Factors Affecting the Effectiveness of Water-Reducing Agents

1. Impact of Cement Properties on the Plasticizing Effect of Water-Reducing Agents
Different types of cement yield different plasticizing effects with water-reducing agents. The mineral composition, chemical composition of the cement clinker, and the form of gypsum (as a setting regulator) all affect the plasticizing effect of the water-reducing agent. Additionally, factors such as cement fineness, the type and dosage of supplementary materials, the freshness of the cement, and the moisture content and temperature of the cement also influence the plasticizing effect.

1.1 Mineral Composition of Cement
The components C₃A and C₂S have the most significant impact. Cement with a high C₃A content exhibits poor water-reducing effects because C₃A has a much higher adsorption capacity for water-reducing agents compared to other mineral components (several times more than C₃S), while slag has the least adsorption capacity. When a water-reducing agent is added to the cement paste, due to the rapid hydration of C₃A and its high adsorption capacity, it first adsorbs a large amount of the water-reducing agent. Therefore, for cement with high C₃A content, increasing the dosage of the water-reducing agent may significantly improve workability.

1.2 Form and Dosage of Gypsum
Gypsum is generally added during the grinding of cement clinker to regulate setting. However, during the grinding process, the temperature increases, which can cause some dihydrate gypsum to lose part of its crystallization water and transform into hemihydrate gypsum or anhydrite (hard gypsum). Some cement plants, in an effort to reduce production costs, often substitute anhydrite or industrial by-product gypsum (also anhydrite) for dihydrate gypsum. Although the differences may not be significant in standard product inspections, the plasticizing effects can vary greatly when a water-reducing agent is added. Notably, cement using anhydrite as a setting regulator may encounter compatibility issues with lignosulfonate or sugar-based water-reducing agents, resulting in not only a lack of expected water reduction but also rapid loss of workability and abnormal setting.

The reason behind the aforementioned anomalies when anhydrite is used as a setting regulator with lignosulfonate or sugar-based water-reducing agents is that different crystalline forms of gypsum have varying adsorption capacities for these agents, in the order of: CaSO₄ > CaSO₄·12H₂O > CaSO₄·2H₂O. When lignosulfonate or sugar-based water-reducing agents are added to cement with anhydrite and mixed with water, the anhydrite immediately adsorbs a large number of lignosulfonate or sugar molecules, forming a membrane layer of the water-reducing agent that tightly surrounds the anhydrite, preventing the release of SO₄²⁻ ions necessary for the cement paste system. This inhibits the formation of substantial amounts of Aft on the surface of C₃A, resulting in excessive hydration of C₃A and a significant number of interconnected hydrated calcium aluminate crystals. The lighter consequence is rapid slump loss, while severe cases can lead to abnormal quick setting of the concrete.

As the gypsum content in the cement increases, the saturation point of the water-reducing agent continuously decreases, leading to a decline in the flowability of the neat paste at saturation. This occurs because gypsum interacts with C₃A to form ettringite covering the C₃A particles' surface, preventing further hydration of C₃A. While sulfates can improve the compatibility of water-reducing agents with cement, excessive sulfate content can adversely affect concrete, and thus sulfates should not be added in large amounts solely to enhance compatibility.

1.3 Alkali Content of Cement
The alkali content of cement primarily refers to the Na₂O and K₂O content. Alkali content significantly affects the compatibility of cement with water-reducing agents. As the alkali content increases, the plasticizing effect of the water-reducing agent deteriorates, which can also shorten the setting time of concrete and increase slump loss.

1.4 Supplementary Materials in Cement
Currently, over 80% of cement in China includes certain supplementary materials such as volcanic ash, fly ash, slag powder, and coal gangue. Due to the variety, properties, and dosage of these supplementary materials, the effectiveness of water-reducing agents varies accordingly. Experiments show that water-reducing agents are more effective in plasticizing cement with fly ash and slag compared to pure portland cement; however, the effectiveness is poorer for cement with volcanic ash or coal gangue. Therefore, for the latter two types of cement, a higher dosage of water-reducing agent is required to achieve satisfactory water reduction.

1.5 Fineness of Cement
Cement particles have a strong adsorption capacity for water-reducing agent molecules. In cement paste mixed with water-reducing agents, finer cement particles translate to a larger specific surface area, resulting in greater adsorption of water-reducing agent molecules. Consequently, for cements with the same dosage, those with finer particles will exhibit less plasticizing effect. Some cement manufacturers, aiming for early strength, often increase the fineness of the cement. For such cements, a higher dosage of water-reducing agent is necessary to achieve adequate plasticizing effects.

1.6 Freshness and Temperature of Cement
Fresher cement tends to have a weaker plasticizing effect from water-reducing agents due to its stronger positive charge, which enhances its adsorption capacity for the agents. Higher temperatures in cement also correspond to poorer plasticizing effects and faster slump loss. Therefore, when some ready-mixed concrete producers use freshly ground cement that has not yet lost heat to make concrete, it may result in low water reduction rates, rapid slump loss, and even abnormal setting in the mixer, which should be taken seriously and avoided.

2. Self-Characteristics of Water-Reducing Agents Affecting Plasticizing Action
The characteristics of naphthalene-based high-efficiency water-reducing agents influence their plasticizing action on cement, including sulfonation degree, relative molecular weight distribution, polymerization degree, and the structure (linear or branched). Additionally, the state of the water-reducing agent (powdered or liquid) also impacts its plasticizing effects:
(1) The more complete the sulfonation of naphthalene-based water-reducing agents during synthesis, the more sulfonic acid groups are introduced, resulting in better plasticizing effects. Hydrolysis reactions are also crucial as they help remove sulfonic acid groups from the α position of the naphthalene ring, facilitating polycondensation reactions.
(2) The degree of polymerization of naphthalene-based water-reducing agents significantly affects their plasticizing effects, generally having one optimal degree of polymerization.
(3) The nature of the counterions used for neutralization during the production of naphthalene-based water-reducing agents also affects their plasticizing effects.
(4) The state of naphthalene-based water-reducing agents impacts their plasticizing action on cement. Experiments show that powdered water-reducing agents have about 5% less plasticizing effect compared to liquid ones. This phenomenon occurs because the molecules of powdered water-reducing agents have a tangled structure, and after dissolving in water for more than one day, they adopt a linear structure, which exhibits stronger plasticizing effects when adsorbed onto cement particles.