Polycarboxylate Superplasticizers in Concrete: Addressing Common Challenges and Implementing Effective Solutions
Polycarboxylate superplasticizers (referred to as PCE), as the third generation of water-reducing agents, have become the most widely used in the construction industry worldwide due to their low dosage, excellent dispersion performance, strong designability of functions, and environmentally friendly preparation process. Similarly, the continuous expansion of PCE application fields, the increasing performance requirements of concrete in practical projects, and the complexity and variability of concrete raw materials have led to many technical challenges in the promotion and application of PCE that need to be resolved urgently. In-depth analysis and effective resolution of these technical issues are not only conducive to the further development of PCE but also enhance the stability and durability of concrete quality, ensuring the safe use of buildings or structures, thus holding significant importance.
1. Issues with Poor Quality of Concrete Raw Materials
The promotion and application of concrete admixtures have directly promoted the improvement of concrete performance and the progress of concrete technology. It is precisely because concrete admixtures can effectively solve the issues of concrete performance requirements that concrete engineers have increasingly relied on them, even considering them a "panacea" for all concrete performance problems.
When any problems occur in concrete projects, people first make demands on admixture manufacturers. Due to the limitations of their position in the industrial chain and their professional habits, admixture manufacturers also habitually believe that adjusting the admixture formula and usage can almost solve all problems, but often achieve a half-effort, half-result effect. In fact, most of the fluctuations in the application effects of admixtures are directly related to the fluctuations in the physical properties of concrete raw materials. Ensuring the stable supply of various raw materials for concrete is key to ensuring the quality of concrete.
The impact of cement and various auxiliary cementitious material properties on the effectiveness of water-reducing agents has been reported in many previous literatures. In recent years, with the increasing scarcity of high-quality sand and stone resources, machine-made sand and sand and stone aggregates with high mud content have been widely used in concrete projects. The use of these raw materials poses a huge challenge to the safe and efficient application of PCE. The following will analyze such issues and propose solutions.
1.1 Wide Application of Machine-made Sand
With the increasing scarcity of natural sand resources and the continuous expansion of construction scale in recent years, using machine-made sand to replace natural sand in preparing concrete has become an inevitable trend. "Building Sand" (GB/T14684-2001) refers to artificial sand made from machine-made sand and mixed sand that has been treated to remove soil, implementing the technical requirements and testing methods for artificial sand. Machine-made sand is defined as "rock particles with a particle size of less than 4.75mm, made by mechanical crushing and screening, but does not include particles of soft rock or weathered rock"; mixed sand is defined as "sand made by mixing machine-made sand and natural sand." Due to the characteristics of mechanical crushing and screening, the basic characteristics of machine-made sand are as follows.
(1) Production characteristics. Machine-made sand is produced using local materials or residual materials from the production of coarse aggregates with a sand-making machine. Therefore, the quality of machine-made sand can be artificially controlled by adjusting the sand-making parameters, that is, the fineness modulus, particle shape, and grading of machine-made sand can be adjusted and improved, which is the essential difference between machine-made sand and natural sand. Similarly, due to different ore sources of machine-made sand in various regions and different equipment and processes for producing machine-made sand, the particle shape and grading of the produced machine-made sand will be quite different.
(2) Appearance characteristics. Natural sand usually appears yellow, and high mud content is not easy to see, while machine-made sand usually appears grayish-white or black, with sharp particles. About 10% stone powder content of machine-made sand produced by dry method looks like it is entirely stone powder, which makes people feel doubtful and reluctant to use it.
(3) Stone powder content. During the production process of machine-made sand, a certain amount of stone powder is inevitably produced, which is normal and one of the most obvious differences between machine-made sand and natural sand. The specific definition of stone powder is the powdery substance with a particle size of less than 75μm formed during the processing of machine-made sand after soil removal, and its mineral composition and chemical composition are exactly the same as the parent rock being processed. Although both are particles with a particle size of less than 75μm, the composition of stone powder is completely different from the mud in natural sand. Mud in natural sand is harmful to concrete and must be strictly controlled. However, an appropriate amount of stone powder in machine-made sand is beneficial to concrete, and the presence of an appropriate amount of stone powder can make up for the poor workability of machine-made sand in preparing concrete. At the same time, the introduction of stone powder is beneficial to improve the fine aggregate grading of concrete (in this regard, natural sand is limited by its formation form, and its fine grading part is not perfect), improve the compactness of concrete, and thus play a role in improving the comprehensive performance of concrete.
(4) Fineness degree. At present, machine-made sand is basically medium-coarse sand, and the fineness modulus is generally in the range of 3.0 to 3.7. If the fineness modulus is too large, there will be too many coarse particles, too few particles less than 300μm, unreasonable grading, and poor workability of concrete. If the fineness modulus is too small, there will be too much stone powder less than 75μm, which may increase the water consumption of concrete, reduce strength, and increase shrinkage. In other words, the change in the content of stone powder in machine-made sand varies with the fineness modulus. The smaller the fineness modulus, the higher the stone powder content, and vice versa, the larger the fineness modulus, the lower the stone powder content.
(5) Grading condition. From the statistical results of the particle composition, there are too many particles in machine-made sand greater than 2.36mm and less than 0.15mm, and too few intermediate particles (especially 0.3mm to 1.18mm), sometimes a certain particle grade is missing. Generally speaking, the grading of machine-made sand can only basically meet the technical requirements of natural sand I area or 11 area sand.
(6) Particle shape. Machine-made sand, due to its mechanical crushing, is mostly triangular or rectangular (some have more flaky particles), with rough surfaces and sharp edges, which is beneficial for the bonding of aggregates and cement, but not conducive to the workability of concrete, especially for concrete with lower strength grades, it can cause serious bleeding phenomena. However, the presence of an appropriate amount of stone powder can make up for this deficiency to a certain extent.
In summary, machine-made sand, as a substitute for natural sand, has its own advantages and disadvantages. When preparing concrete, the following points should be noted.
(1) Pay attention to the design of the concrete mix ratio of machine-made sand. Due to the poor grading of machine-made sand, problems such as large slump loss, bleeding of the mixture, and high concrete pumping pressure are easy to occur when preparing concrete. By adjusting the concrete mix ratio and improving the particle grading, the above problems can be improved. Sometimes, to solve the bleeding problem, it is also possible to consider using air-entraining agents or viscosity regulators and other technical measures.
(2) Strictly control the MB value of machine-made sand. The MB value of machine-made sand is an important index to determine whether the fine powder contained in the machine-made sand is stone powder or mud powder according to the national standard. In practical projects, people often think that a higher content of stone powder will affect the effect of water-reducing agents, especially the effect of PCE, but in fact, it is the mud powder contained in the machine-made sand that is harmful to the effect of PCE. Some experts have studied the impact of six types of stone powder from quartzite, gneiss, granite, basalt, limestone, and marble salt on the effect of water-reducing agents, and their research results show that stone powder does have an adverse effect on the effect of water-reducing agents, especially on the effect of PCE, but the overall impact is small. When using machine-made sand to prepare self-compacting concrete of C50, it was found that even for self-compacting concrete with a stone powder content of 17%, appropriately increasing the dosage of PCE will significantly reduce the pumping pressure. This shows that by appropriately adjusting the dosage of water-reducing agents, the adverse impact of stone powder on its dispersion effect can be resolved.
(3) Pay attention to the impact of high stone powder content in machine-made sand on the durability of concrete. At present, most of the research on the impact of stone powder content in machine-made sand on concrete performance is still on its workability and strength, and the research on the impact of stone powder content on the durability of concrete needs to be further in-depth. Some experts' research results show that appropriately increasing limestone powder can improve the grading of machine-made sand, increase the amount of paste, and fill the gaps between particles, thereby improving the workability of machine-made sand concrete. However, for concrete of different strength grades, the optimal content of stone powder in machine-made sand is different. At the same time, an appropriate amount of stone powder is beneficial to improve the performance of machine-made sand concrete in terms of chloride penetration resistance, freeze-thaw resistance, sulfate erosion resistance, and wear resistance. However, when the stone powder content in machine-made sand exceeds 7% to 10%, it will not be conducive to controlling the plastic shrinkage and drying shrinkage of concrete.
1.2 High Mud Content in Sand and Stone Aggregates
In the actual engineering application process, concrete companies often require admixture companies to use admixture technology to solve the problem of poor workability of concrete mixture caused by high mud content in sand and stone. This issue has attracted widespread attention in both the engineering and academic communities. Many people believe that using admixture technology to solve the problem of poor workability caused by high mud content in sand and stone is a false proposition, because even if the inconvenience of construction caused by high mud content in sand and stone is solved by admixture technology, the large amount of soil in the concrete will bring great harm to the strength and durability of the hardened concrete. Sand and stone, as the largest amount of raw materials in concrete, have a significant impact on the performance of concrete. Chinese standards have clear constraints on the mud content of sand and stone, but sand and stone companies, in order to save costs, do not wash the sand and stone with high mud content as required. Although some experts insist that the problem of high mud content should be first strictly controlled by the sand and stone companies to ensure that the quality of sand and stone meets the requirements of the concrete mix for the mud content index, the following analyzes the mechanism of clay inhibiting the dispersion performance of PCE and introduces some admixture measures to improve the dispersion of sand and stone with high mud content.
1.3 Mechanism of Clay Inhibiting the Dispersion Performance of PCE
Clay is mainly composed of layered silicate minerals, which have a variety of types, complex structures, and significant performance differences. The minerals commonly contained in clay include montmorillonite, kaolinite, feldspar, muscovite, illite, and attapulgite, etc. Many scholars have studied the inhibitory effect of different clay minerals on the dispersion performance of PCE and found that montmorillonite has the most significant inhibitory effect on the dispersion performance of PCE. When sand and stone contain about 2% montmorillonite, it will have a significant adverse effect on the workability of the concrete mixture, and feldspar, kaolinite, and muscovite will also have a significant adverse effect on the workability of the mixture when the content reaches 4% to 5%. The reason for the significant inhibitory effect of montmorillonite minerals on the dispersion performance of PCE is the strong variability of the interlayer spacing of montmorillonite minerals. The research results of Wu Hao and others show that the calcium sodium bentonite with montmorillonite as the main component has a layer spacing d(001) of only 1.093nm in a completely dehydrated state, and the d(001) value after adsorbing water and reaching equilibrium at a relative humidity of 50% to 60% is 1.443nm, and the d(001) value after adsorbing PCE and drying naturally to the same state is 1.863nm, indicating that PCE has entered the interlayer of montmorillonite. However, because the overall spatial size of the PCE molecule is much larger than the interlayer spacing of montmorillonite after water absorption and swelling, PCE is not adsorbed into the interlayer as a whole, but is intercalated and adsorbed in the form of side chains being adsorbed between the clay layers, causing the dispersion performance of PCE to fail. The research results are consistent with those of Ng S. and others. Ng S. and others have even provided a schematic diagram of the intercalation and adsorption of the side chains of PCE between the layers of montmorillonite.
In summary, the main reasons for the poor workability of concrete mixture caused by high mud content in sand and stone are: first, the volume expansion of clay minerals after absorbing water and intercalating and adsorbing PCE, resulting in an increase in the total "solid phase" volume fraction in the system; second, the reduction of liquid phase volume fraction; third, the consumption of PCE by intercalation adsorption between clay layers, reducing the effective PCE in the liquid phase that plays a dispersion role.
1.4 Measures to Improve the Effect of PCE on Concrete Made with Sand and Stone with High Mud Content
To improve the effect of PCE on concrete made with sand and stone with high mud content, the following measures are often taken.
(1) Increase the dosage of water-reducing agents. This method is the simplest and most effective, but because the dosage of water-reducing agents often needs to be increased by one or even two times to show effects, the cost of water-reducing agents per unit of concrete is relatively high, which is usually difficult to be accepted by concrete companies. Moreover, for sand and stone with high mud content, and the clay is mainly montmorillonite, the effect is limited.
(2) Add inhibitors to reduce the consumption of PCE by clay. These inhibitors are usually cationic polymers with a straight chain, which are more easily adsorbed on the surface of clay compared to PCE, thereby reducing the intercalation adsorption of PCE on clay.
(3) Change the concrete preparation process. Engineering technicians have proposed that when mixing concrete, it is possible to first mix and stir the cementitious materials, PCE, and mixing water (1 to 2 minutes), and then add sand and stone to continue stirring, which is conducive to reducing the impact of sand and stone with high mud content on the dispersion effect of PCE. However, in actual production, this process may have some inconveniences, especially extending the mixing time of concrete.
2. Problems with Unreasonable Concrete Mix Proportion Design
Concrete is an artificial engineering material made by mixing cementitious materials, aggregates, and water. It is a typical granular suspension system, and the mixture reaches a more uniform state through the mutual stacking and filling of materials of different particle sizes, thereby ensuring the excellent mechanical properties and durability of concrete after hardening. However, if the concrete mix ratio is unreasonable, it will cause problems such as segregation and bleeding of the mixture, poor slump retention, and high air content in the mixture, which also "affects" the effect of PCE on the surface.
2.1 The Impact of Mix Ratio on System Viscosity
The famous rheological Kireger-Dougherty (K-D) formula indicates that the viscosity of a suspension dispersion system is mainly affected by the viscosity of the medium, the volume fraction of the dispersed phase, and the maximum close packing volume fraction of the dispersed phase.
It can be seen that it strongly depends on the particle size distribution. For two suspension dispersion systems with the same solid volume fraction, the system with a higher maximum close packing volume fraction will have a smaller viscosity.
The above view can be expressed in the concrete mixture system as follows: optimizing the concrete mix ratio is conducive to improving its maximum close packing volume fraction. Under the condition of adding an equal amount of mixing water, the concrete mixture with a good mix ratio will have a lower system viscosity, thus having good workability.
2.2 The Impact of Mix Ratio on System Stability
A good concrete mix ratio is not only conducive to reducing the viscosity of the concrete mixture and improving the workability of the concrete but also conducive to improving the stability of the concrete mixture. According to Stokes' formula, when the viscosity of the liquid is determined, the sedimentation rate of the sphere increases with the increase of the sphere radius. For the concrete mixture, the particle sizes of coarse aggregate, fine aggregate, and cementitious materials decrease in turn, and the particle size of the cementitious materials is much smaller than the former two, so the sedimentation rate of the three materials in the mixing water gradually decreases.Similarly, when the aggregate grading is unreasonable, the gaps between aggregates will inevitably be filled by cementitious materials or mixing water. When the amount of cementitious material is high, it is not only uneconomical but also easy to cause an increase in the system viscosity. If the mixing water amount is increased, it will increase the trend of bleeding in the system and reduce the strength of the concrete. Therefore, a reasonable concrete mix ratio is of great significance to the stability of the concrete mixture.
3. Problems with Poor Slump Retention of Concrete
The slump retention of concrete is of great significance for the promotion and application of commercial concrete, as commercial concrete needs to meet the construction slump requirements after a long time of transportation to the construction site. In addition, ultra-high and long-distance pumping also puts forward very high requirements for the slump retention of concrete. Taking the Shanghai Center as an example, the extreme height of concrete pumping reached 606m, and the concrete needs to maintain good workability in the pumping pipeline. Once the slump is lost, it is very easy to cause blockage of the pump, which will affect the construction process. This poses a great challenge to the slump retention performance of water-reducing agents.
3.1 Reasons for the Loss of Concrete Slump
(1) Rapid consumption of internal water in the concrete mixture. Free water in the concrete mixture has an important impact on the workability of the paste, and the hydration of cement and evaporation of water will both cause a reduction in free water, leading to a decrease in the distance between particles inside the paste, an increase in paste viscosity, and ultimately a decrease in concrete slump.
(2) Rapid consumption of water-reducing agent molecules in the concrete mixture. The forms of water-reducing agents in the mixture mainly include: first, water-reducing agents that are interspersed in the cement hydration products or form organic mineral phases with the hydration products, whose function is to change the morphology of the hydration products, but do not play a dispersion role; second, water-reducing agents adsorbed on the surface of cement particles, which directly play a dispersion role; third, water-reducing agents remaining in the paste pore solution, which maintain a dynamic balance with the second part of the water-reducing agents, continuously supplementing the second part of the water-reducing agents consumed by cement hydration, and playing an important role in maintaining the dispersion performance of the water-reducing agents.
3.2 Measures to Solve the Problem of Excessive Slump Loss of Concrete
(1) Reduce water evaporation. In the production, transportation, and pumping of commercial concrete, the direct exposure of concrete should be minimized to reduce the evaporation of free water.
(2) Slow down cement hydration. Using cement with lower C3A content, lower alkali content, appropriate gypsum/C3A ratio, and appropriate specific surface area (referring to not exceeding 360m2/kg) is an important measure to slow down cement hydration and improve the slump retention of concrete. In addition, the compound addition of retarders is also an important means to improve the slump retention of concrete. For concrete construction in high-temperature environments in summer, it is even necessary to lower the temperature of the concrete mixture by adding ice cubes and other methods, thereby reducing the cement hydration rate and improving the slump retention of the mixture.
(3) Reasonable use of mineral admixtures. The use of mineral admixtures is not only beneficial to improve the durability of concrete, but also reduces the amount of cement used, and is conducive to optimizing the particle size distribution of cementitious materials, thus helping to improve the slump retention of the concrete mixture.
(4) Choose slump-retaining PCE. As mentioned earlier, retarders are an effective means to improve the slump retention of concrete, and their use in combination with air-entraining agents often achieves better results, but they usually have a certain impact on the strength development and final strength of concrete. At present, choosing slump-retaining PCE is an important means to improve the slump retention of the concrete mixture.
4. Problems with Slow Early Strength Development of Concrete
With the rapid advancement of residential industrialization in various countries, the demand for concrete precast components is increasing. Therefore, improving the rate of early strength development of concrete can accelerate the turnover rate of molds, thereby improving the production efficiency of concrete precast components. Using PCE to prepare concrete precast components can improve the appearance quality of the components, and due to the excellent dispersion performance of PCE, using it in the production of high-strength precast components can give full play to its performance and cost advantages, so the application prospect is broad.
The compounding technology of admixtures has always been one of the simplest and most effective methods to solve concrete problems. The commonly used concrete early strength agents can be divided into three categories: inorganic salt type, organic type, and composite type early strength agents. Among them, inorganic salt type early strength agents include chlorides, sulfates, lithium salts, and calcium salts, etc., and the organic type is mainly alcohol amines, which provide many compounding options for composite type early strength agents. Compounding technology can usually achieve good early strength effects, but its disadvantages are also obvious. For example, chloride type early strength agents have a greater risk of corrosion to steel reinforcement, sulfate type early strength agents are prone to crystallization at low temperatures, and the dosage of triethanolamine early strength agents is not easy to control, etc. When PCE is compounded with early strength agents, there are also problems with poor solubility, and early strength agents usually reduce the dispersion effect of PCE.
Some experts have studied the impact of comb-like copolymers with different functional groups on cement hydration, and their experimental results show that the adsorption of negatively charged copolymers on the cement surface is significantly greater than that of positively charged copolymers, and the adsorption ability of -COO- is significantly higher than that of -SO3-. This is mainly because the adsorption of -COO- on the cement surface is driven by both electrostatic attraction and complexation, while the adsorption of -SO3- on the cement surface is driven only by electrostatic attraction. Cationic copolymers only slightly affect cement hydration, but -COO- significantly delays cement hydration, and the effect of -SO3- in delaying cement hydration is relatively weaker than that of -COO-. The mechanism of anionic groups delaying cement hydration is that the higher adsorption amount significantly reduces the diffusion rate of cement mineral surface particles and water, and the complexation of anionic groups with calcium ions greatly inhibits the nucleation of hydration products. It can be seen that the anionic groups on the main chain of PCE all have a delaying effect on cement hydration. Therefore, increasing the number of -SO3- and cationic groups on the main chain of PCE helps to improve the early strength effect of PCE, but it usually reduces the dispersion effect of PCE. In addition, increasing the side chain length of PCE is also conducive to improving the early strength effect of PCE, which may be related to the reduction of the mass percentage of anionic groups on the main chain of PCE caused by long side chains.
4.3 Application of Nucleating Early Strength Agents
Professor Plank from Germany once proposed at the "Polycarboxylate Superplasticizers and Application Technology Exchange Meeting" that nano-scale C-S-H seed crystals can be prepared under the condition of PCE existence by using calcium nitrate, sodium silicate, and nitric acid. These substances added to the cement paste help to reduce the generation barrier of C-S-H gel, promote the generation of C-S-H gel, consume Ca2+ and silicate ions in the cement paste, and further promote the dissolution of cement mineral phases, accelerating the early hydration of cement.
5. Problems with High Air Content in Concrete Mixture with PCE
As a surfactant, the hydrophilic side chain in the molecular structure of PCE has a strong air-entraining property. That is, PCE reduces the surface tension of the mixing water, making it easy to introduce and form bubbles of uneven size and easy to aggregate during the mixing process of concrete. If these bubbles are not expelled in time, they will affect the appearance quality of concrete and even pose a threat to the strength of concrete, so it should be paid enough attention.
5.1 Reasonable Selection of Raw Materials and Standardization of Construction Methods
Attention should be paid to the adaptability of PCE with cement and mixed materials/admixtures to avoid bubbles caused by poor adaptability. At the same time, attention should be paid to the rationality of the mix ratio design, as excessive cementitious material, large sand ratio, insufficient water consumption, and thickening components in the admixtures will cause the newly mixed concrete to have a high viscosity, which is not conducive to the expulsion of harmful bubbles. Finally, the vibration intensity and time should be strictly controlled.
5.2 Use of Defoamers and Air-Entraining Agents for 'Defoaming Before Entraining' Treatment of Concrete
Some studies have shown that after treating concrete with suitable defoamers and PCE-specific air-entraining agents, the air content loss in the concrete is small and the bubble structure is good. The requirements for defoamers and air-entraining agents are as follows:
When defoamers are used in combination with PCE, defoamers that are insoluble in PCE and can stably exist in the system should be selected. According to the specific preparation process of the mixing station, oil-type, emulsified, dissolved, and solid defoamers can be selected. In addition, the dosage of defoamers should be reasonably controlled. If the dosage of defoamers is too much, it will cause a large reduction in the beneficial bubbles in the newly mixed paste, leading to poor workability and rapid loss of fluidity of the concrete mixture. Air-entraining agents generally need to use PCE-specific air-entraining agents, and their dosage should also be strictly controlled. If the air-entraining agent is selected improperly or the dosage is too large, and too many small bubbles are introduced, it is likely to form large connected bubbles again, resulting in an increase in the air content and bubble diameter of concrete, forming serious defects, and affecting the strength and durability of concrete.
There is a problem of poor solubility between PCE and defoamers in solving the problem of high air content in concrete by compounding defoamers with PCE. Therefore, directly synthesizing low air-entraining PCE through the design of PCE molecular structure can solve the problem of high air content in concrete from the source. Zheng Gang used methoxy EO/PO block polyether with defoaming function to esterify with acrylic acid, first synthesizing a methoxy EO/PO block polyether acrylate macromonomer with polymerizability and low air-entraining function, and then copolymerizing the above macromonomer with acrylic acid, polyoxyethylene acrylate, and isobutyl acrylate by free radical polymerization, successfully preparing a low air-entraining PCE. The concrete prepared with this PCE has the advantages of low air content and high compressive strength.
6. Problems with Poor Workability of Fresh Concrete
The workability of fresh concrete includes fluidity, cohesion, and water retention. Fluidity refers to the ability of the concrete mixture to flow and fill the mold evenly and densely under its own weight or mechanical vibration. Cohesion refers to the certain cohesive force between the various components of the concrete mixture, which can prevent stratification and segregation during construction. Water retention refers to the ability of the concrete mixture to retain water and prevent bleeding during construction. In the actual preparation process of concrete, on the one hand, for low-strength concrete, the amount of cementitious material is not high, and the water-cement ratio is relatively large. In addition, the aggregate grading of this type of concrete is usually poor. Using high water reduction rate PCE for the preparation of this type of concrete is prone to segregation and bleeding of the mixture; on the other hand, high-strength concrete prepared by using low-strength grade cement, increasing the amount of cementitious material, and reducing the water-cement ratio is prone to high viscosity of the concrete, poor fluidity of the mixture, and slow flow speed. Therefore, the viscosity of the concrete mixture is too low or too high, which will lead to poor workability of concrete, reduce construction quality, and is very unfavorable to the mechanical and durability performance of concrete.
6.1 Increasing the Viscosity of Concrete Mixture
Viscosity Modifying Agent (VMA) is an admixture used to increase the viscosity of the concrete mixture. It can significantly improve the cohesion and stability of the cement-based cementitious material system. K.H. Khayat classifies commonly used viscosity modifiers into the following categories.
(1) Water-soluble synthetic and natural organic polymers. This type of viscosity modifier mainly includes xanthan gum, warm wheel gum, cellulose ethers, polyoxyethylene, polyacrylamide, and polyvinyl alcohol, etc., which can increase the viscosity of the mixing water.
(2) Water-soluble organic flocculants. This type of viscosity modifier mainly includes styrene copolymers containing carboxyl groups, polymeric high molecular weight electrolytes, and natural tree gums, etc., which adsorb on the surface of cement particles and increase the viscosity of the paste by enhancing the interaction between cement particles.
(3) Organic latex materials. This type of viscosity modifier mainly includes acrylic emulsions and water-soluble clay dispersants. They can enhance the interaction between cement particles and provide additional ultra-fine latex particles for the cement paste system.
(4) Hygroscopic and swelling inorganic materials. This type of viscosity modifier mainly includes bentonite, silica fume, and ground asbestos, etc. These viscosity modifiers have a large specific surface area, greatly enhancing the water retention ability of the paste.
(5) Non-hygroscopic and non-swelling inorganic materials. This type of viscosity modifier mainly includes fly ash, lime powder, metakaolin, limestone powder, and diatomite, etc. These viscosity modifiers also have a high specific surface area, which can increase the content of fine particles in the paste.
Regarding the mechanism of action of viscosity modifiers, many scholars have conducted research and analysis. Many studies believe that viscosity modifiers improve the stability of concrete (anti-bleeding, anti-segregation) through a combination of various mechanisms, while reducing its fluidity (increasing shear stress and plastic viscosity). The main mechanisms of action of viscosity modifiers can be summarized in the following three aspects:
First, water retention effect. The hydrophilic long side chains in the molecular structure of viscosity modifiers adsorb and fix free water molecules, and the swelling effect causes their apparent volume to increase, thus increasing the viscosity of the mixing water.
Second, interaction and entanglement between polymers. The side chains of adjacent viscosity modifiers attract each other, leading to the formation and entanglement of a gel network structure, which prevents the migration of free water and increases the viscosity of the entire system.
Third, the interaction between polymers and cement particles. Polymers in the paste solution adsorb on the surface of cement particles, causing an increase in particle size and an increase in the resistance to the movement of polymer chains. In addition, higher concentrations can lead to bridging between cement particles, forming a rigid network structure.
6.2 Reducing the Viscosity of Concrete Mixture
The biggest disadvantage of high-strength concrete in terms of workability is that its mixture viscosity is often too high, which is not conducive to the construction of high-strength concrete. The common measures to reduce viscosity in actual projects include the following.
(1) Use high-quality fly ash. High-quality fly ash contains a large number of small spherical glassy substances, which can play a good roller effect, thereby reducing the viscosity of the mixture. The particle diameter of ultra-fine fly ash is about 2μm, and it is a good measure to reduce the viscosity of concrete mixture when mixed with silica fume (particle diameter about 0.2μm) and cement, ordinary particle size fly ash, slag powder, etc., when preparing high-strength concrete and ultra-high-strength concrete.
(2) Use an appropriate amount of air-entraining agents. Air-entraining agents can significantly increase the content of tiny bubbles in the concrete mixture. These tiny bubbles occupy the space of free water in the paste, allowing more free water to exist in the gaps between particles, improving the lubricating effect of water. At the same time, the introduced tiny bubbles also play a good roller effect, thereby reducing the viscosity of the mixture.
(3) Development of viscosity-reducing PCE. At present, the mechanism of action of viscosity-reducing PCE is not clear, and most explanations are that this type of PCE can reduce the binding of its hydrophilic groups to free water in the paste, allowing more free water to participate in the lubrication between particles, thus playing a viscosity-reducing effect. However, many researchers have questioned this, believing that it may be because this type of PCE increases the content of beneficial bubbles in the paste, thereby reducing the viscosity of the mixture. A.Lange and others believe that the hydrophilic-lipophilic balance (HLB) value of PCE molecules determines the viscosity-reducing effect of PCE. PCE with a higher HLB value has higher hydrophilicity, and the concrete mixture prepared by it has lower viscosity. At the same time, A. Lange and others also pointed out that APEG typePCE has the best viscosity-reducing effect, while the viscosity-reducing effects of IPEG type PCE and MPEG type PCEare weakened in turn.
(4) Other measures. At the "Sixth Polycarboxylate Superplasticizers and Application Technology Exchange Meeting", the research results of Professor Plank's team pointed out that non-ionic polymer small molecules, although they do not have adsorption properties, improved the dispersion effect of PCE under low water-cement ratio conditions (<0.30). That is, non-ionic small molecules can act as co-dispersed agents to play a dispersion role together with PCE. These small molecules dissolve in the cement pore solution, providing a lubricating effect as a separating agent, keeping cement particles apart and preventing particle flocculation. These small molecule polymers provide a possibility for viscosity reduction of high-strength concrete, but their effect still needs further experimental verification.
At present, the share of PCE in the Chinese admixture market has far exceeded that of several other water-reducing agents, and people have high hopes for PCE, expecting this "high-performance" water-reducing agent to solve all technical problems of concrete. However, things often go contrary to expectations, and PCE encounters more technical problems in actual application, including biases in the understanding of concrete admixtures by concrete engineering technicians, as well as a decline in the quality and stability of concrete raw materials. We should be committed to correctly analyzing the problems and solving these technical issues step by step from a technical level, which is a major responsibility of researchers, developers, producers, and applicators of concrete admixtures.
