Indian sandstone not sticking to mortar - What could be the issue?

[Karanbajwa]

Hi there all, I am laying calibrated 22mm indian sandstone 900 x 600 slabs on a full bed 1 to 6 mix of cement and sharp sand. I used a bond bridge (sbr and cement painted on the underside of each slab) and the mix was moist I believe s2 slump.
The next morning almost all of the slabs were still loose and moving when walked on. Though this was less than 24 hours later.
What could of gone wrong ? Or did I just not give it enough time to go off?

[dig dug dan]

You walked on them next day???
I think you found the problem!

[Karanbajwa]

Ah I see being a complete novice I thought it might be okay. How long should you typically give before walking on?

Also I've noticed my bedding mix is turning grey now that it is drying ( and was dark brown when wet) is colour anything to go by? Online videos seem to show a much more yellow bedding mix but I assumed it was because they were using building sand rather than sharp?

[dig dug dan]

Ah I see being a complete novice I thought it might be okay. How long should you typically give before walking on?

Also I've noticed my bedding mix is turning grey now that it is drying ( and was dark brown when wet) is colour anything to go by? Online videos seem to show a much more yellow bedding mix but I assumed it was because they were using building sand rather than sharp?

Have you used washed river sand, or sharp sand? There is a difference.The mix should have been fairly wet, to a point where when you tap the slab down, the muck oozes out from round it to a degree.
You need to leave it a week ideally before walking on it!

[dig dug dan]

Ah I see being a complete novice I thought it might be okay. How long should you typically give before walking on?

Also I've noticed my bedding mix is turning grey now that it is drying ( and was dark brown when wet) is colour anything to go by? Online videos seem to show a much more yellow bedding mix but I assumed it was because they were using building sand rather than sharp?

Have you used washed river sand, or sharp sand? There is a difference.The mix should have been fairly wet, to a point where when you tap the slab down, the muck oozes out from round it to a degree.
You need to leave it a week ideally before walking on it!

[Karanbajwa]

Hi there my friend. I used sharp sand. Is this the norm? Co incidentally they seem much more sturdy now a few days on. Just 1 mild rocker rather then them all. Thanks for your help and which is actually correct river washed or sharp sand?

[dig dug dan]

Riverwash sand is correct. Round here we have sharp sand, which is sometimes called plastering sand. Some people mix it with soft sand to point with, but it's not great for laying on.

[Tony McC]

There is too much confusion over terminology regarding sands. To be fair, the 'official' terminology doesn't help either as it is far too technical and overly-complicated for everyday use - fore example: the 'sharp' sand that should be used for bedding paving is officially referred to as .... Gf 85 0/4 (MP) fine aggregate

https://www.pavingexpert.com/aggs01#sand

Building sand tends to be small rounded or sub-rounded grains, with plenty of fines and no limit on clay content.

Plastering sand is, in many cases, just a little coarser than building sand, but there's not a lot in it.

Riverwashed sand is usually rounded or sub-rounded grains of a wide-ish range of sizes, and can be relatively 'coarse' but the grain shape is often unsuitable for bedding.

Sharp/Grit/Concreting sand tends to be coarser, with more larger grains of 2-4mm, and many of the grains will be angular or sub-angular, providing far greater interlock for a bedding material.

And then we have the grits, the dusts, the whin, the waste and dozens of other "aggregates" used for bedding. It's enough to drive you mad!

I know only too well about all the "special" mixes various contractors, installers, groundworkers and everyone else seems to have for bedding paving - 2 shovels of this sand to 3 shovels of that sand, and so on - but I also know all too well the science, the testing, the evaluations, the long-term case studies that have been done, in Britain, in Europe, in Asia and North America, and time after time the same conclusion is reached: the best performance for any bedding sand comes from a coarse, gritty, 2-4mm aggregate with angular or sub-angular particles and limited fines. All these tales of using a shovel of building sand to "improve" adhesion - they are delusions! The people coming up with these apocryphal tales may well believe them, swear on their mother's life or whetever, but the data simply doesn't back them up.

What does stand up, and is fully supported by decades of research, is that paving has a longer and less troubled service life when it has been palce donto a laying course that is based on a coarser, gritter, sharper sand (or grit).