Fireplaces, Heating Stoves, Flues and Chimneys - Replaceing Brick Hearth with Slate

I am in the process of replacing a brick fireplace hearth with slate tile. I had to roto-hammer out the old brick so that the new tile will be flush with the new laminate flooring being installed in the room (had carpet). I've removed the brick, but also had to remove the front row of fire brick from the firebox. The front row of fire bricks were installed at an angle to meet the old hearth. Do I have to replace these with new fire bricks, or am I safe to to use concrete to level this out? I'll be leveling out the old hearth area with concrete before installing the slate tiles. This is a wood burning fireplace. Thanks.