Using ettringite-based expansive agents in heat-cured self-compacting concrete (SCC) can significantly improve strength and durability without the need for additional cement, according to a new study published in Buildings.
Study: Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes. Image Credit: Minakryn Ruslan/Shutterstock.com
The research evaluated the performance of SCCs containing expansive agents (EAs) under various curing conditions. By examining mechanical properties, expansion behavior, and pore structure, the authors demonstrated how EAs can help overcome the strength reductions typically caused by high-temperature curing. The findings offer a practical solution for producing more durable, sustainable precast concrete elements.
Background
SCC is widely used in precast concrete due to its ability to flow into complex forms, improve surface finishes, and reduce labor and vibration during placement. To accelerate production, heat curing is commonly applied. However, elevated temperatures often lead to cracking and increased porosity, which can compromise long-term mechanical performance.
To compensate, manufacturers typically increase the cement content in SCC. While this helps maintain strength, it also drives up production costs and carbon emissions. As a more sustainable strategy, many producers now partially replace cement with supplementary cementitious materials (SCMs), such as fly ash (FA) and limestone filler (LF). These additives can reduce the negative effects of heat curing, but their interaction with expansive agents remains largely unexplored.
This study addresses that gap by investigating how two types of EAs perform in combination with FA and LF under heat-curing conditions.
Methods
The researchers developed six SCC mixes—three with EAs and three without. Each mix used siliceous gravel (4–12 mm) and sand (0–4 mm), along with a polycarboxylate-based superplasticizer. FA or LF was used to increase the fine aggregate content, and EAs were added at 10 % of the cement weight.
Two types of expansive agents were tested:
- Type-K: based on calcium sulfoaluminate
- Type-G: based on calcium oxide
Each mix was subjected to one of three curing regimes:
- Standard curing: 98 ± 2 % relative humidity at 20 ± 2?°C
- Dry curing: 50 ± 5 % RH at 20 ± 2?°C
- Standard heat curing: typical for precast concrete production
The team assessed compressive strength at 2 and 28 days. Porosity and pore size distribution were measured using mercury intrusion porosimetry (MIP), and long-term expansion was tracked over 80 days using prismatic samples under uniaxial restraint.
Results and Discussion
The addition of expansive agents slightly reduced workability, especially in mixes with type-K. Despite this, all expansive SCCs met the fresh-state requirements for self-compaction.
Slump flow varied depending on the EA–cement combination. Mixes with type-K EA showed lower slump flow, likely due to its anhydrous composition interfering with superplasticizer performance. The higher sulfate content of type-K may have also played a role.
Meanwhile, SCCs with type-G EA showed greater slump flow and higher air content compared to those with FA or LF. However, these mixes maintained similar densities to the reference SCCs, suggesting a packing effect from the EA particles. Importantly, no surface irregularities were observed, indicating that the increased air content did not negatively affect finish quality.
Both types of EAs improved compressive strength. This was attributed to the higher total binder content from combining cement, SCMs, and EA. Notably, type-K mixes achieved the highest strength, due to the formation of ettringite within restrained pores—an effect that reduced porosity and enhanced structural integrity.
Conclusion
The study clearly shows that ettringite-based expansive agents can offset the strength loss typically seen in heat-cured SCC without requiring additional cement. By incorporating either type-K or type-G EA, concrete producers can maintain or even enhance mechanical performance while using more sustainable material combinations.
For the precast industry, this presents a cost-effective path to producing high-performance concrete elements that meet strength and durability standards with a lower environmental footprint.
However, the authors note that further microstructural analysis is needed to fully understand how EAs interact with SCMs like FA and LF under heat curing. These insights will be essential for optimizing mix designs and validating the long-term performance of expansive SCCs in real-world applications.
Journal Reference
García Calvo, J. L., & Carballosa, P. (2025). Use of Expansive Agents to Increase the Sustainability and Performance of Heat-Cured Concretes. Buildings, 15(17), 3128. DOI: 10.3390/buildings15173128. https://www.mdpi.com/2075-5309/15/17/3128
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