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The influence of a concrete accelerator on the porosity of concrete

The porosity of concrete directly affects its strength, durability, and permeability. A concrete accelerator is widely used to shorten setting time and improve early strength, and its effect on porosity is dual: an appropriate amount can reduce total porosity and improve pore structure; excessive or improper use can increase porosity and enlarge pore size.

This article provides an in-depth analysis of the relationship between concrete accelerators and porosity, explaining their mechanisms, consequences, and optimal mitigation measures.

What is a concrete accelerator?

Concrete accelerators are additives added to concrete mixtures to accelerate the chemical reaction (hydration reaction) between cement and water. Its main functions are:

Shorten the initial and final setting times: The speed at which concrete transitions from the plastic state to the solid state is much faster.
Improve early-strength development: Concrete can achieve higher compressive strength in the first few hours or days.

This makes them indispensable in the following applications:

Cold weather concrete construction: In order to counteract the slowing effect of low temperature on hydration.
Spray concrete (sprayed concrete): used for rapid hardening and stacking on vertical or top surfaces.
Quick maintenance project: used for quickly reopening sidewalks, runways, or industrial floors.

Understand the porosity of concrete

Porosity refers to the total volume of voids or pores within the hardened concrete matrix. These pores are mainly filled with air and water.

Porosity is a key factor determining the quality of concrete, as it directly affects:

Strength: Generally, the higher the porosity, the lower the compressive and tensile strength. Pores play a role in stress concentration and reduce the load-bearing cross-section.
Durability: Porosity has the most significant impact on durability. Porous concrete is more susceptible to environmental degradation.
Permeability: Although related to porosity, permeability varies. It measures the ease of flow of liquid and gas through interconnected pores in concrete. High porosity usually leads to high permeability.
Anti-freezing and thawing cycle performance: Water in pores can freeze and expand, leading to internal microcracks that, in turn, cause peeling and deterioration.
Chemical corrosion resistance: The porous structure allows corrosive chemicals such as sulfates and chlorides to penetrate faster and deeper, leading to internal decay or steel corrosion.

The influence of a concrete accelerator on the porosity of concrete

Concrete accelerators (such as calcium formate, triethanolamine, and chloride accelerators) change the porosity of concrete by adjusting the hydration kinetics of cement and the formation of hydration products:

Positive impact: reduce porosity and improve pore structure

Mechanism: Accelerators promote the hydration reactions of tricalcium silicate (C3S) and tricalcium aluminate (C3A), thereby accelerating the formation of dense hydration products (CSH gel, ettringite). These products fill the capillary pores, reduce the total porosity, and minimize macropores to the greatest extent possible.

When no accelerator is used, the porosity of concrete at 28 days is 18-22%, with large pores (>50nm) accounting for 40-50% of the total pores.

Using the optimal amount of accelerator: After 28 days, the porosity decreases to 14-19%, and the high porosity decreases to 25-35%, thereby improving the density and impermeability of concrete.

Negative effects: increased porosity and coarsening of pores

Excessive addition (>3% of cement mass): Excessive addition can lead to uneven hydration – local rapid reactions result in incomplete hydration of cement, leaving unreacted particles and forming connected pores. Compared with the optimal addition amount group, the porosity may increase by 10% to 15%.

Chloride-based accelerator (misuse): High chloride content can lead to salt crystallization, creating additional microcracks and pores and reducing long-term durability.

How does a concrete accelerator change the porosity of concrete

Accelerate hydration and pore filling

Accelerators can reduce the activation energy of cement hydration and shorten the induction period. At an early stage, more hydration products (CSH gel) will be generated to fill the pores formed by water evaporation and those of unreacted cement.

For example, calcium formate (1% dosage) can accelerate C3S hydration by 30-40%, increase early CSH gel content, and reduce capillary porosity by 15-20%.

Changes in the morphology of hydration products

Accelerators can change the morphology of hydration products: ettringite crystals become smaller and more uniform, while CSH gel forms a more dense network. This reduces the gap size between particles and improves pore connectivity.

Comparison: Without a promoter, the calcium aluminate crystals become rough and dispersed, resulting in large gaps between the hydration products.

Effects on solidification and hardening rates

Rapid solidification can reduce surface moisture loss and internal leakage, thereby minimizing the formation of capillary pores to the greatest extent possible. However, excessive solidification speed can cause rapid hardening of concrete, hinder full compaction, and lead to the formation of pores.

The best practice of using accelerators to control porosity

Although accelerators can increase porosity, their use is often unavoidable. The key is to control its impact through reasonable design and construction methods.

Choose appropriate accelerators: Non-chloride accelerators must be used for reinforced concrete construction. Chloride accelerators are suitable only for non-reinforced concrete and do not require consideration of corrosion issues.
Follow dosage recommendations: Excessive use of accelerators is a common mistake, which can lead to a significant increase in porosity, flash setting, and a significant decrease in strength. Be sure to strictly follow the manufacturer’s dosage range.
Maintenance is crucial: this is the most critical step. Proper maintenance (maintaining humidity and a suitable temperature) is crucial for all types of concrete, but especially for rapid-setting concrete. Maintenance can promote continuous hydration of concrete, help fill some initial capillary pores, improve microstructure, and thus enhance the long-term strength and durability of concrete.
Optimize mix design: Consider using auxiliary cementitious materials (SCMs) such as silica fume, fly ash, or slag. These materials have a slower reaction rate, but over time, they will form a finer, less permeable pore structure, which helps counteract the negative effects of accelerators.
Testing and validation: Before a comprehensive application, it is necessary to conduct batch tests using specific materials used on site to verify solidification time, strength development, and other properties.

Conclusion

Concrete accelerators affect concrete porosity by altering hydration kinetics and product morphology. Moderate accelerators can reduce total porosity and improve pore structure, whereas excessive or improper use can increase voids and reduce durability.
By understanding this fundamental trade-off, engineers and contractors can leverage the advantages of accelerators while making informed decisions to ensure the long-term integrity and service life of concrete structures.