Water reducers are a type of high-efficiency admixture that utilizes surfactant technology to reduce cement dosage and enhance the fluidity of concrete mixtures. During construction, due to factors such as varying mix proportions and differences in on-site personnel operation skills, concrete often experiences significant or unstable slump loss, which seriously affects project quality. To ensure project quality, water reducers are currently added during the concrete mixing process. These products offer functions such as slump retention, segregation resistance, and bleeding resistance, improving the workability of concrete and reducing concrete slump loss. The quality of concrete performance is closely related to its slump retention performance; fluid paste greatly promotes concrete slump retention. Therefore, concrete slump retention performance is closely associated with the following factors.
1. Mineral Composition of Cement
1.1 Level of C3A Content
When cement is mixed with water, complex physical and chemical reactions—hydration reactions—occur between cement particles and water molecules. The cement paste forms a flocculent structure, which traps 10% to 30% of the mixing water between cement particles. This trapped water does not participate in the free flow and lubrication of the cement paste, thereby affecting the fluidity of the cement paste.
If a water reducer (cationic surfactant) is added during mixing, the hydrophobic groups of the water reducer are adsorbed onto the surface of cement particles in a directional manner, while the hydrophilic groups point toward the aqueous solution, forming a monomolecular or multimolecular adsorbed film. Due to the directional adsorption of the surfactant, the surface of cement particles carries the same charge and repels each other. This not only keeps the water-cement system in a relatively stable suspended state but also disperses and breaks down the initially formed flocculent structure. As a result, the trapped mixing water is released to participate in the C3A hydration reaction and paste flow. If the cement has a high C3A content, the hydration reaction intensifies, consuming more water and leading to significant concrete slump loss. Therefore, to control concrete slump, the first priority is to control the C3A content of the cement.
1.2 Form and Content of Gypsum
Gypsum is a setting regulator. It releases SO42- to react with C3A, forming ettringite and monosulfoaluminate, thereby regulating the setting time and hardening rate of cement. The form of gypsum has a significant impact on the compatibility between cement and polycarboxylate. The most common types include natural gypsum, FGD gypsum, and PG (phosphogypsum), with the SO42- release rate decreasing in the order: natural gypsum > FGD gypsum > PG. Results show that polycarboxylate has the best compatibility with cement mixed with natural gypsum, and poorer compatibility with cement mixed with FGD gypsum or PG. This is mainly related to the SO42- release rate and content.
In the early stage of hydration, the hydration rate of C3A is fast. Therefore, SO42- is introduced to react with C3A to form ettringite and monosulfoaluminate, which controls the reaction rate of C3A; otherwise, flash setting will occur. If the SO42- release rate is slow (i.e., the amount of SO42- in the paste is low) while C3A hydrates rapidly, the molecular density of the water reducer will decrease, resulting in poor dispersion effect and greater slump loss. Excessively high SO42- content will accelerate the reaction rate between SO42- and C3A, leading to false setting. When the SO42- release amount is equivalent to the C3A content, the cement and water reducer exhibit good adaptability and slump retention performance.
1.3 Alkali Content
Alkali content has a significant impact on adaptability. As alkali content increases, the workability of concrete improves. However, excessive alkali reacts with highly reactive SiO2 in aggregates to form water-soluble alkali silicate gel, causing cracks. At the same time, the plasticizing effect of polycarboxylate weakens, leading to reduced fluidity and a significant shortening of setting time.
Alkali accelerates the dissolution of C3A, and with the participation of gypsum, a certain amount of AFt (ettringite) crystals are quickly formed. These crystals seal the surface of C3A, preventing C3A from directly hydrating to form calcium aluminate and thus improving the fluidity of the paste. When the alkali content is high, a large number of AFt crystals are generated in the early stage, reducing the fluidity of the paste and its adaptability. This results in insufficient water reduction rate, poor plasticizing effect, and significant time-dependent slump loss of the mixture.
1.4 Fineness and Particle Gradation
Finely ground cement accelerates hydration, absorbs more water reducer in the early stage, and reduces the number of free water reducer molecules in the solution, leading to poor dispersibility and dispersion retention. In addition, cement exhibits a flocculation effect, which becomes more pronounced as the cement becomes finer. Therefore, with the same dosage of water reducer, finer cement results in poorer dispersibility and dispersion retention.
If fine cement fineness is pursued, high temperatures in the ball mill cause natural gypsum to lose water, reducing the content of natural gypsum. This worsens the adaptability with the water reducer, intensifies slump loss, and may even lead to false setting.
1.5 Admixtures
Admixtures such as fly ash, slag powder, limestone, zeolite, and coal gangue vary in type, property, and dosage, resulting in differences in the dispersion and plasticizing effects of polycarboxylate on cement.
Fly ash particles are mostly spherical with large pores and a dense vitreous surface. The "ball-bearing effect" of these particles gives concrete good fluidity. Therefore, Grade I fly ash can effectively reduce cement dosage and improve the cohesion, strength, and durability of concrete. However, Grade II fly ash has a stronger adsorption effect on admixtures than cement particles. Therefore, under the action of admixtures, cement paste with a large amount of ordinary fly ash may have acceptable initial fluidity, but over time, fly ash particles absorb more water reducer molecules—especially greater adsorption of air—leading to significant time-dependent slump loss of concrete.
Slag powder particles have irregular angular shapes. After grinding, their shape changes significantly, reducing the contact area with cement and creating water-draining properties, which lowers the absorption of the water reducer. Therefore, replacing cement with an appropriate amount of slag powder can improve the fluidity of the paste and reduce the slump loss of the mixture.
Limestone has a weak adsorption capacity for water reducers. Therefore, incorporating limestone into cement not only improves the compatibility between cement and water reducer but also reduces slump loss. In contrast, admixtures such as zeolite and coal gangue have a strong adsorption capacity for plasticizers in cement, leading to slump loss during cement use.
1.6 Freshness and Temperature
Fresh cement is dry, with a high initial hydration rate, high hydration heat, high water demand, and strong adsorption capacity. These characteristics result in reduced water reduction rate and significant slump loss. Additionally, when the cement temperature is below 70°C, its plasticizing effect remains basically unaffected. When the cement temperature exceeds 80°C, the plasticizing effect of the water reducer decreases significantly. At even higher temperatures, natural gypsum loses water and turns into anhydrite, and abnormal phenomena such as false setting may occur during mixing. This leads to a significant increase in water demand and adsorption capacity, rapid intensification of slump loss, and reduced compatibility between the admixture and cement.
2. Water Reducer Compound Blending
The compound blending of polycarboxylate high-performance water reducers represents the culmination of concrete technology. It involves raw material performance, mix proportion design, understanding of concrete workability, on-site concrete construction control, and comprehensive compound blending of water reducers.
2.1 Performance of Water Reducer Mother Liquor
Typically, the molecular weight, molecular chain structure, and functional groups of the water reducer mother liquor determine its performance. Therefore, mother liquors from different manufacturers exhibit different properties (including slump retention performance). When concrete performance is unsatisfactory, testing mixing with a different mother liquor is a sensible choice.
2.2 Defoaming and Air Entrainment
If the mother liquor is not defoamed, its pH value is low and its adaptability is poor. Large bubbles are unstable and prone to bursting, resulting in significant air loss. This reduces the amount of paste, makes concrete prone to bleeding, and impairs wrapping and fluidity, thereby affecting slump retention performance. Adding an appropriate amount of defoamer to the water reducer can inhibit the formation of bubbles in concrete, break down existing bubbles, and not affect other performance of concrete. This removes internal bubbles, effectively improves the service performance of the mother liquor, enhances the appearance of concrete structures, and increases production efficiency.
Air entraining agents can reduce the interfacial tension of the solid-liquid-gas three-phase system and increase the strength of the foam film. During concrete mixing, they generate many small bubble-like "rolling balls," which reduce friction between aggregates and improve concrete fluidity. If the flow state is not changed, water can be saved. At the same time, since water in concrete is evenly distributed on the surface of bubbles, less water is available for free movement, reducing concrete bleeding and improving water retention and cohesion. Air entraining agents generate fine, uniform microbubbles that remain stable even after concrete hardens. These bubbles can improve the workability of concrete mixtures and enhance the frost resistance, impermeability, and erosion resistance of concrete. In the compound blending of water reducers, polyether-based air entraining agents with saponin as the main component are usually selected.
Huangyin (a type of air entraining agent) can effectively reduce the surface tension of the solution, forming closed and independent bubbles with a large quantity, narrow spacing, long stability time, and low time-dependent loss. It significantly improves the workability of plastic concrete (especially paste lifting and wrapping performance) and enhances the durability of concrete. It is a promising air entraining agent and plays an important role in engineering construction. Polyether-based air entraining agents have good compatibility with polycarboxylate water reducers. They combine the advantages of anionic and non-ionic air entraining agents, exhibit strong foaming ability, and are surfactants that can effectively reduce surface tension and interfacial energy, easily forming closed bubbles with a size of less than 200μm. The molecules of the air entraining agent are adsorbed on the surface of bubbles in a directional manner, forming a relatively strong liquid film that makes the bubbles more uniform and stable. This improves the lubrication and fluidity of concrete and greatly promotes slump retention performance. It also has a good effect on improving the durability and freeze-thaw cycle resistance of concrete and significantly enhances the workability of concrete.
2.3 Paste Lifting
Paste lifting is the core technology of water reducer compound blending, which improves adaptability. Paste lifting is a comprehensive process with various methods and measures, each with different mechanisms, including chemical and physical paste lifting methods. The chemical method utilizes the mechanism of matching C3A and gypsum to form ettringite for paste lifting.
Water reducers achieve paste lifting through defoaming and air entrainment: eliminating unstable large bubbles and introducing closed, independent small bubbles. The large number of small bubbles with narrow spacing increases the amount of concrete paste. By fully utilizing the "micro-bead effect" of bubbles, the wrapping and fluidity of concrete are improved, and the workability of plastic concrete is significantly enhanced. In on-site compound blending of water reducers, Huangyin is often compounded with polyether air entraining agents for paste lifting.
In addition, many retarders such as sodium hexametaphosphate are good dispersants. Belonging to the alkali metal phosphate category, they can enhance and improve the dispersion performance of solid or liquid substances, help disperse cement particles, and prevent agglomeration of dispersed particles, thereby maintaining the stability of the dispersion system and achieving good paste lifting effects. When sodium hexametaphosphate is used as a retarder in compound blending with polycarboxylate water reducers, it not only provides a good retarding effect but also exhibits excellent dispersion performance (especially suitable for concrete with high stone powder content). It promotes concrete paste lifting and slump retention performance and maintains the stability of concrete performance.
2.4 Dosage of Retarder
Under normal circumstances, regardless of spring, summer, autumn, or winter, the dosage of the retarder must ensure that the initial setting time of concrete is approximately 6 hours and the final setting time is approximately 9 hours. For this reason, during the compound blending of the water reducer, the dosage of the retarder is determined based on the maximum temperature, minimum temperature, and average temperature. If the dosage of the retarder is insufficient, it not only affects the setting time of concrete but also impairs the slump retention performance of concrete—especially the slump retention performance within the first 30 minutes. Practice has shown that the slump retention performance of concrete within the first 30 minutes is largely determined by the type and quantity of the mother liquor and retarder.
2.5 Proportion and Dosage of Slump Retention Agent
Concrete slump retention performance occurs in stages: slump retention within the first 30 minutes mainly relies on the properties of the mother liquor and the type and quantity of the retarder; slump retention between 30 and 60 minutes mainly depends on medium-term slump retention agents; and slump retention after 60 minutes mainly relies on long-term slump retention agents. To enhance concrete slump retention performance and save costs, the quantity of the retarder should not be reduced; instead, it can be increased by 5 to 10 kg compared to the normal dosage. This follows the principle of "no compromise on the early stage to avoid gaps in the later stage." Medium-term slump retention agents take over the "baton" of medium-term concrete slump retention and must further consolidate slump retention performance to lay the foundation for later slump retention. Therefore, the dosage of medium-term slump retention agents must not be insufficient. Adding long-term slump retention agents extends the later slump retention performance of concrete. If the medium-term slump retention performance of concrete is excellent, adding an appropriate amount of long-term slump retention agents will ensure good later slump retention performance. The dosage of long-term slump retention agents is approximately one-third of that of medium-term slump retention agents.
3. Conclusion
With the development of the national economy, concrete raw material resources have become increasingly scarce, and their quality has deteriorated. This has led to a sharp decline in water reducer adaptability, poor concrete workability, and difficult construction. Through the analysis of cement mineral composition and water reducer compound blending tests, adjusting the effective components of the water reducer can ensure that the concrete slump retention performance meets construction requirements, while its workability, mechanical properties, and durability comply with design and standard requirements.
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