you can use muriatic acid. Home Depot or local hardware stores carry it. it's used by masons, tile setters or for cleaning swimming pools, great for cleaning stone and brick not so good for metal, very corrosive with metal.
either dip it or spray it with a squirt bottle, wash it off well when you have your desired look.
CAUTION! wear gloves, protective eyewear, mask and use only outdoors. very bad fumes.