keystone repair

I have an arch of cut stone that creates the front door to my house. One of the stones at the top of the arch has slipped from its mortar and sticks down an inch or two. Is there an adhesive I can use to stick it back into place rather than using mortar? The orginal mortar is intact around it (not broken, cracked.) I am hoping to use an adhesive that will keep me from having to try to match mortar color.
Any suggestions? Thanks
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Keystone says it all, the " Key"... Jack it in place , dig out bad mortar and mortar it in place, You realy can`t think caulk will do, can you. Loose that stone and all will follow.
You are best to get a pro, since you even consider caulk, if the Key mortar is bad so is alot more. Do not learn on this job.
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As Ransley says, the key is essential to the whole arch holding up, so there is some danger involved. If you are handy and experienced, you could us a bottle jack and 4x4 posts and T-joint to hold up both sides of the adjoining stones, adjusting the pressure just enough to hold them securely, then remove the keystone laterally, if possible. Chisel off mortar on butting stones and on keystone, then pre-moisten all surfaces with water, soak keystone in water a few minutes, then butter all surfaces with bagged ready mix brick mortar mixed to a thick paste. Place keystone in gap, trowel and finish. Brace the stone so it is exactly flush with the underside of the arch, and let mortar set a few days before removing braces. You will need finishing tools for the mortar joints, and don't even consider this repair a DIY project if it is your first attempt to repair brick!
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