I layed old St. Louis solid Brick on the exterior porch and patio floors and interior kitchen floors of my house 6 months ago. The bricks are laid on a concrete slab.
After laying and mortaring, I acid washed with Muratic Acid, Let dry for 24 hrs, then applied Prosoco Limestone and Marble Protector (StandOff a Silicone based sealer). Within 6 weeks, the bricks have turned from a deep red to a light red/grey. It appears the bricks have effloresced under the sealer, and I have not been able to remove it using Prosoco's Safety Clean or Muraitic Acid.
It seems the only way to get to the efflorescence for removal is to remove the StandOff sealer with Prosoco's Dicone NC9. This will be a messy process, especially inside.