Sandstone Slab Stains/Marks

[DIY Dummy 2809]

Hello all

Had new grey riven sandstone slabs laid around 7 weeks ago and notice some marks/stains are now visible.

Didn’t lay the slabs myself and not particularly handy, so looking for some third party input.

Combination of darker areas and white chalky residue. I believe the white residue to be efflorescence.

Generally, the darker marks seem to correlate with solid points, surrounded by hollow areas so believe this may be reflective staining from where the slab is in contact with the mortar. Slabs were laid on full bed (not spots) so not sure what’s caused this.

Just wondering whether:
a) my assumptions in respect of cause/type of marks seem correct (based on the information and pictures you have access to)?
b) there is any way to resolve or minimise short of lifting and relaying?
c) there is any long-term impact resultant from these issues? E.g will marks stay constant or become more noticeable, will underlying cause impact longevity of slabs etc.?

Thanks

 

[John D v2.0]

Should have been laid on a full bed of sharp sand with a little cement in the mix, as lean as possible, not wet - as dry as possible.
Any water in the mix increases the chance of the cement soaking through and setting at the surface.
A little staining is possible but it should wash out again as long as it's not cement based.
Was the mortar strong and wet?