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Aggregate defects.

Most aggregates are stable but a few exhibit changes that can affect the performance. In addition, some concrete mixes may be contaminated, leading to long-term deterioration. Quality control and compliance with appropriate standards should ensure that unsuitable materials are not used. However, residual impurities may cause local problems. For example, small quantities of coal, lignite, hydrated lime or pyrites may cause small pop-outs or staining on the surface.

Acknowledgement: The Concrete Society

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Power-trowelled concrete floor and lignite

Staining by pyrites and lignite on concrete surfaces