Salt DISSOLVES and the water carries it into the surrounding soil. How far depends on the spoil and the amount of rain, but it's not a recommended weed killer.

Use glyphosate or other herbisides, boiling water, or household vinegar ... but not salt.

The simplest method of keeping unwanted growth from the cracks in sidewalks is to clean out the accumulated dust, dirt and organic matter and not pour poisons there. Salt can adversely affect plant growth much beyond the sidewalk and herbicides can get to be quite costly. Boiling water is a hazard if not carefully used and household vinegar (3 to 5 percent Acetic Acid) is not strong enough to control most plants even if applied at high noon. Horticultural Vinegar (20 percent Acetic Acid) is but that can be quite harmful to the applicator if due care is not used.

Return your bag of rock salt, that's not a good idea. 'Crack' between lawn and sidewalk...Are you describing edging, there all several ways to address it. Weedeater, gas or electric edger, half moon shovel as part of regular lawn maintenance. When I had a smaller yard, I've done it on hands and knees with rechargeable cordless hand shears ;)

For actual cracks within driveway or sidewalks, not running the outer length of the pavement, a bottle of premixed ready to use Roundup carefully spritzed on will do a lot less damage than rock salt. Read the label, then apply exactly as directed.

If you clean those cracks of what those "weeds" are germinating in with the hose you probably will not need to do anything more for at least two years. No poison sprays, no boiling water, no extra out of pocket expense, except the water used.

If you could add a pic of the trouble area, suggestions could be much more accurate. Cracks in sidewalk may just need to be spackled with concrete. I don't understand the crack between the lawn and sidewalk either. If there's an actual gap, you could get some dirt & grass seed. My dad used to scalp those edges too close with weed trimmer, which would kill the grass and what took its' place was Oxalis stricta. Maybe you've inadvertently created a situation where the grass can't survive at the edge? Many weeds tolerate salt just fine, I would want to know what the weeds are first, to see if they are sensitive to salt. If not, salt won't kill them. As a long-term solution, this isn't practical, as said above. Rain will move the salt in the direction of the flow of water when it rains, which could affect the lawn and/or large landscape plants like trees and shrubs.