Can you use muriatic acid to kill the grass root between the bricks?

Answer 1

Muratic acid = hydrochloric acid, and it will kill pretty much anything if suitably concentrated, including the roots of plants, and your skin if it touches it. Grass roots are shallow compared to some other weeds and easily killed.

USDA does not recommend a concentration over 20% HCl, and you should wear protective clothing (gloves, goggles, and clothes you don't care about) and only use it in a well-ventilated area because the fumes are corrosive. For comparison, your stomach acid is about 0.5% HCl. You will have felt how much that burns if you've ever thrown up. :)

When diluting, add acid to water and NOT the other way around - that could spit concentrated acid back at you.