Definition of Water-Reducing Agents
Water-reducing agents are substances that significantly reduce the amount of mixing water while maintaining the flow value of refractory castables relatively unchanged. They are also known as water-reducing agents. These agents do not chemically react with the components of the material; instead, they function through surface physicochemical actions and are classified as surfactants or electrolytes.
Working Principle of Water-Reducing Agents
When dissolved in water, water-reducing agents adsorb onto the surface of particles, increasing the ζ potential of the particle surfaces and enhancing the repulsive forces between particles. This process releases free water that is encapsulated within the cohesive structure formed by fine particles, thereby providing wetting and dispersing effects, which is why they are also referred to as dispersants. By releasing free water from the cohesive structure, they can reduce the amount of water used per unit while maintaining the rheological properties (workability) of the castable, or improve workability without changing the amount of water used, making the material easier to work with and shape.
Types of Water-Reducing Agents
By Appearance
Liquid agents and powder agents. Liquid agents typically have solid contents of 20%, 40% (also known as mother liquor), and 60%, while powder agents generally have a solid content of 98%.
By Water Reduction and Enhancement Capability
Ordinary water-reducing agents (also known as plasticizers, with a water reduction rate of no less than 8%, represented by lignosulfonates).
High-efficiency water-reducing agents (also known as superplasticizers, with a water reduction rate of no less than 14%, including naphthalene-based, melamine-based, amino sulfonate-based, and aliphatic series).
High-performance water-reducing agents (with a water reduction rate of no less than 25%, represented by polycarboxylate-based agents), which can be further categorized into early-strength, standard, and retarding types.
By Composition
Lignosulfonates, polycyclic aromatic salts, water-soluble resin sulfonates, naphthalene-based high-efficiency water-reducing agents, aliphatic high-efficiency water-reducing agents, amino high-efficiency water-reducing agents, and polycarboxylate high-performance water-reducing agents.
By Chemical Composition
Lignosulfonate-based water-reducing agents, naphthalene-based high-efficiency water-reducing agents, melamine-based high-efficiency water-reducing agents, amino sulfonate-based high-efficiency water-reducing agents, fatty acid-based high water-reducing agents, and polycarboxylate-based high-efficiency water-reducing agents.
Role of Water-Reducing Agents in Ultra-Low Cement Corundum Castables
After determining the mix ratio for ultra-low cement corundum castables, sodium polyphosphate (A), melamine-based condensates (B), and naphthalene sulfonate condensates (C) were used as water-reducing agents. Suitable dosages were selected to prepare refractory castables, and their process parameters and performance were evaluated. The castables without water-reducing agents exhibited spontaneous agglomeration of ultrafine powders, which could not effectively fill the pores and were distributed very unevenly. A significant amount of water was trapped in the flocculates or filled the pores, leading to increased water usage, lower bulk density, higher porosity, and lower strength after heat treatment, which also hindered sintering. Sodium polyphosphate showed a certain dispersing and water-reducing effect, which could partially prevent the spontaneous agglomeration of ultrafine powders, allowing for better filling in the pores and improving water utilization, resulting in a water reduction rate of approximately 17%.
Consequently, the bulk density of the castables increased, and the porosity decreased. Compared to the castables without water-reducing agents, the compressive strength after firing increased by 0.6 to 1.9 times, and the high-temperature flexural strength improved by 1.25 times. B and C, as organic high-efficiency water-reducing agents, exhibited particularly significant dispersing and water-reducing effects, with water reduction rates of 25% and 28%, respectively. Compared to the castables without water-reducing agents, their bulk density increased by approximately 3.5%, porosity decreased by 15%, compressive strength after firing improved by 1 to 4 times, and high-temperature flexural strength increased by more than 3.5 times. It was also observed that the effect of water-reducing agent C was better than that of B.
Conclusion
In the formulation of ultra-low cement refractory castables, it is essential to incorporate water-reducing agents, with a preference for organic high-efficiency water-reducing agents.
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