Hi @heba_jamaa,

This seems peculiar. Have you spoken to the builder who carried out the work since?

he has no clean what is happening, any ideas?

Auto corrector!! meant no clue

How dry was the mortar mix used to bed the slabs? If a very wet mix was used this can be absorbed by the slabs from below and can show through on the surface. A semi dry mortar is best for sandstone.

Hi Heba I am assuming that mortar was used to lay the stones. Did your contractor specify what method would be used? I can't tell much from the photo. If cement has been absorbed into the slabs there is nothing you can do to rectify that. But as I say, it is difficult to tell from a photograph, sorry.

It looks as though the stone was not sealed after being laid which can cause all of the issues you have mentioned. If the sand was wet the stone would have absorbed some of the colour and stained it I'm afriad. The best way to rectify is to get a professional to clean the stone as much as possible then ensure they are sealed with a high quality sealer.

Hi Heba

I concur with what a lot of people have said about the sealant. Some paving stones are more porous than others and will need sealing to prevent them from absorbing water. I am also concerned, from the photo, that the water has nowhere to drain to. All paved areas need to be laid on a fall (a uniform gradient) to allow water to run off them. This fall should be directed to a drain or planting border so that it has somewhere to drain to. I recommend pavingexpert.com as the most concise and simple reference for information on these matters. Regarding the colour change, I'm afraid this natural weathering happens to all stone. The stone can be brought back to it original colour by jet-washing.

Is it yorkstone OP? The colouring looks like Scoutmoor yorkstone (a sandstone) which does have grey and tan colour variations. The wetness does look like a sealant issue.