Concrete surface preparation

I need to patch some voids in a concrete foundation that had a bentonite clay waterproofing material applied to it. The bentonite clay is quite sticky and seems difficult to remove; is there an easy way to get it off? It clogs up a wire brush quickly, and a normal water hose isn't powerful enough to get it all off. I don't have a power washer and would prefer to avoid renting one for the small area I have to do. Any suggestions would be helpful.
Cheers, Wayne

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