Resolving the precise compatibility challenge of polycarboxylic acid superplasticizers in high-performance concrete—eliminating large bubbles while introducing small bubbles—centers on admixture blending technology and meticulous control of construction processes. By scientifically adjusting the types and dosages of defoamers and air-entraining agents, and optimizing mixing and pouring procedures, it is possible to simultaneously eliminate harmful large bubbles while retaining or introducing beneficial microbubbles.
Key solutions encompass the following aspects:
I. Admixture Blending Technology
This represents the pivotal solution to the contradiction.
Blending Defoamers and Air-Entraining Agents: Precisely blend appropriate amounts of defoamers and air-entraining agents into polycarboxylate-based water-reducing agents. Defoamers disrupt surface tension, causing large, unstable bubbles to collapse or coalesce and escape. Air-entraining agents introduce numerous uniform, stable microbubbles (approximately 50-200 micrometers in diameter) to enhance concrete's freeze-thaw resistance. Research indicates that at appropriate defoamer concentrations, large bubbles generated by polycarboxylate can be effectively reduced while stabilizing the introduced microbubbles.
Selecting Suitable Defoamer Types: Choose highly efficient defoamers with good compatibility with air-entraining agents (e.g., organosilicon, polyether types) for polycarboxylate-based systems. The defoamer dosage must be precisely controlled; excessive amounts impair the air-entraining agent's effectiveness, while insufficient amounts fail to eliminate large bubbles effectively.
Selecting high-efficiency air-entraining agents: Choose agents that generate independent, closed, and uniformly distributed microbubbles (e.g., sodium rosinate, alkyl sulfonates). Ensure these microbubbles exhibit excellent stability within the cement paste, resisting rupture or aggregation during use.
II. Material Selection and Mix Proportion Optimization
Control cement and admixtures: Certain cements and admixtures (e.g., fly ash, slag powder) may affect bubble stability. Optimize material combinations through testing to ensure stability of the bubble system.
Sand Ratio and Aggregate Gradation: Optimizing sand ratio and aggregate gradation improves concrete workability and bubble structure. Proper gradation facilitates uniform slurry coating of aggregates, reducing retention of large bubbles.
Water-Cement Ratio Control: Strictly control the water-cement ratio. Minimizing it while maintaining workability enhances concrete density and stabilizes bubble structure.
III. Construction Process Control
Mixing Process Optimization: Adjust mixing time, speed, and method. Appropriate mixing promotes uniform bubble distribution and facilitates large bubble escape, but excessive mixing may rupture microbubbles.
Vibration Technique: Employ appropriate vibration methods and duration. Correct vibration eliminates large harmful voids within concrete without excessively disrupting the structure of micro-air bubbles.
On-site Air Content Testing and Control: Continuously monitor concrete air content at the mixer discharge point or prior to pouring. Adjust admixture dosage or mixing process based on results to ensure air content meets freeze-thaw resistance requirements.
Mix Design Validation: Conduct comprehensive mix design testing prior to actual application. This includes evaluating fresh concrete air content and workability, as well as hardened concrete compressive strength and freeze-thaw durability to ensure compliance with design specifications.
In summary, combining chemical methods (compound defoamers and air-entraining agents) with physical methods (optimized mix design and construction techniques) effectively resolves the compatibility challenge between “eliminating large bubbles” and “introducing fine bubbles.” This approach produces high-performance concrete with both excellent workability and high durability.
