I have a pecan tree that’s around 100 yrs old in my yard and wanting to pour a 24x24 concrete slab on one side of it, will be adding to the existing concrete in pic, my question is will it eventually hurt or Kill the tree? The concrete will be about 2 feet from it.
Right now, that pecan's roots go out from the trunk at least 1.5X the width of the crown. In the location pictured, they go under the gravel to the grassy-ish area beyond. Given that the garage is obviously fairly new, and given that vehicles drive over that gravel, the ground under the gravel has already been extremely compacted by the construction vehicles during the garage construction, vehicles driven on the gravel since then and, if the new garage replaced an older garage, vehicles driven on the gravel before the new garage was built. All of this activity has probably already stressed the tree a bit. You can tell by looking at the lower branches of the tree - are they dead on that side but not on the other? Do they have sparser foliage/fewer pecans on the driveway side? If so, then yes, the tree is stressed.
Pecans, like many nut-bearing trees, have taproots and an extensive system of shallow feeder roots. This site says the feeder roots are 6 to 18 inches deep. It also says that pecans should be planted 35 feet or more from a building (or in your case, the garage should have been placed 35 feet or more away from the tree, which it wasn't). This older paper from the Georgia Agricultural Experiment Station gives a ton of information about pecan root structure, none of it encouraging when looked at in light of your project.
So, in your case, replacing the gravel with concrete will almost certainly result in the destruction of all feeder roots to a depth of at least 12 inches, assuming six inches of compacted base gravel under four-six inches of concrete. Since a large proportion of the tree's feeder roots on that side of the trunk are in that depth, and given that the gravel extends for approximately 45% of the tree's crown, then yes, adding concrete will, in my opinion, severely damage the tree and more than likely slowly kill it.
Another concern is that, if the tree is only damaged by the construction, the roots under half the tree will eventually die. And because you're going to place the concrete within two feet of the crown, you will almost certainly kill at least some of the deeper structural roots when you do this (the concrete pourers will probably remove the soil to within 18 inches of the trunk in order to "form" the pad properly). I think there is a very strong chance that a severe windstorm in a few years will blow the tree over.
Now, I am not a certified arborist, so before you do anything I strongly recommend that you contact a certified arborist and get their professional opinion. Note that an Arborist is not the same as a "tree-cutter" or "tree removal expert".