July 30, 2026
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1 Mineral Composition of Cement
1.1 C3A Content Levels
When cement is mixed with water, cement particles undergo complex physical and chemical reactions—hydration—forming a flocculent structure within the cement paste. This structure traps 10%–30% of the mixing water between cement particles, preventing it from contributing to the paste’s free flow and lubrication properties, thereby affecting the paste’s fluidity.
When a water-reducing agent (cationic surfactant) is added during mixing, its hydrophobic groups orientally adsorb onto the cement particle surfaces, while the hydrophilic groups face the aqueous solution, forming a monolayer or multilayer adsorption film. This directional adsorption causes cement particles to carry like charges and repel each other. This not only maintains the water-water system in a relatively stable suspension state but also disperses and breaks down the initially formed flocculent structures. Consequently, the entrapped mixing water is released to participate in the C3A hydration reaction and slurry flow. When cement contains higher C3A levels, accelerated hydration consumes more water, leading to significant slump loss in concrete. Therefore, controlling cement C3A content is paramount for maintaining concrete slump.
1.2 Gypsum Form and Content
Gypsum acts as a setting regulator by releasing SO42- to react with C3A, forming calcium aluminate hydrates and calcium aluminate monosulfate. This process controls cement setting time and hardening rate. Gypsum form significantly impacts compatibility between cement and polycarboxylate. The most common types include raw gypsum, FGD gypsum, and PG. Their SO42- release rates, from highest to lowest, are: raw gypsum, FGD gypsum, and PG. Results indicate that polycarboxylate has the best compatibility with cement blended with raw gypsum, while compatibility with FGD gypsum and PG is poorer. This is primarily related to the release rate and content of SO42- .
During the initial hydration stage, the rapid hydration rate of C3A necessitates controlling its reaction with SO42- through the formation of calcium aluminate hydrate (CAH) and calcium monosulfate (CaSO4·2H2O). Failure to do so may cause flash setting. If the release rate of SO42- is slow—meaning low SO42- content in the paste—but C3A hydrates rapidly, the molecular density of the water-reducing agent decreases. This reduces its dispersing effect, leading to greater slump loss. Excessively high SO42- content accelerates its reaction with C3A, causing false setting. When SO42- release matches C3A content, cement and water-reducing agents exhibit optimal compatibility and slump retention.
1.3 Alkali Content
Alkali content significantly impacts compatibility. Higher alkali levels enhance workability; however, excessive alkali reacts with highly reactive SiO2 in aggregates to form water-soluble alkali-silicate gel, causing cracking. Simultaneously, the plasticizing effect of polycarboxylates weakens, reducing flowability and drastically shortening setting time.
Alkali accelerates C3A dissolution and, with gypsum participation, rapidly generates a certain amount of AFt crystals. These crystals seal the C3A surface, preventing direct C3A hydration into calcium aluminate and thereby enhancing paste flowability. At high alkali levels, excessive AFt crystallization occurs early on, reducing paste flowability and adaptability. This leads to insufficient water reduction, poor plasticizing effects, and significant slump loss over time in the mixture.
1.4 Fineness and Particle Gradation
As cement becomes finer, hydration accelerates, absorbing more water-reducing agent in the early stages. This reduces the concentration of free water-reducing agent molecules in solution, impairing dispersion and dispersion retention. Additionally, cement exhibits flocculation effects, which intensify with finer particle size. Therefore, at consistent water-reducing agent dosages, finer cement demonstrates poorer dispersion and dispersion maintenance.
Pursuing excessive cement fineness can cause high temperatures in ball mills, leading to dehydration of gypsum and reduced gypsum content. This diminishes compatibility with water-reducing agents, exacerbates slump loss, and may even cause false setting.
1.5 Blending Materials
The variety, properties, and dosage of blending materials—such as fly ash, slag powder, limestone, zeolite, and coal gangue—differ significantly. Consequently, the dispersion and plasticizing effects of polycarboxylate on cement also vary.
Fly ash particles are predominantly spherical with large pores and a dense glassy surface layer. Their rolling action enhances concrete flowability, enabling Type I fly ash to effectively reduce cement dosage while improving cohesion, strength, and durability. However, Type II fly ash exhibits greater adsorption of admixtures than cement particles. Consequently, under admixture influence, cement paste with high ordinary fly ash content may exhibit acceptable initial flowability. Yet over time, fly ash particles absorb more water-reducing agent molecules—particularly gas—causing significant slump loss in concrete.
Slag powder particles exhibit angular irregular shapes. After grinding, their morphology changes significantly, reducing the contact area with cement and enhancing drainage properties, thereby decreasing absorption of water-reducing agents. Thus, replacing an appropriate amount of cement with slag powder improves paste flowability and reduces slump loss in the mixture.
Limestone exhibits weak adsorption of water-reducing agents. Incorporating it into cement not only improves compatibility between cement and water-reducing agents but also reduces slump loss. Additionally, materials like zeolite and coal gangue strongly adsorb plasticizers in cement, causing slump loss during use.
1.6 Freshness and Temperature
Fresh cement is dry, exhibits high initial hydration rates, generates substantial hydration heat, requires more water, and possesses strong adsorption capacity. This leads to reduced water-reduction rates and significant slump loss. Furthermore, when cement temperature is below 70°C, its plasticizing effect remains largely unaffected; When cement temperature exceeds 80°C, the plasticizing effect of water-reducing agents significantly diminishes. At higher temperatures, gypsum hydrate dehydrates into anhydrous gypsum, potentially causing abnormal phenomena like false setting during mixing. Water demand and adsorption capacity both increase markedly, leading to rapid slump loss and reduced compatibility between admixtures and cement.
- Admixture Blending
The blending of polycarboxylic acid high-performance water-reducing agents represents the pinnacle of concrete technology, encompassing raw material properties, mix design, understanding of concrete workability, on-site concrete construction control, and comprehensive admixture blending.
2.1 Properties of Admixture Concentrate
Typically, the molecular weight, molecular chain structure, and functional groups of the mother liquor determine its properties. Consequently, mother liquors from different manufacturers exhibit varying characteristics (including slump retention). When concrete performance is suboptimal, switching to a different mother liquor for trial mixing is a prudent approach.
2.2 Defoaming and Air Entrainment
An undefoamed mother liquor with low pH exhibits poor adaptability; Large bubbles are unstable and prone to rupture, leading to significant air loss (i.e., reduced paste volume), concrete bleeding, and diminished wrapping and flow properties, which in turn affect slump retention. Adding an appropriate amount of defoamer to the water-reducing agent suppresses bubble formation in concrete and disrupts existing bubbles without affecting other concrete properties. This removes internal air bubbles, effectively improving the masterbatch’s performance, enhancing the concrete’s physical appearance, and increasing production efficiency.
Air-entraining agents reduce interfacial tension between solid, liquid, and gas phases, increasing foam film strength. During concrete mixing, this generates numerous fine, ball-like air bubbles that decrease friction between aggregates and enhance concrete workability. Without altering flow characteristics, this approach conserves water usage. Simultaneously, since water in concrete is evenly distributed across bubble surfaces, less water remains freely mobile. This reduces bleeding, improves water retention, and enhances cohesion. Air-entraining agents generate fine, uniform microbubbles that persist even after concrete hardens. These bubbles enhance the workability of concrete mixes and improve the concrete’s freeze-thaw resistance, permeability resistance, and erosion resistance. When compounding water-reducing agents, polyether air-entraining agents primarily composed of saponins are typically selected.
Yellow air-entraining agents effectively reduce solution surface tension, forming numerous, tightly spaced, stable bubbles with minimal loss over time. They significantly enhance the workability of plastic concrete (particularly slurry wrapping performance) while improving durability, making them a highly promising agent with significant engineering applications. Polyether air-entraining agents exhibit excellent compatibility with polycarboxylate superplasticizers. Combining the advantages of both anionic and nonionic air-entraining agents, they possess strong foaming capabilities. As surfactants that effectively reduce surface tension and interfacial energy, they readily form numerous closed bubbles smaller than 200μm. Air-entraining agent molecules adsorb directionally on bubble surfaces, forming a robust liquid film that stabilizes bubbles uniformly. This enhances concrete lubrication and flowability, significantly improving slump retention. It effectively boosts concrete durability and freeze-thaw resistance while markedly improving workability.
2.3 Slurry Enhancement
The core technology in compounding water-reducing agents is slurry enhancement, which improves adaptability. Slurry enhancement is multifaceted, involving various methods and mechanisms—both chemical and physical approaches exist. The chemical method leverages the compatibility between C3A and gypsum to generate calcium aluminate hydrate (CAH) for slurry enhancement.
Water-reducing agents achieve slurry enhancement through defoaming and air-entraining. They eliminate fragile large air bubbles while introducing sealed, independent microbubbles. The increased bubble count and reduced inter-bubble spacing boost the volume of concrete paste. This maximizes the microbead effect of bubbles, enhancing concrete’s enveloping capacity and flowability, and markedly improving the workability of plastic concrete. On-site compounding often employs yellow-based composite polyether air-entraining agents for slurry enhancement.
Additionally, many retarders like sodium hexametaphosphate serve as effective dispersants. As alkali metal phosphates, they enhance dispersion in solid or liquid materials, aid cement particle dispersion, and prevent agglomeration of dispersed particles, thereby maintaining dispersion stability and delivering superior slurry enhancement. When using sodium hexametaphosphate as a retarder in polycarboxylate superplasticizer formulations, it not only provides effective retarding but also exhibits excellent dispersing properties (particularly suitable for concrete with high stone powder content). This promotes slurry enhancement and slump retention in concrete, ensuring stable concrete performance.
2.4 Dosage of Retarders
Under normal conditions, regardless of season, the retarder dosage must ensure an initial setting time of approximately 6 hours and a final setting time of approximately 9 hours. When blending water-reducing agents, the retarder dosage should be determined based on maximum, minimum, and average temperatures. Insufficient retarder not only affects setting time but also compromises slump retention—particularly during the initial 30 minutes. Practical experience confirms that concrete’s early-stage slump retention (first 30 minutes) is largely determined by the type and quantity of both the base liquid and retarder.
2.5 Proportion and Dosage of Retarding Agents
Concrete workability retention occurs in stages: the first 30 minutes rely primarily on the characteristics of the base mixture and the retarding agent (type and quantity); the 30–60 minute window depends mainly on medium-range retarders; and retention beyond 60 minutes relies primarily on high-range retarders. To enhance concrete slump retention while reducing costs, the quantity of retarder should not be reduced but rather increased by 5–10 kg beyond the standard dosage. This adheres to the principle of “not exceeding the upper limit while ensuring the lower limit is met.” The mid-retarder takes over the baton for mid-stage slump retention, further solidifying retention performance and laying the groundwork for late-stage retention. Therefore, the dosage of mid-retarder must also be strictly maintained. Adding high-range water-reducing agents extends the concrete’s late-stage slump retention. With excellent mid-stage slump retention, incorporating an appropriate amount of high-range agents significantly improves late-stage performance. The dosage of high-range agents is approximately one-third that of mid-range agents.
3 Conclusion
With the development of the national economy, concrete raw materials are becoming increasingly scarce and of poorer quality, leading to a sharp decline in the adaptability of water-reducing agents. This results in deteriorated concrete workability and construction difficulties. Through cement mineral composition analysis and water-reducing agent compounding experiments, adjusting the effective components of water-reducing agents can ensure that concrete slump retention meets construction requirements while its workability, mechanical properties, and durability comply with design and standard specifications.
Related Reading
For a broader overview of definitions, density, chemistry, product forms and selection, read What Is a Superplasticizer? Definition, Density, Types and Concrete Use.
To learn more about this topic, read Polycarboxylate Superplasticizer Powder Guide.
To learn more about this topic, read PCE Powder vs Liquid Selection Guide.
