Author(s)
Zhanzhao Li, Christopher A. Gorski, Aaron Thompson, Jeffrey R. Shallenberger
Abstract
Deleterious materials, particularly those containing iron sulfide minerals such as pyrrhotite and pyrite, pose a significant threat to the long-term durability of concrete structures. These minerals, when present in aggregates, can trigger destructive reactions that compromise structural integrity, leading to cracking, spalling, and costly repairs. While their behaviour in acidic environments has been widely studied, understanding their dissolution and reaction mechanisms in highly alkaline conditions—typical of concrete pore solutions—remains critical for effective mitigation.
Research into alkaline environments with pH values of 13–14 reveals that pyrrhotite dissolves at a rate several magnitudes higher than pyrite, especially as temperature and pH increase. This accelerated dissolution releases sulphur species, which subsequently form expansive reaction products within the concrete matrix. The resulting internal stresses can drive progressive deterioration over time. Interestingly, the type of alkali present—whether potassium or sodium—does not significantly alter dissolution behaviour, suggesting that temperature and mineral type are the dominant factors influencing reaction kinetics.
The dissolution process for pyrrhotite is governed by both chemical and diffusion-controlled mechanisms. Oxidation of iron and sulphur species occurs alongside diffusion through an Fe(III)-(oxy)hydroxide surface layer, which partially restricts further reactions. By slowing this dissolution process, engineers can potentially reduce the formation of harmful secondary products such as gypsum, ettringite, and thaumasite, which are known to exacerbate damage through internal sulphate attack.
Current standards vary in their approach to limiting iron sulfide content in aggregates. For example, European guidelines set total sulphur thresholds, while North American standards often lack specific limits. However, aggregate suitability cannot be determined solely by sulphur content—factors such as mineral type, pore solution chemistry, and environmental exposure conditions must also be considered.
A deeper understanding of dissolution kinetics provides the foundation for developing robust testing methods, refining acceptance criteria, and implementing preventative measures. Through targeted mineral characterisation and control of aggregate sources, the concrete industry can significantly reduce the risk of damage caused by deleterious materials. This not only extends service life but also minimises maintenance costs and environmental impact, supporting the creation of more resilient infrastructure worldwide.
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