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Lignite is a soft, weak, black or brown substance, akin to coal, naturally occurring in some aggregate sources. For most concrete applications, e.g. foundations, mass concrete, concrete covered by finishes, the effect of lignite is relatively insignificant. The main problems relate to exposed power-trowelled slabs and architectural finishes.
The particles are less dense than those of the aggregate, rising to the top surface during placing and compaction. The finishing process may cover the lignite particles with a layer of mortar. As the concrete dries and is worn, the mortar skin break down, leaving unsightly black marks and holes in the slab surface. There may be only a few particles per square metre, but in extreme cases there may be tens or hundreds of small specks. While they do not have structural implications, the particles may be detrimental to surface performance in terms of appearance, wear, abrasion, cleanliness, and compatibility with finishes.
The British Standard for natural aggregates for concrete, BS EN 12620: 2002 +A1 2008 Annex G.4 gives the limits for the lignite content in aggregates which should not normally be exceeded.
Where appearance is important or the concrete is for power-trowelled slabs (see Concrete Advice No. 12, Power-trowelled concrete floors and lignite), the specification should state that aggregates should be free of lignite.
Acknowledgement: The Concrete Society
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