Concrete is normally embedded with mold, mildew and dirt. Spray the concrete with a 1:3 ratio of muriatic acid and water (one part acid/three parts water). The bubbling you see will stop in one to three-seconds. Once the bubbling stops, the solution is farily neutral. Powerwash the surface that you want to clean.
Barry's Tip: "Quick rinse first to prevent any chemicals from getting on you."
Spray the entire surface with sodium hypochloride. A 50% (one part sodium hypochloride/one part water) is usually OK. If you notice dark green round clusters of mold, you may need to use straight sodium hypochloride. Powerwash the surface again, making sure you change directions while washing.
Example: If you start washing with a north/south stroke, the second powerwash should be in an east/west direction. Once the surface is clean, it needs to dry for 24 to 48 hours.
Caution: Never mix muriatic acid and sodium hypochloride together. They create poisonous gas. Don't mix them in a sprayer or on the concrete. Always have a rinse cylce between these applications.
Barry's Tip on Cleaning Pebblestone: "The acid wash stage is necessary to burn the concrete film off the rocks. This film is a by-product of the process used to create pebblestones. It is sometimes necessary to acid was twice to completely clean the rocks."
Now you are ready to seal your concrete, pebblestone or flagstone. To create a golden wet look, seal the surface with an acrylic sealer. To create a natural look use Tuff Stuff Sealer.