Is there a water source near? Sprinkler, etc? If it is lime it will have either a flat white layer appearance or a rocky white pepple appearance. Another possibility is that it is grout from when the chimney was built - simple sulfamic or phosphoric acid wash. If it is lime - good luck. I have tried Hydrochloric, Muratic, Phosphoric, Mineral Shock, turbo blasting at 4,000psi, Calcite presoak, NMD80, etc. The only thing I have not tried is Effortless from EcoChem. I had good luck with the Calcite presoak, NMD 80, and a wire brush on a house a week ago but it took three trips and applications to get the calcium off of the beige brick.
Houston, Texas