What sulfate thresholds prevent concrete damage?

Fine aggregates with up to 1.5% SO 3 and coarse aggregates with up to 3% avoid damage.

Deterioration Process of Concrete Exposed to Internal Sulfate Attack

Weifeng Chen^1,2, Bei Huang^1,2, Yuexue Yuan^1,2

¹College of Mateials Science and Engineering, Nanjing Tech University, Nanjing 211800, China.

|March 19, 2020

Summary

This study examined how gypsum-contaminated aggregates affect concrete durability. Researchers tested concrete with varying sulfate levels in fine and coarse aggregates. They found that low sulfate content prevents expansion and strength loss. Concrete with up to 1.5% sulfate in fine aggregates remained stable. Coarse aggregates with up to 3% sulfate also avoided damage. The study used XRD, TG, DSC, SEM, and EDS to track changes over time. They found that ettringite formation causes expansion. Gypsum-paste interface cracking leads to aggregate separation. These findings suggest guidelines for sulfate content in construction materials.

Area of Science:

Concrete durability research in civil engineering
Material degradation analysis in construction science

Background:

Concrete structures in regions with gypsum-rich aggregates face deterioration risks. Gypsum contamination is common in southern China's aggregates. Prior research has shown that sulfate exposure can lead to expansion and strength loss. However, the exact thresholds for damage remain unclear. This gap motivated a study of sulfate effects on concrete. No prior work had resolved how varying gypsum content affects expansion. Researchers wanted to determine safe limits for sulfate in aggregates. The study aimed to clarify how sulfate concentration influences concrete durability.

Purpose Of The Study:

This study aimed to assess how gypsum-contaminated aggregates impact concrete durability. Researchers focused on expansion and compressive strength changes. They wanted to identify safe sulfate thresholds in aggregates. The motivation came from frequent concrete damage in gypsum-rich regions. The goal was to prevent structural failures in construction. They also sought to understand mineral composition changes over time. The study aimed to provide data for material specifications. The findings could inform construction standards in sulfate-prone areas.

Main Methods:

Researchers designed two concrete groups with different aggregate types. Fine and coarse aggregates had varying sulfate levels. SO₃ contents ranged from 0% to 7% by weight. XRD, TG, and DSC analyzed mineral composition changes. SEM and EDS studied microstructure evolution. The study spanned one year to capture long-term effects. Researchers monitored expansion and strength loss. They tracked sulfate ion concentrations and phase formation.

Main Results:

Concrete with SO₃ below 1.5% in fine aggregates showed no significant expansion. Coarse aggregates with up to 3% SO₃ also avoided damage. Sulfate ion concentrations did not form new gypsum phases. Gypsum content decreased during internal sulfate attack. Ettringite levels increased over time. Ettringite was the primary cause of expansion damage. Fractures started at gypsum-paste interfaces. Aggregate separation from paste occurred in damaged samples.

Conclusions:

The study showed that low sulfate levels in aggregates prevent concrete damage. Concrete with SO₃ below 1.5% in fine aggregates remained stable. Coarse aggregates with up to 3% sulfate did not cause expansion. Ettringite formation was linked to expansion damage. The researchers propose that gypsum-paste interface cracking precedes failure. Aggregate separation followed crack propagation. These findings suggest guidelines for sulfate content in aggregates. The authors state that these thresholds can inform construction practices.

Keywords:

concrete gypsum internal sulfate attack performance Concrete sulfate damage Aggregate contamination Concrete expansion Ettringite formation

Frequently Asked Questions

The researchers propose that ettringite formation causes expansion. Gypsum-paste interface cracking precedes damage.

XRD, TG, DSC, SEM, and EDS were used to analyze mineral and microstructure changes.

Sulfate levels above 1.5% in fine aggregates may lead to expansion and strength loss.

Gypsum content decreases while ettringite increases, leading to expansion damage.

Cracks start at gypsum-paste interfaces and end with aggregate separation from paste.

Fine aggregates with up to 1.5% SO<sub>3</sub> and coarse aggregates with up to 3% avoid damage.

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