Midnight storm quartzite paving - Laying advice requested

[42Neil]

Does anyone have any experience laying Brett Midnight Storm (Elite textured) quartzite paving?

I am a DIYer about to attempt to lay these in the next few weeks or so. Based on the advice on this site, I have prepared a sub-base of 100mm compacted MOT1, and plan to use a cement bound laying course. So far, so good. However, I am still unsure on a few aspects that I am hoping someone familiar with this type of paving may be able to help with:

1) How permeable/porous is quartzite? Is it known/likely to have issues with picture framing (from absorbing cement during the curing/setting of the laying course) and/or absorbing colouration from the laying course or pointing material if normal grit sand & OPC are used (rather than silver sand and white cement)?

2) Should I be using a bond bridge? This site seems somewhat contradictory on the need for these, and on the need for adhesion between paving and laying course more generally. i.e. Some pages suggest that adhesion between the paving and laying course is important, and others (such as the "bedding material" page) state that paving doesn't necessarily need to adhere to the laying course. Is the importance of adhesion more related to the mass of the slabs being used? i.e. for thick, heavy slabs good adhesion isn't that important as they aren't going to budge anyway, but for thinner, lighter tiles good adhesion is more important as they are more easily moved?

3) Should I use a normal bedding mix (sharp sand/cement) or a class IV mortar (building sand, cement & plasticiser)? The pavers are only 20mm thick (and up to 900mm long), so may be considered a tile rather than a slab, and on the "slate paving" page of this site, it is recommended that for slate tiles (<30mm), a stiff class IV mortar laying course be used rather than the normal bedding mix. Is this advice also applicable to other thin natural stones (such as quartzite) or is this specifically more of an issue with slate?

Any advice appreciated. I'd rather ask the questions now and (hopefully) get it right rather than balls it up and then try to figure out what went wrong.

[London Stone Paving]

Hello Neil

Sounds like a well planned out project and very glad to see your using MOT for a sub base and not concrete.

1. In my experience picture framing is not usually caused by the bedding layer per se but can be common when the bedding layer is too wet and the moisture from it is squeezed up through the flag joints. In this situation the moisture sits around the edges of the flags and absorbs causing the picture framing. the best way to avoid this is to keep the mix semi dry. Also using the correct amount of mortar in your bedding layer so that excess doesn't get squeezed up through the joints.
As you suggest it can be also be caused by the pointing material. One way to prevent this is to give the slabs a good drink before pointing. This will fill the stone capillaries with water and prevent them from absorbing the muck from the pointing mix

2. Quartz is generally very hard and as such will not be very porous. A bond bridge would help to aid adhesion so in my opinion it would be well worth doing

3. Normal sharp sand and cement will be adequate

Steve

[42Neil]

Thanks for the reply.

Out of curiosity, I placed a sample tile into a small amount of water (1cm or so) to see if it would soak any through to the surface. As you can see from the photo, it did soak water through to the surface in a couple of places. So it seem these quartzite tiles *are* permeable in places.



Therefore I am wondering if it would be best to go with silver sand and white cement for the laying course to mitigate against possible colour staining (since the tiles are a pretty light grey colour).

Any thoughts? Don't want to throw money away on expensive sand if its unnecessary, but don't want to have to redo 35 square metres of patio if it stains.

As a side note, I have found it quite difficult to source silver grit sand. The closest that my local builders merchant has is "sharp washed sand" which is sourced from Wareham and is pretty light in colour. It seems a bit finer than the "grit sand" sold at the same merchant though, so i wonder if it is suitable for a laying course sand.

[mickg]

add SBR to the mix and use white cement with a sharp sand or grit sand as its also known, as an extra precaution you can paint the rear of the paving with SBR just as you are about to lay it but make sure any surplus SBR is removed with clean water immediately