Sequence of choices:
1) Powerwashing - safe, quick and easy. It removes dirt, mold, moss, etc. Generally, It does not remove stains.
2) Bleach - dangerous to lawns & plants. It is also pretty harmful to auto & home carpets when tracked in. Note that "dry" bleach is still dangerous - the bleached area should be rinsed extremely well. Bleach removes many stains such as the brown stains on concrete from leaves in the fall. It also kills mold, moss, etc. It does not remove dirt.
3) Sandblasting or sand-entrained powerwashing. This removes everything listed above since it works by removing a layer of the concrete itself and everything attached to it. If used intelligently and sparingly, it is somewhat safe, but I'd recommend it only as a last resort. Any form of sandblasting is similar to using rubbing compound (abrasive) to clean automobile paint - you are removing part of what you are cleaning.
Good luck, Gideon
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any info would be appreciated.
thanks mike