Concrete Bluestone Patio Installation - Mortar or Polymeric Sand

by JASON
(CLEVELAND, OHIO)

I AM HAVING A BLUESTONE PATIO INSTALLED ON A CONCRETE FOUNDATION THAT IS ALREADY POURED. THE CONTRACTOR THAT I AM USING IS BUSY AND IS TRYING TO CONTRACT OUT THE WORK. THE SUBCONTRACTOR WANTS TO USE MORTAR TO ADHERE THE BLUESTONE TO THE CONCRETE. THE ORIGINAL CONTRACTOR IS CONCERNED THAT WILL CAUSE PROBLEMS IN THE FUTURE DUE TO FREEZE/THAW. WHAT IS YOUR OPINION ON WHETHER OR NOT TO USE MORTAR. THE OTHER OPTION IS TO USE NOTHING AND JUST USE A POLYMERIC TO FILL JOINTS.

ANSWER

Hi Jason,
If you are referring to polymeric sand, I would not suggest that for various reasons and definitely use mortar. There is always a chance of mortared joints cracking with any concrete application, but if the concrete base was installed correctly, the chances of the mortar cracking in freeze thaw conditions are minimized. Besides, you have already chosen a concrete foundation.

I found that polymeric sand can crack. Also, this type of joint filler should be used in a dry laid application. This would include a loose stone base of anywhere from 4 to 8 inches. On top of that stone dust or sand is applied and then the bluestone. The joints are then filled with the same material (stonedust or sand) or some contractors will use polymeric sand. See my page Building a Patio.

For your purposes, I won't go into the reasons that I prefer stonedust over polymeric sand when installing a dry laid patio. However, I can tell you that just as with mortar, in freeze thaw conditions, the polymeric sand can crack. Also, if it starts deteriorating, water will seep down to your concrete base beneath the bluestone, and this will not be good at all!

Here are some other pages with information on polymeric sand. They are written mostly for travertine pavers, but much of the information applies to your question.
Polymeric Sand
Travertine Filler

So definitely use the mortar for your bluestone patio.