Disintegrating Flagstone

I just received a load of flagstone from a Tennessee quarry, and it was covered in a residue from the stone. It looked like dry mortar. We rinsed it off but noticed that every time we tried to rinse it the top surface of the flagstone turned to mud. We called the quarry, and we were told to pressure wash it. We tried that with a low-pressure washer, but it continues to be a problem.

This flagstone is going on a covered patio that will get wet from rain that blows under the roof. I am very concerned about the mess it will make and the softness of the stone. I have never had this problem with flagstone from PA or Alabama. Should I try to seal it or use it somewhere else. I do not want to pay someone to install it and have on-going problems. Please advise.

Thank you!