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Concrete is the most widely used building material in the world. Still, its basic formula – cement, water, and aggregates – often needs to be fine-tuned to meet the needs of different projects, from cold winters to hot summers, from skyscrapers to sidewalks. This is where additives shine.
Simply put, admixtures are used in concrete to improve one or more of its properties, enabling it to perform better during mixing, construction, hardening, and long-term use. They are an essential component of high-performance concrete formulations.
Admixtures are chemicals or minerals added to concrete during the mixing process (usually accounting for 0.5-5% of the weight of cement) to alter its properties. Unlike the main components, they work in small doses to address specific environmental challenges.
Problem: If the concrete is too dry and thick, it won’t be easy to pump, pour, and compact, making construction extremely challenging. But adding water directly will seriously reduce its strength.
Solution: Plasticizers and Superplasticizers, also known as water-reducing agents.
Working principle: They disperse and agglomerate cement particles through chemical reactions, releasing the encapsulated moisture, thereby significantly increasing the fluidity of concrete without increasing the total water content.
The ultimate goal is to make the concrete smooth and easy to pump and pour, ensuring that it can fill every corner of the formwork without sacrificing strength.
The time between the completion of concrete mixing and the onset of hardening is limited. According to project requirements, we need to accurately “fast forward” or “slow down” the process.
Delay condensation
Solution: retarders
Working principle: The retarder will slow down the hydration reaction rate of cement.
Final objective:
Prevent premature solidification of concrete in hot weather.
Provide sufficient time for long-distance transportation or complex pouring operations.
Avoid creating “cold joints” during continuous pouring.
Accelerated Condensation
Solution: Accelerators
Working principle: The Early strength agent will accelerate the hydration reaction of cement.
Final objective:
In cold weather, counteract the impact of low temperatures on strength development.
Used for emergency repair projects to restore roads or structures to use as soon as possible.
Accelerate the turnover speed of templates and shorten the overall construction period.
Solution: High-efficiency water reducer, silica fume, fly ash, and slag.
Working principle:
Efficient water-reducing agents significantly reduce the water-cement ratio, making the internal structure of concrete denser and directly improving its strength.
Mineral additives such as silica fume, fly ash, and slag generate additional cementitious substances through micro-filling effects and long-term secondary hydration reactions, which not only improve the later strength of concrete but also significantly reduce its permeability, thereby resisting chemical erosion and environmental damage.
The ultimate goal is to manufacture concrete with high strength, ultra-high strength, and even ultra-high performance, and to make it resistant to chemical corrosion, sulfate attack, and chloride ion penetration, greatly extending the service life of the structure.
Problem: In cold regions, the water that seeps into the pores of concrete will expand by about 9% when it freezes, generating enormous internal pressure. Repeated freeze-thaw cycles will gradually cause the concrete to peel off and be damaged.
Solution: Air Entraining Admixtures
Working principle: The air entraining agent will introduce billions of disconnected tiny bubbles inside the concrete. When water freezes, these bubbles act like miniature ‘pressure relief valves’, providing space for the expansion of the ice.
The ultimate goal is to protect concrete from freeze-thaw cycles in harsh and humid environments, which is crucial for the durability of infrastructure in northern regions.
In addition to the main functions mentioned above, there are many additives used to meet specific engineering challenges:
Waterproofing Admixtures: Reduce the water absorption and permeability of concrete, which is used in basements, water tanks, etc.
Shrinkage Reducing/Expansion Agents: Compensate for the shrinkage of concrete during the hardening process and reduce the risk of cracking.
Corrosion inhibitors: slow down or prevent the corrosion of internal steel bars, especially suitable for marine environments or de-icing salt areas.
Coloring Admixtures: Provide long-lasting, integrated colors for concrete that are used for decorative flooring or walls.
Concrete admixtures are no longer an option in construction, but a necessity in modern engineering technology. From minor repairs to skyscrapers, they make concrete versatile enough for any project. It is precisely because of these additives that we can build safer, more economical, and more durable architectural wonders.
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