the fluidity of the clean slurry, it is likely to be misjudged; and for winter-type polycarboxylic acid, in order to avoid reverse growth of slump Or the occurrence of delayed bleeding phenomenon, the slump-preserving and retarding components are greatly reduced, and the initial fluidity of the clean slurry may be larger at this time . It is also inaccurate to use the test results to measure the loss of concrete slump over time. It is recommended to use the same batch of raw materials and different batches of additives to conduct a comparison test of the fluidity of the clean slurry. The data accumulated by this method can be used as long-term tracking, and can check the fluctuation of the additives entering the factory to a certain extent. Its advantage is that it can be quickly , Save manpower. When both parties have doubts about the test results, they can use the method of trial mixing of concrete for testing. Although this method is more accurate and time-consuming, each mixing station can choose the method of on-site inspection based on its own situation and test the storage time of the glue used. It is recommended not to exceed half a month.
b. Requirements for raw materials The Polycarboxylic acid-based high-performance water-reducing agent currently used in the market have high requirements on raw material quality and stability, and have high requirements on cement adaptability, sand and gravel mud content, water absorption, stone powder content, sand and gravel gradation, sand rate, and coal ash burning loss. The quantity and water demand are relatively sensitive, so it is particularly important to check the raw materials entering the site. Good material stability promotes the stability of production control and greatly reduces the probability of quality accidents.
- Cement
It is biased to evaluate the adaptability of cement and admixtures only based on the test results of the fluidity test of the clean slurry. It should be evaluated in conjunction with the trial mixing of concrete. There are many factors that affect the adaptability of cement and admixtures, including the content of C3A and C4AF , the type of cement mixture, the shape of gypsum, etc. C3A and C4AF adsorb more water-reducing agents, resulting in less water-reducing agents on the silicate phase surface. Commonly used mixed materials include limestone powder, fly ash, and mineral powder. However, in the actual production process, cement manufacturers In the pursuit of low cost, admixtures such as volcanic ash and coal gangue that have a large adsorption capacity for polycarboxylic acids are often added , resulting in poor adaptability of cement and admixtures; semi-hydrated gypsum generates dihydrate gypsum when it encounters water, which consumes part of the Free water leads to accelerated loss of concrete. In response to the above problems, when selecting polycarboxylic acid varieties, matching tests should be conducted based on the characteristics of cement.
- Aggregate
The particle gradation and sand rate of aggregates in concrete also have a great impact on the water-reducing effect of polycarboxylate-based water-reducing agents. Tests have shown that when other conditions remain unchanged and only the sand rate changes between 40% and 50% , the water-reducing rate of the same polycarboxylate water-reducing agent can differ by up to 4% due to the change in sand rate . The use of continuously graded aggregates can reduce the sand rate and single water consumption, increase the concrete flow effect, and reduce the water reduction rate requirement of admixtures.
The water absorption rate of aggregates also has a great impact on polycarboxylic acids. During the mixing process, a large amount of free water enters the aggregate pores, reducing the fluidity and slump of concrete, and the advantages of polycarboxylic acids cannot be reflected.
The mud content in the aggregate is mainly various types of clay minerals, which are silicate minerals containing mainly aluminum, magnesium, etc. It has a layered structure, fine particles, and strong adsorption. Its adsorption is mainly mineral surface adsorption and intercalation adsorption. The main types of clay include illite, kaolin, montmorillonite, etc. Their adsorption of polycarboxylic acid is stronger than that of glue. Among them, montmorillonite has the strongest adsorption, which will significantly reduce the workability of concrete. It is reflected in the decrease of slump and fluidity, the enhancement of viscosity, and the increase of pump loss.
Compared with traditional high-efficiency water-reducing agents such as naphthalene series, polycarboxylate water-reducing agents are more sensitive to clay (especially montmorillonite) in sand and gravel aggregates, which greatly restricts its promotion and application in concrete. On the one hand, clay minerals have a strong adsorption effect on polycarboxylic acid-based high-performance water-reducing agents, which reduces the number of polycarboxylic acid molecules used to disperse cement, reduces water-reducing performance, and increases fluidity loss; Due to the high specific surface area of clay minerals, they can absorb more water and reduce the amount of free water in the slurry, which leads to reduced fluidity and increased viscosity of concrete.
During the production process, in response to changes in the mud content of sand and gravel, the dosage of admixtures is often adjusted to ensure working performance. However, it should be noted that when the mud content is significantly reduced, the dosage of admixtures should be reduced in time to reduce the concrete quality.
-Admixtures
When selecting relatively high-quality Class I and II fly ash and mineral powder, as the dosage increases, the dosage of Polycarboxylic acid-based high-performance water-reducing agent will gradually decrease. Taking C35 as an example, comparative trial mixing tests have proven that when the total amount of admixtures is increased from 20% to 50% , the admixture content will decrease by 0.4% to 0.6% (basic content is 1.8% , solid content is about 11% ). Studies have shown that cement, mineral powder, and fly ash have different adsorption rules for admixtures, and the order of adsorption amounts is: cement > mineral powder > fly ash.
The water demand ratio and loss on ignition index of fly ash have a more important impact on the performance of polycarboxylic acid. The loss on ignition of fly ash becomes larger. While the carbon particles in fly ash absorb water, they also absorb part of the polycarboxylic acid. Water-reducing agent will reduce the amount of effective water-reducing agent in the concrete slurry, causing the effect of polycarboxylate water-reducing agent to become worse. The water demand ratio mainly affects the amount of free water in concrete. When the water demand ratio decreases and the formula water remains unchanged, the amount of free water increases, and the amount of admixture needs to be reduced. On the contrary, the amount of admixture should be increased. Generally speaking, Grade I and II fly ash has little effect on polycarboxylic acid, while fly ash of Grade III or above will significantly increase the amount of admixtures, resulting in a severe loss of concrete slump.
Granulated blast furnace slag powder can have a good effect of increasing the initial fluidity of the slurry and reducing fluidity loss. The fluidity ratio parameter of mineral powder has a greater impact on polycarboxylic acid, which is similar to the water demand ratio of fly ash.
3 Precautions for use
a.Water reduction rate Although a high water-reducing rate can significantly save the cost of a single admixture, polycarboxylic acids with high solid content and high water-reducing rate are particularly sensitive to fluctuations in the moisture content and mud content of sand and gravel, as well as the adaptability of adhesive materials. While bringing great challenges to production control, the risk of segregation also becomes difficult to control. At present, there are not many companies that have production lines to detect moisture content in real time and central large-screen videos to control the concrete conditions at each pouring site. There are also not many companies that can well control concrete production under high solid content and high water reduction rate. It is recommended to combine The company's actual situation chooses the water reduction rate range suitable for the company's production.
b. Saturation dosage Due to its high water-reducing properties, polycarboxylic acid admixtures will cause bleeding, slurrying, bottom-grabbing, segregation, etc. when the admixture is added to obtain good fluidity, cohesion, and workability. , the admixture dosage at this time is the saturation dosage. In the saturated state, although excellent working performance can be obtained, it will also increase the risk of concrete segregation . On the premise that the workability of concrete is not significantly affected, a small amount of dosage can generally be artificially deducted to increase the ability to resist concrete segregation.
A large number of experiments have shown that the water-reducing effect of polycarboxylic acid-based high-performance water-reducing agents is highly dependent on its dosage, and as the dosage of cementitious materials increases, this dependence becomes even greater. When the amount of gelling material is the same, the relationship between the water-reducing effect of Polycarboxylic acid-based high-performance water-reducing agent and the amount generally increases with the increase in the amount of water-reducing agent, but the amount of gelling material When the dosage is low, after reaching a certain dosage, the water-reducing effect even "decreases" as the dosage increases. This does not mean that the water-reducing effect decreases when the dosage is increased, but because the concrete at this time has serious segregation and bleeding phenomena, the concrete mixture is hardened, and the fluidity is difficult to reflect by slump.
c. Dosage width Since the water consumption of concrete is greatly reduced after using Polycarboxylic acid-based high-performance water-reducing agent the water consumption of a single concrete is mostly 140 to 175kg/m 3 , and the commonly used water-binder ratio ranges from 0.26 to 0.45 , which can even be reduced to 0.2. At low water usage, small fluctuations in water usage may result in large changes in slump, but will have a small effect on strength. It is precisely because water consumption is sensitive to slump. When testing the slump loss of concrete mixed with polycarboxylic acid-based high-performance water-reducing agent, it is easier to lose water and fluctuate the sand moisture content due to steel plates, tools, evaporation, etc. Causes errors, especially in the case of low slump or low water-binder ratio. In order to overcome the shortcoming of the polycarboxylate water-reducing agent being too sensitive to the water consumption of low-strength grade concrete during use, the concentration of the polycarboxylate water-reducing agent can be reduced to about 10% of the solid content .
Due to the strong sensitivity of polycarboxylic acid admixtures, how to obtain good concrete conditions, such as enhancing the sensitivity of polycarboxylic acid admixtures to concrete raw materials, reducing the difficulty of production control, increasing water retention, plasticity retention and slump retention, has become a difficult problem , so the concept of increasing the content width of polycarboxylic acid admixtures and improving the robustness of concrete has been put on the work agenda by more and more people. Only with a good level of management and control, combined with an in-depth understanding of the characteristics of raw materials, and the selection of more suitable polycarboxylic acid masterbatch and small materials can we prepare admixtures with higher width.
d. Requirements for production control A good level of production control is required, including the operator's due diligence, close cooperation between the tester and the operator, and on-site tracking at the pouring site. Opening appraisal is necessary because normal fluctuations in raw materials exist at any time, and it is difficult to control the state and production of concrete with one dosage over a long period of time to meet production needs. Using opening appraisal combined with on-site tracking measures can maximize production needs and ensure the strength and working performance of concrete. This is a dynamic adjustment process. Polycarboxylic acid is more sensitive to water, and its sensitivity is higher than that of naphthalene grease admixtures. This requires that the sand loaded on the forklift be loaded with sand with a stable water content. At the same time, due to health assessment requirements, drivers should not flush the tanker hopper or reduce the flushing time. The measurement system must be accurate. It is recommended to increase the frequency of calibration of the measurement system to ensure the accuracy of the measurement system.
e. Storage issues When producing and storing polycarboxylic acid-based high-performance water-reducing admixtures, long-term contact with iron materials should be avoided. Since polycarboxylic acid-based high-performance water reducing agent products are weakly acidic, and an initiator is added during the polymerization process, long-term contact with iron materials will cause a slow reaction, causing the molecular weight of the polycarboxylic acid to change. Therefore, the polycarboxylic acid-based high-performance water-reducing agent has a high Performance water-reducing agents should be transported and stored in clean plastic, glass or stainless steel containers instead of iron containers. In high-temperature seasons, polycarboxylic acid-based high-performance water-reducing agents should be placed in a cool place to prevent exposure to the sun; in low-temperature seasons, anti-freeze measures should be taken for polycarboxylic acid-based high-performance water-reducing agents.
f. Compatibility issues with other admixtures At present, some companies use aliphatic or naphthalene ester series and polycarboxylic acid in alternating ways. At this time, you need to pay close attention to the impact of the admixtures in the previous truck of concrete on the subsequent truck of concrete when switching to admixtures. The main effects are on the Residual concrete in weighing scales, production hosts and tank trucks, and the impact of concrete on pump trucks and pouring contact surfaces, including cracks caused by differences in setting time, loss of concrete slump and fluidity, and abnormal increases in production water consumption, etc.
Studies have shown that after compounding polycarboxylic acid and aliphatic additives, the strength of concrete does not decrease significantly, and the crack resistance of aliphatic admixtures can be improved. Other studies have shown that under the same water-cement ratio conditions, the concrete mixture mixed with polycarboxylic acid and aliphatic composite water-reducing admixtures is better than the composite water-reducing agent mixed with polycarboxylic acid and the other two high-efficiency water-reducing agents. The slump is large and the loss over time is small, and as the amount of polycarboxylic acid increases, the slump also increases. When polycarboxylic acid is compounded with amino groups, the greater the polycarboxylic acid content, the smaller the slump loss is. However, when polycarboxylic acid is compounded with naphthalene series, the slump loss increases as the polycarboxylic acid content increases.
Studies have shown that from the perspective of the mutual solubility of the solutions, Polycarboxylic acid-based high-performance water-reducing agentand melamine-based high-efficiency water-reducing agents or sulfamate-based high-efficiency water-reducing agent solutions cannot be mixed together in actual engineering. Without considering the effect of combined use, polycarboxylic acid -based water-reducing agents may be used in combination with lignosulfonate water-reducing agents, naphthalene-based high-efficiency water-reducing agents, and sulfamate-based high-efficiency water-reducing agents; Based on the test results of pure slurry and concrete, the compound blending of polycarboxylic acid-based water-reducing agent and lignosulfonate water-reducing agent or sulfamate-based high-efficiency water-reducing agent has a superimposed effect in terms of plasticizing and reinforcing effects. And on the basis of 0.1% Polycarboxylic acid-based high-performance water-reducing agent, the compound addition of lignosulfonate water-reducing agent and sulfamate-based high-efficiency water-reducing agent can also help improve the slump retention of concrete; poly The compound blending of carboxylic acid-based water-reducing agent with naphthalene-based high-efficiency water-reducing agent, melamine-based high-efficiency water-reducing agent, and sulfamate-based high-efficiency water-reducing agent will weaken its plasticizing effect, and polycarboxylic acid-based water-reducing agent When combined with naphthalene series superplasticizer, it has the greatest negative effect on the plasticizing effect and slump retention.
g. Analysis of abnormal conditions of concrete For concrete using polycarboxylic acid, it is necessary to comprehensively analyze the reasons for the abnormal state of the concrete. Generally, the elimination method is used, that is, the production line takes out the existing adhesive materials and conducts comparative analysis tests on the fluidity of the clean slurry to eliminate the differences between different batches of adhesive materials and different batches. Second, the cause of the admixture; then do an admixture compatibility test to rule out the cause of sand; thirdly, combined with the opening status to determine whether the admixture is insufficient, whether there are metering factors, temperature factors, etc., only by combining the test to find the reasons can we make a decision Targeted adjustments. It also plays a decisive role in preventing similar phenomena from occurring.
4 Conclusion
The above is some experience in the application of polycarboxylic acid admixtures. Although the larger section explains the precautions, it does not mean that the disadvantages of polycarboxylic acid outweigh the advantages. Polycarboxylate Superplasticizer(PCE) have the characteristics of excellent dispersion and large flow state. They also have excellent properties such as low alkali, low chloride ions, low shrinkage, no pollution, low environmental load and obvious economic benefits, and are increasingly used In projects such as high-speed rail, subway, viaduct, airport, nuclear power and other projects, it is clearly stated in the bidding documents that the admixtures used must be polycarboxylic acid-based admixtures. These will inevitably make polycarboxylic acid the inevitable future development of high-efficiency water-reducing agents for concrete,and how to select and use Polycarboxylate Superplasticizer(PCE) according to raw materials has become the top priority to control the quality, performance and cost of concrete.
