Lignite in block paving

[Kimdi]

I have recently had a driveway laid with Bradstone Beamish cobbles. Three days after being laid the driveway was covered in Black spots. We complained to Bradstone who inform me this is lignite within the blocks and is natural and not detremental. I am not happy with this explanation. The supplier inform me this is the first time they have experienced this problem and the contractor who laid it has not experienced this in the past. Are Bradstone pulling the wool over my eyes?? Can anyone help.

[Carberry]

Got any pictures? upload them to a site like photobucket and post link here

[lutonlagerlout]

isnt lignite some sort of coal?
why would that be in block paving?
LLL ???

[ken]

Because ash from coal burning power stations gets used as aggregates in block making sometimes….. So the gaffa told me once upon a time.

[Big Phil]

lignite is 'immature coal' and bradstone are indeed shitting you as it is a contaminant that should not be in aggregate used for aesthetics. it is natural in some sand deposits when quarrying near woodland, but it should be removed if used for decorative purposes. lignite has a lighter density than sand and floats up to the surface in block production. it may not have any structural effect, but can swell when soaked in water and when soft can lead to staining & streaking. they need to check the aggregate quality used in their production as you shouldn't accept this.

[Brucieboy]

Lignite is a general term for fossilised wood and coal. You'll find most concrete companies will add a clause in their quotations or other documents stating that lignite (and pyrites aka ironstone) are naturally occurring within some aggregate bearing locations and therefore difficult to eliminate completely. Because its relative density is about 1.2 to 1.6 and therefore lighter than cement (3.1) and aggregates (2.5 to 2.7), it tends to rise to the surface during the placing the placing and compacting process. The offending particles, anywhere in size from 3mm to 10mm (or larger in 20mm aggregate), sit under the surface covered by a thin layer of laitence (cement/fine sand). This wears away after a few days exposing the particles. They are generally dark brown/black in colour and often mistaken for pyrites because they can produce a rust like stain.

Unless severely contaminated, the particles are more of an aesthetics problem rather than detrimental to the performance of the concrete (block or slab). In very severe cases, we've had to employ drill and fill techniques to remove them, i.e. drill out the particles, remove the dust from holes and fill with a proprietary colour matched repair mortar. It's a very labour intensive process and can be very costly depending on the scale of the problem.

Lignite is quite common in areas close to coal seams, i.e. Trent Valley, and also prevalent in some marine dredged locations, particlularly in certain areas around the coast of southern England. A few aggregate processing plants employ flotation systems to try and remove as much lignite as possible.

Basically, it is almost impossible for an aggregate supplier of flint/quartz type gravel to categorically guarantee that they won't suffer lignite at some time or another. It doesn't occur with hard rocks such as limestone or granite.

A few photos would help to appreciate the scale of your particular problem.