From what it sounds like, it's a good thing that concrete, itself, didn't harden on the brick.
If this is the case, perhaps a good medium-to-heavy power washing will work. You don't want to use any type of tool or chemical that will harm the brick. Bricks are fired to harden and seal them. The insides are, basically, soft and vulnerable to absorbing anything should the outer 'seal' be breached. Take care in this job unless you want to have a new job of replacing broken or chipped bricks.
As an aside, I wouldn't put any type of sealer on walkway bricks. Again, they should be somewhat sealed by the natural firing process. They will absorbe 'liquid' to some degree because they do 'breathe' (I failed to mention earlier, so the sealing I was referring to isn't 100% absolute.) If pressure washing doesn't work, you may have to experiment with a little scrubbing by hand. DON'T use a wire brush; stay with a bristle-brush or a nylon brush.